# Understanding HDR from a photographer's perspective.



## Moglex (Aug 31, 2008)

It's apparant from looking at a few of the HDR posts of the last week or so that there are some serious misunderstandings of what HDR actually is, at least as far as photographers use the term. I want to try and give a very simple overview of what HDR attempts to achieve for the photographer wanting to create a screen or paper representation of a scene. I want to try and do this without obfuscating the issue with details of the non-linear response of our eyes and other sensors.

I hope it will stimulate discussion and perhaps others will want to expound on some of the deeper technicalities.

The problem.

When we talk about the range of a photographic film or sensor we are already creating a certain confusion with terminology because there really isn't a problem with range. If you photgraph a scene with very bright elements and completely dark elements, provided you use somewhere vaguely the correct exposure you will end up with a photograph where the bright parts are white and the dark parts are black.

What we are really concerned with is the fact that because of the tonal resolution of the sensor (and, indeed, the medium on which we view the resultant photograph) we cannot see detail in both the darkest and the lightest areas at the same time.

As I've already said, explaining this properly would involve detailed discussion of the type of non linearity of the sensors and the methods used to overcome the problems this causes, including the possibility of using floating point numbers in the internal representation of the scene.

Instead of that I'd like to try and explain what an HDR photograph on a screen or a piece of paper actually is by showing how you could create an HDR image without the use of anything more than an Instamatic and some glue.

Deep breath!

Imagine that you want to take a photograph of one of those doll's houses where the front opens and you can see all the rooms layed out inside.

Imagine there are nine rooms and that each room is lit differently from the brightest ballroom in (apparantly) even steps to the darkest, moonlit bedroom.

You eyes can probably make out the detail in each room all at once and certainly if you shield them you can see each room clearly, one at a time.

If, however, you take a photograph of the whole house, exposing for the mid point, the darkest room is black and the brightest almost plain white.

So what you do is photograph each room individually, exposing for its own lighting (your Instamatic has unusually close focusing capabilities).

You now send off your cassette to the lab and get back a set of prints and paste them in the correct positions onto a sheet of paper and voila! you have a (rather odd) HDR representation of the interior of the dolls house where you can see the detail of each room just as well as any other, all at the same time.

Of course, this is not what we actually do with photographic HDR as we want to retain an idea of the relative brightness of the rooms.

So now suppose you take the exposures with your digital camera and this time you print them yourself with each room a little darker in proportion to its actual illumination. You will have sacrificed some detail but now have pretty much manually produced 'HDR' picture of the doll's house which you can photograph and compress using the JPEG algorithm (or any other) and display as either a print or a screen image.


This is effectively what HDR software does but in at a much smaller scale and using an intermediate internal representation of the image that has a much greater bit depth than we typically use followed by a mapping back to a suitable output bit depth.


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## Helen B (Aug 31, 2008)

I think that you need to explain where HDR ends and tone mapping begins - that seems to be the main cause of confusion and based on your post above, you appear to be just as confused as many other people.



> What we are really concerned with is the fact that because of the tonal resolution of the sensor (and, indeed, the medium on which we view the resultant photograph) we cannot see detail in both the darkest and the lightest areas at the same time.



I also think that your use of the term 'tonal resolution' is a little misleading - perhaps you need to define it, because it sounds like you are using it for 'dynamic range' - the relative limits of luminance that are recorded. I would interpret 'tonal resolution' to mean the smallest difference in luminance that is recorded or stored, not the overall range.

Best,
Helen


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## Moglex (Aug 31, 2008)

Helen B said:


> I think that you need to explain where HDR ends and tone mapping begins - that seems to be the main cause of confusion and based on your post above, you appear to be just as confused as many other people.



Perhaps you could explain where the doll's house example falls down.

As I cannot see any evidence of confusion that would help to pinpoint it, if the confusion is in my mind rather than yours.



> I also think that your use of the term 'tonal resolution' is a little misleading - perhaps you need to define it, because it sounds like you are using it for 'dynamic range' - the relative limits of luminance that are recorded.



Again, perhaps you could explain how you get that from what I wrote as that is precisely what I do *not* mean.

I'm not quite sure how you can make the jump from 'resolution' to 'range' as the two things are quite different and I risked using a term (resolution) that has an already clearly understood meaning specifically to make that point.



> I would interpret 'tonal resolution' to mean the smallest difference in luminance that is recorded or stored, not the overall range.



Well done!

You are 100% correct.

It just remains for you to explain why you thought I meant anything else.


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## Moglex (Aug 31, 2008)

Helen B said:


> I think that you need to explain where HDR ends and tone mapping begins
> 
> Best,
> Helen



I was trying to avoid getting this technical but as I seem to have confused Helen, I'll have a stab at explaining where this boundary lies as simply as possible.

I'm going to completely ignore non-linearity here as I can guarantee it will confuse the issue for a lot of people.

Suppose you have a sensor that can resolve 100 levels of brightness.

And you have a printer that can display 100 levels of brightness.

And you have a scene that has 150 levels of brightness.

So you take one shot where the bottom 100 levels are resolved and the top 50 are burned out.

You take another shot where the top 100 are resolved and the bottom 50 are all black.

Your software then uses these two images to create an internal representation with the full 150 levels of brightness.

That is the HDR part of the process.

However, as your output device cannot handle all 150 levels the software now performs some tone mapping. For example it could simply divide each value by 1.5 and output it resulting in an image which has compressed detail from all the brighness range but is displayable on a device that can only resolve 100 levels.

As I said above, that is an absurdly simplified explanation, mainly because it ignores the non linear nature of things. The problem with a full explanation is that it requires several pages and the maths gets quite involved for the non-mathematically inclined. I hope it helped at least someone.


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## Jim Benton (Aug 31, 2008)

So what does all this tell us about making an HDR image from one RAW exposure 'developed' different ways?


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## reg (Aug 31, 2008)

Jim Benton said:


> So what does all this tell us about making an HDR image from one RAW exposure 'developed' different ways?



That's not HDR


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## Helen B (Aug 31, 2008)

Moglex said:


> Perhaps you could explain where the doll's house example falls down.



OK.



> So what you do is photograph each room individually, exposing for its own lighting (your Instamatic has unusually close focusing capabilities).
> 
> You now send off your cassette to the lab and get back a set of prints and paste them in the correct positions onto a sheet of paper and voila! you have a (rather odd) HDR representation of the interior of the dolls house where you can see the detail of each room just as well as any other, all at the same time.


I wouldn't call that an HDR image at all. It has already had a form of tone mapping applied to render it as an LDR image. The information about the total dynamic range has been lost, even though it was made from the total dynamic range. Had each print been illuminated at different, appropriate levels then it would be an HDR image. That's really the key: the HDRI file contains information about the full range of scene luminance.



> Of course, this is not what we actually do with photographic HDR as we want to retain an idea of the relative brightness of the rooms.
> 
> So now suppose you take the exposures with your digital camera and this time you print them yourself with each room a little darker in proportion to its actual illumination. You will have sacrificed some detail but now have pretty much manually produced 'HDR' picture of the doll's house which you can photograph and compress using the JPEG algorithm (or any other) and display as either a print or a screen image.


That's where I suggest that tone mapping gets mentioned. The HDR image is turned into an LDR image by tone mapping because of the impracticality (but not impossibility) of displaying the HDR image. It's no longer an HDR image, but an LDR image produced from an HDR image by tone mapping. It's often called an HDR image, of course, because many photographers call tone mapping 'HDR'. An HDR-originated image that is shown as an LDR image with simple tonal compression instead of tone mapping would probably not be recognised as an HDR-originated image by many photographers.



> What we are really concerned with is the fact that because of the tonal resolution of the sensor (and, indeed, the medium on which we view the resultant photograph) we cannot see detail in both the darkest and the lightest areas at the same time.


That sounds very much like a dynamic range problem, not a tonal resolution problem. If the dynamic range of the sensor was adequate, you would be able to see detail in both the brightest and darkest parts.

Best,
Helen


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## jamesino (Aug 31, 2008)

What if you did not tone map the HDR image?

Could you post an example of a before tone-map and after tone-map image please?

Thanks.


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## Moglex (Sep 1, 2008)

Helen B said:


> I wouldn't call that an HDR image at all. It has already had a form of tone mapping applied to render it as an LDR image. The information about the total dynamic range has been lost, even though it was made from the total dynamic range. Had each print been illuminated at different, appropriate levels then it would be an HDR image. That's really the key: the HDRI file contains information about the full range of scene luminance.


All I can really say to that is: If you want to continue to use HDR in a way that is at odds with the way it is generally used photographically, go ahead. No one can stop you.

Do not, however, expect the rest of the photgraphic world to go along with you. That isn't going to happen. At best you will be considered a tiresome pedant and at worst simply wrong.



> That's where I suggest that tone mapping gets mentioned. The HDR image is turned into an LDR image by tone mapping because of the impracticality (but not impossibility) of displaying the HDR image. It's no longer an HDR image, but an LDR image produced from an HDR image by tone mapping. It's often called an HDR image, of course, because many photographers call tone mapping 'HDR'. An HDR-originated image that is shown as an LDR image with simple tonal compression instead of tone mapping would probably not be recognised as an HDR-originated image by many photographers.



I think you are confusing yourself because you still don't seem to understand the difference between range and resolution.

Photographers know what they mean by HDR. People who use computer representations of HDR to do analyses that do not involve physical output of the HDR image mean something else, but this is a photography forum not, for example, a nuclear explosion analysis forum.



> That sounds very much like a dynamic range problem, not a tonal resolution problem. If the dynamic range of the sensor was adequate, you would be able to see detail in both the brightest and darkest parts.



LOL.

You've just demonstrated with perfect clarity that you don't understand the relationship between range, resolution, and the ability to record an image!

Clue: what you say above is exactly wrong.

You've further muddled up the distinction between the source range, the recorded range, and the output range.

I would also suggest that you have a bit of a think about exactly what is involved in originating  and HDR image from multiple exposures to get a more holistic view of the whole shebang.


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## Moglex (Sep 1, 2008)

jamesino said:


> What if you did not tone map the HDR image?
> 
> Could you post an example of a before tone-map and after tone-map image please?



I'm afraid that wouldn't be very helpful as, whilst you could display the 'after' image, the only way you could display the 'before' image would be to use some process that involved silently dumping the very information that makes the HDR image HDR.


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## Helen B (Sep 1, 2008)

Moglex said:


> All I can really say to that is: If you want to continue to use HDR in a way that is at odds with the way it is generally used photographically, go ahead. No one can stop you.
> 
> Do not, however, expect the rest of the photgraphic world to go along with you. That isn't going to happen. At best you will be considered a tiresome pedant and at worst simply wrong.
> 
> ...



We're going to have to differ on this entire subject because further debate is pointless. I'll stick to my version and I guess that you will stick to yours.

Best,
Helen


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## Moglex (Sep 1, 2008)

Helen B said:


> We're going to have to differ on this entire subject because further debate is pointless. I'll stick to my version and I guess that you will stick to yours.



That's fine.

I'll stick to mine as will the the rest of the photographic community (since it's actually theirs).

I really would recommend that you bone up on what tonal resolution and range really mean and how they interrelate, though. Once you get a proper handle on that things should become a lot clearer.


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## Helen B (Sep 1, 2008)

Moglex said:


> ...
> 
> I really would recommend that you bone up on what tonal resolution and range really mean and how they interrelate, though. Once you get a proper handle on that things should become a lot clearer.



I've stopped arguing with you. Why don't you stop the condescension?

Best,
Helen


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## Arch (Sep 1, 2008)

Guys, this is a classic case of both sides having valid and indeed true information, but you are divided by common language and its uses.

Helen is right that without the full luminance values the tone mapped output file is tachnically not a true HDRI.
But... i still maintain my personal view that HDR as it relates to us photographers (and not computer game designers or graphic artworkers) can be used to discribe the tone mapped _representation_ of the finished HDRI.

How else would we as photographers share our HDR images?.... we couldn't.

It would be way too painful to have to go around correcting everyone who displays thier HDR images.... and to give thier images a different name... and what would those images be called?.... this is where a huge amount of confusion would begin....

A guy uses 1 RAW to create an 'HDR'... this can be called tone mapping as all the information used to create the image came from 1 file...

Another guy used 7 exposures to create a HDR.... your going to tell him his image is merly tone mapped as well?.... it may (very) technically be true but if the image was a representaion of the actual finished HDRI he has at home (provided he used all the correct methods) then how can it not be called a HDR?.... it would be very confusing to put this in the same basket as the example above.

Anyway, this is my thinking towards the subject and i have maintined this idea for over 2 years when i first started to experiment with HDR... untill another term is invented to name a tone mapped representation of a _true_ HDR... i will continue to use these terms as i see them.

I would always expect tho to see, as above, people with different appraoches to language and the use of specific terms to disagree... but like you have done, you will just have to agree to disagree... no harm done.


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## Moglex (Sep 1, 2008)

Helen B said:


> I've stopped arguing with you. Why don't you stop the condescension?



I'm sorry. I really wasn't trying to be condescending.

When you posted:



> That sounds very much like a dynamic range problem, not a tonal resolution problem. If the dynamic range of the sensor was adequate, you would be able to see detail in both the brightest and darkest parts.



It really did demonstrate that you are confused about the part played by resolution and the part played by range and how they interrelate.

I shall cease trying to discuss the matter with you so as not to cause further offence.


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## Helen B (Sep 1, 2008)

Arch said:


> But... i still maintain my personal view that HDR as it relates to us photographers (and not computer game designers or graphic artworkers) can be used to discribe the tone mapped _representation_ of the finished HDRI.



I'd like to clarify what I am trying to say here: I see nothing very wrong with the generic term 'HDR' for LDR renderings of HDR-originated images, or even calling things 'fake HDR' - it's an easy shorthand. As I understand it this thread is, however, specifically intended to go further than that loose, almost meaningless description. That's why I suggested that, if you wish to explain things, it may be worth being clear about the role of tone mapping because it is tone mapping that gives most photographic 'HDR' images their characteristic appearance. Moglex did, after all, invite further contributions: _"I hope it will stimulate discussion and perhaps others will want to expound on some of the deeper technicalities."_

Best,
Helen


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## Moglex (Sep 1, 2008)

Arch said:


> Helen is right that without the full luminance values the tone mapped output file is tachnically not a true HDRI.
> But... i still maintain my personal view that HDR as it relates to us photographers (and not computer game designers or graphic artworkers) can be used to discribe the tone mapped _representation_ of the finished HDRI.
> 
> How else would we as photographers share our HDR images?.... we couldn't.
> ...



You are taking a sensible, pragmatic, approach.

It's the only way to deal with these things.

Many years ago people used to drive me to distraction by using the wrong term to describe something. I soon realised that trying to stem the tide was merely tilting at windmills.

If a 'mainfarme' was originally a name that refered to a box containing the CPU and possibly the memory and its antonym was 'peripheral' rather than 'mini' or 'micro', once sufficient people started using it in its more recent sense it was pointless trying to tell them all they were wrong.

This particular subject is complicated because HDR, as used by photographers, is fundamentally different to HDR as the term was originally defined.

There is a very significant extra process that the software must put the image through to create what we call an HDR result that simply does not exist in HDR in the original sense.

This is something that neither Helen B nor Bifurcator seem to be aware of - they certainly haven't mentioned it when defending the original definition of HDR even though it seperates photographic HDR in a very fundamental way.


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## Moglex (Sep 1, 2008)

Helen B said:


> I'd like to clarify what I am trying to say here: I see nothing very wrong with the generic term 'HDR' for LDR renderings of HDR-originated images, or even calling things 'fake HDR' - it's an easy shorthand. As I understand it this thread is, however, specifically intended to go further than that loose, almost meaningless description. That's why I suggested that, if you wish to explain things, it may be worth being clear about the role of tone mapping because it is tone mapping that gives most photographic 'HDR' images their characteristic appearance. Moglex did, after all, invite further contributions: _"I hope it will stimulate discussion and perhaps others will want to expound on some of the deeper technicalities."_



Actually, it is *not* simply tone mapping that gives HDR it's distinctive appearance.

There is a further process in photographic HDR that is fundamental to producing the results we see but which neither you nor Bifurcator seem to be aware of.

Your description of HDR as used by photographers as 'loose, almost meaningless' is just a rather petulant way of disagreeing with the stance that Arch has so sensibly taken.

Photographic HDR has a clear and consistent meaning which is well understood by photographers.

It is simply pointless to try and insist that photographers are misusing a term which once had a single, somewhat different scientific meaning but which has now bifurcated into two terms that are not easily confused because of their wholly different applications.


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## Bifurcator (Sep 1, 2008)

@Helen,
That's how I see all threads where the person is specifically asking about HDR from a state of confusion.


 It's not a case of both sides being right. It's a case of fact vrs. fantasy. But like Helen, in regular discussion if the photo-group wants to misuse a term or two it's no sweat to me. As long as they don't start calling cameras WMDs I'm good.


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## Helen B (Sep 1, 2008)

Moglex said:


> I'm sorry. I really wasn't trying to be condescending.
> 
> When you posted:
> 
> ...



You won't leave it alone, will you? As you still maintained your stance that you are right and I am wrong in two posts after I suggested discontinuing the discussion, I'll explain my point further. I do realise that there is a link between range, detail, tonal resolution and bit depth - in fact there are a number of my posts on the web about that issue, particularly as I am interested in making prints with a very high density range (in excess of 2.5 OD) that are challenging for 8 bpp printing systems.

Nowhere in you original post do you mention bit depth. Bit depth has a lot to do with shadow detail and shadow tonal resolution, but rarely causes a problem with highlight detail - it was the mention of loss of highlight detail that suggested to me that you were referring to a dynamic range problem rather than a bit depth (tonal resolution) problem. 

If the sensor has an adequate dynamic range (ie the shadow and highlight luminances are clear of the sensor's threshold above the noise floor and saturation values respectively) then the ability to record detail in the shadows, _but not the highlights_, will be limited by the bit depth of the ADC (not always the sensor itself, but I appreciate that you are using 'sensor' to mean the sensor-ADC combination, as is common practice). The over-riding factor is that the sensor must have adequate dynamic range.

Best,
Helen


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## Moglex (Sep 1, 2008)

Bifurcator said:


> @Helen,


There's a PM facility for personal messages. 



> That's how I see all threads where the person is specifically asking about HDR from a state of confusion. It's not a case of both sides being right. It's a case fact vrs. fantasy.



All you are saying here is that you are taking a dog in a manger approach and insisting that you and only you are correct when that simply isn't the case.

You are insisting that HDR means one thing: a specific scientific meaning that cannot really relate to photography as practiced by the members of this group since there is no current, practical, way to display the result.

In actual fact the term long ago bifurcated to mean, in addition to its original meaning: a series of processes to allow an photographic output to represent a *H*igher *D*ynamic *R*ange than it would otherwise be able to. (Personally I consider the 'dynamic' to be little more than a noise word originally included to make the TLA more impressive :mrgreen.

I wonder what sort of a state you get into when someone used the term 'ATM' to mean 'Asynchronous Transfer Method'.

Do you jump up and down telling them that it means 'Adobe Type Manager' (or Automatic Teller Machine) and they are living in a fantasy world?


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## Bifurcator (Sep 1, 2008)

jamesino said:


> What if you did not tone map the HDR image?
> 
> Could you post an example of a before tone-map and after tone-map image please?
> 
> Thanks.



This is an EXR formatted HDR: http://tesselator.gpmod.com/Private_/PICT4482_PIZ_hdri.exr Load this into CS3 or another HDR editor (there are many) and select the exposure tool.  Load it into Photomatrix and look ad the HDR histogram or use the exposure up/down tool in Photomatrix. 


Had I saved it as a Radiance file it would have an .hdr file extension - as HDR is a file format. The difference between EXR and HDR is that the EXR format has more compression options, it's a little more robust (in terms of options), and it's an "open" file format meaning us developers can get twisted.   See: www.OpenEXR.org


And here's the tone-mapped jpeg as requested:


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## Helen B (Sep 1, 2008)

As we're all chatting away so nicely here, I'd love to see some of Moglex's photos. Where might I find some on the web? That's what photography is all about, after all, and it helps to give a perspective.

Many thanks,
Helen


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## Bifurcator (Sep 1, 2008)

Helen B said:


> As we're all chatting away so nicely here, I'd love to see some of Moglex's photos. Where might I find some on the web? That's what photography is all about, after all, and it helps to give a perspective.
> 
> Many thanks,
> Helen



Is he raising a ruckus again?   I still have him on ignore so I can't see anything except what you quote.


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## Moglex (Sep 1, 2008)

Helen B said:


> You won't leave it alone, will you? As you still maintained your stance that you are right and I am wrong in two posts after I suggested discontinuing the discussion, I'll explain my point further.



Come on now, be fair, you started up again after you said we should agree to differ by arguing with Arch's very sensible post trying to insinuate you were right and everyone else was wrong by using expressions like: 'loose, almost meaningless description'.



> I do realise that there is a link between range, detail, tonal resolution and bit depth



Good, that's a start.



> Nowhere in you original post do you mention bit depth.



Indeed.

That is because I was trying to make the explanation as simple as possible and it is not actually necessary to refer to bit depth to give a simple explanation of what is going on.



> Bit depth has a lot to do with shadow detail and shadow tonal resolution, but rarely causes a problem with highlight detail - it was the mention of loss of highlight detail that suggested to me that you were referring to a dynamic range problem rather than a bit depth (tonal resolution) problem.
> 
> If the sensor has an adequate dynamic range (ie the shadow and highlight luminances are clear of the sensor's threshold above the noise floor and saturation values respectively) then the ability to record detail in the shadows, _but not the highlights_, will be limited by the bit depth of the ADC (not always the sensor itself, but I appreciate that you are using 'sensor' to mean the sensor-ADC combination, as is common practice) {Moglex: "_Indeed - as I have said I was trying to make the explanation as simple as possible, not show off my knowledge with irrelevant pedantry_"}. The over-riding factor is that the sensor must have adequate dynamic range.



I'm sorry to have to say this and run the risk of getting accused of condescention again, but you still don't seem to realise how 'dynamic range' relates to resolution.

Once again, you inject a raftload of impressive technical sounding terms: "_luminances are clear of the sensor's threshold above the noise floor and saturation values respectively_" (BTW, I assume you intended *below* the saturation levels rather than above?), without seeming to grasp the underlying truth: 

*The sensor is mounted in a camera so it's raw range is irrelevant. What is important is its resolution because that is what determines the amount of shadow detail it can capture at the point that highlight detail is about to be lost.*


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## Helen B (Sep 1, 2008)

Moglex said:


> Come on now, be fair, you started up again after you said we should agree to differ by arguing with Arch's very sensible post trying to insinuate you were right and everyone else was wrong by using expressions like: 'loose, almost meaningless description'.



I wasn't arguing with Arch, I was agreeing with him: _"I'd like to clarify what I am trying to say here: I see nothing very wrong with the generic term 'HDR' for LDR renderings of HDR-originated images, or even calling things 'fake HDR' - it's an easy shorthand."_

Best,
Helen


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## Bifurcator (Sep 1, 2008)

Wow... a butt kiss to the admin and throwing you under the bus all in the same sentence.  

I guess Arch is smarter than that. He types like an intelligent guy anyway.   I guess he knows we're all just here trying to communicate our ideas. It's all good, it's all fun, it's all good fun.


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## Moglex (Sep 1, 2008)

Bifurcator said:


> Wow... a butt kiss to the admin ...
> 
> I guess Arch is smarter than that. He types like an intelligent guy ...



ROFLMAO.

Did you mean to be that funny?


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## tirediron (Sep 1, 2008)

Helen B said:


> As we're all chatting away so nicely here, *I'd love to see some of Moglex's photos*. Where might I find some on the web? That's what photography is all about, after all, and it helps to give a perspective.
> 
> Many thanks,
> Helen


 
I'll second this.  

Moglex you are certainly using some very impressive terminology, and I have to admit that when we start getting into the technical aspects of how sensors actually 'sense' I start to get out of my depth.  I'd like to see some supporting references for  your statements.  Not saying I don't believe you, but I have seen Helen's work and I know from other posts she's made that she is extremely knoweldgable, and I'm more than a little reticent to doubt her.

I'd also like to comment on a statement made earlier in the post refering to the distinctive appearance of HDRs.  A properly executed HDR should NOT have a distinctive appearance.  Discussions of bit-depth, dynamic range and so forth aside, just because you can produce an image with a mind-boggling dynamic range doesn't mean it's a good image.


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## Arch (Sep 1, 2008)

come on guys.. im sure we are all smart enough to know that disagreeing with someone only makes you learn more about looking at the subject from another perspective, even if you think the other person is dead wrong.... after all disagreeing is where real progress is made... however it doesn't need to get personal... im sure we can all agree that.

So please show a little self restraint and bite your toungs


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## Moglex (Sep 1, 2008)

tirediron said:


> I'll second this.
> 
> Moglex you are certainly using some very impressive terminology,



I'd actually tried to keep the technicalities to a minimum, They only intruded when I quoted what Helen had said.



> and I have to admit that when we start getting into the technical aspects of how sensors actually 'sense' I start to get out of my depth.  I'd like to see some supporting references for  your statements.



I'm afraid that really, if you want to know whom to believe, there is no substitute for going to texts and actually understanding what is going on.

There is absolutely no way you can possibly gain any knowledge about sensor function from examining my photographs. Or Helens.

Frankly, Helen's mentioning them is nothing more than a cheap diversionary tactic designed to shift attention away from the fact that she has got out of her depth on the technicalities.



> Not saying I don't believe you, but I have seen Helen's work and I know from other posts she's made that she is extremely knoweldgable, and I'm more than a little reticent to doubt her.



What Helen tends to do is to regurgitate large chunks of photographic and optics texts in what appears to be an attempt to impress people with her knowledge (which I dare say *is* impressive, though not inexhaustible).

Answering questions on fora such as these should be about looking at what the OP has asked, understanding why they are having difficulty with their problem and explaining it in the simplest terms that will get the message across (perhaps with pointers to extra reading).

It should not be taken as an opportunity to show off the fact that you can read an optics or photographic text and paraphase it. If the person asking the question had the time and/or capability to do that they would have done so.

If you wish to determine the truth or otherwise of what I have written on the basis that Helen has typed a great deal of theory (which you admit you do not always understand) in the past, then I can't really stop you. (Although all you are really saying is that she's typed a load of technical stuff, I've typed a load of technical stuff and because she's been doing it longer you're inclined to believe her).

I would point out, however, that after I last pointed out the flaw in her understanding of range and resolution as it relates to a sensor in a camera system she failed to come up with any counter argument and limited herself to a denial of disagreeing with Arch.



> I'd also like to comment on a statement made earlier in the post refering to the distinctive appearance of HDRs.  A properly executed HDR should NOT have a distinctive appearance.  Discussions of bit-depth, dynamic range and so forth aside, just because you can produce an image with a mind-boggling dynamic range doesn't mean it's a good image.



It is true that if you use HDR sparingly you can achieve a perfectly natural result that just looks as if you have been lucky/careful with your lighting.

I was really refering more to those shots where HDR is used in such a way that is has a sort of signature look.

In both cases, however, there is actually much more going on than simple tone mapping which is what Helen/Bufurcator seem to be implying.


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## Moglex (Sep 1, 2008)

Arch said:


> come on guys.. im sure we are all smart enough to know that disagreeing with someone only makes you learn more about looking at the subject from another perspective, even if you think the other person is dead wrong.... after all disagreeing is where real progress is made... however it doesn't need to get personal... im sure we can all agree that.
> 
> So please show a little self restraint and bite your toungs



Sorry, I started my last post before I read this (I had lunch between starting and finishing it).

I entirely agree with you but I really feel that trying to get ahead in a technical argument by introducing irrelevancies such as looking at someone's photographs is going a bit far.

I'd like to keep this discussion going in a steady, non confrontational way as I think there is much of interest to be brought out.

Even though I haven't changed my opinion I've certainly learned by being forced to think about it in ever greater detail to try and get my point across as clearly and concisely as possible.


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## Bifurcator (Sep 1, 2008)

Arch said:


> bite your toungs



Ouch!  <bleeds excessively on the floor>  Thanks allot!


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## tirediron (Sep 1, 2008)

> I'm afraid that really, if you want to know whom to believe, there is no substitute for going to texts and actually understanding what is going on.


That's one way of doing it, however generally in a debate persons making statements also cite references; to what page(s) of which publications shall I look to find corroborating evidence for your statements?



> There is absolutely no way you can possibly gain any knowledge about sensor function from examining my photographs. Or Helens.


Granted, and to be quite honest, nor do I care. As a film photographer, I knew that the my film was comprised of silver halide in a gelatin base and that's about all. What I can get from your images is a sense your abilities as a photographer and from that make an assessment on the likely accuracy of your posts. If for instance I look at a gallery of your work and say, My god, this man is the next Ansel Adams or O. Winston Link, then it's much more likely you do know what you're talking about and not simply regurgitating paragraphs out of textbooks, whereas if I go to your gallery and see a half-dozen poorly exposed holiday snaps with fingers and toes cut off, I'm going to [reasonably I think] have some doubt about your level of expertise. I have seen Helen's work, readily available by the links in her signature, and while it's by no means all to my taste, it is definitely the work of a skilled photographer.



> What Helen tends to do is to regurgitate large chunks of photographic and optics texts in what appears to be an attempt to impress people...


Unless you can substantiate these allegations, please refrain from personal attacks; it's hardly the mark of a gentleman!


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## Helen B (Sep 1, 2008)

Moglex said:


> ...
> 
> I would point out, however, that after I last pointed out the flaw in her understanding of range and resolution as it relates to a sensor in a camera system she failed to come up with any counter argument and limited herself to a denial of disagreeing with Arch.



That's because I have suggested dropping the topic. Although you can't let it go, you can't come up with a reasoned, technical argument that can be discussed to show that you are correct. You simply say that I am wrong, and make a personal attack on my motives and knowledge, and make false claims about me that you cannot know the truth about.

Best,
Helen


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## Bifurcator (Sep 1, 2008)

tirediron said:


> Quote:
> 
> 
> > What Helen tends to do is to regurgitate large chunks of photographic and optics texts in what appears to be an attempt to impress people...
> ...




Hey, no picking on Helen!  Helen has got to be one of the most helpful and knowledgeable people here!


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## Moglex (Sep 1, 2008)

Helen B said:


> That's because I have suggested dropping the topic.



And then started up again. 



> Although you can't let it go, you can't come up with a reasoned, technical argument that can be discussed to show that you are correct.



No, I explained exactly where your error was and you simply ignored that.



> You simply say that I am wrong,



No, I explained:

*The sensor is mounted in a camera so it's raw range is irrelevant. What is important is its resolution because that is what determines the amount of shadow detail it can capture at the point that highlight detail is about to be lost.*

Let's drop the personal stuff.

I'm quite prepared to admit that I could be wrong. It happens.

Just explain what is incorrect about the above, bolded, statement. It's not a complicated statement so it should not be hard to point out any error.

If you can't, admit that it was you who was confused.

Remember this 'discussion' started because you accused me of being confused in what I wrote in the OP.


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## ksmattfish (Sep 1, 2008)

Back to the initial point (I think)...  Yes, there is a difference between a high dynamic range photo and local contrast manipulation (such as tonemapping), and when many photographers refer to the look of HDR they are really concerned with modified local contrast.  Google "dynamic range" and "local contrast" to learn a lot without the bickering.


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## tirediron (Sep 1, 2008)

Moglex said:


> *The sensor is mounted in a camera so it's raw range is irrelevant. *


 
Okay, for my edification, what is the "raw range" referred to and why is it irrelevant?  




Moglex said:


> *What is important is its resolution because that is what determines the amount of shadow detail it can capture at the point that highlight detail is about to be lost.*


 
Which resolution?


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## Moglex (Sep 1, 2008)

tirediron said:


> That's one way of doing it, however generally in a debate persons making statements also cite references; to what page(s) of which publications shall I look to find corroborating evidence for your statements?



You cite references to back up facts.

Your audience has to use their own brains to decide if your reasoning is correct.

I'm not arguing about facts, I'm arguing about the deductions that can be made from known facts.

If someone had made the same argument already (that I was aware of) I'd simply point to it. 




> What I can get from your images is a sense your abilities as a photographer and from that make an assessment on the likely accuracy of your posts. If for instance I look at a gallery of your work and say, My god, this man is the next Ansel Adams or O. Winston Link, then it's much more likely you do know what you're talking about and not simply regurgitating paragraphs out of textbooks, whereas if I go to your gallery and see a half-dozen poorly exposed holiday snaps with fingers and toes cut off, I'm going to [reasonably I think] have some doubt about your level of expertise.



I'm afraid that's logically nonsensical.

I could be a superb photographer and talk complete and utter garbage about technicalites or I could never have taken a photograph in my life and yet be a scientifc geniius who knows the technicalities inside out. (Actually, I'm not that good at either :blushing.

If you think you can get any idea of someone's technical knowledge at this sort of level of abstraction by looking at their photographs you really are deluding yourself.

This is somewhat different to someone discussing the technicalities of, say, lighting placement, where you could glean some knowledge of their competance from their results.


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## Moglex (Sep 1, 2008)

tirediron said:


> Okay, for my edification, what is the "raw range" referred to and why is it irrelevant?



A sensor will have a certain range of light values for which it will respond in a consistant fashion. Above that level it will saturate and be unable to produce useful information.

This is the equivelant of overexposing a film. You lose information in the highlights because once the film is completely exposed that's it and anything brighter will only result in the same density on the -ve.

It's irrelevant for the same reason that a film's range (not its speed) is irrelevant in that once it is in the camera you have various means at your disposal to shift the *effective* range over a vast field (exposure control).



> Which resolution?


The resolution of the sensor-ADC (hereafter refered to as the sensor).

If this resolution is low, then you will not be able to expose for very bright objects and retain detail in the shadows.

As it increases you can decrease the exposure to handle brighter subjects and the extra resolution will mean that shadow detail can be kept the same.


I hope I've made that clear.

It's not an easy concept to distill into a few lines.


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## tirediron (Sep 1, 2008)

> I'm arguing about the deductions that can be made from known facts.


Fine; I'm an eager little beaver, I would like to learn more about this (and no, that's not sarcasm); I'm asking you as the person who is alleging expertise, where I should go.



> I'm afraid that's logically nonsensical... <snipped> ...This is somewhat different to someone discussing the technicalities of, say, lighting placement, where you could glean some knowledge of their competence from their results.


Not at all; you purport to be very knowledgeable in the matter, so it doesn't seem unreasonable of me to assume that you must have produced one or two HDR images, I would submit that those would serve very well as indicators of your skill level, unless of course this is supposed to be a purely academic discussion based on theory, in which case I believe you want www.thephilosphyforum.com.


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## Moglex (Sep 1, 2008)

tirediron said:


> Fine; I'm an eager little beaver, I would like to learn more about this (and no, that's not sarcasm); I'm asking you as the person who is alleging expertise, where I should go.



For starters, look at the answers I gave to the two specific questions you asked.

If anything is unclear, please say so and I'll try and explain further.



> Not at all; you purport to be very knowledgeable in the matter, so it doesn't seem unreasonable of me to assume that you must have produced one or two HDR images,



I'm afraid on its own it's completely unreasonable.

Would you expect someone who was a expert scalple maker to have performed an operation?



> I would submit that those would serve very well as indicators of your skill level



They would indicate photographic skill, not the underlying knowledge of the technology which is why I say you are deluding yourself if you think you can ascertain whom is correct about an aspect of sensor technology by looking at photographs.


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## tirediron (Sep 1, 2008)

> For starters, look at the answers I gave to the two specific questions you asked. If anything is unclear, please say so and I'll try and explain further.


You in fact only answered the former; to the latter you simply said, "The resolution of the sensor". Does term as you intend it refer to the number of pixels on the sensor? Does it refer to the sensors ability to distinguish between different wavelengths of light, or? Again, there was no sarcasm intended, but the term 'resolution' when applied to the sensor of a digital camera can be interpreted in several different ways.



> They would indicate photographic skill, not the underlying knowledge of the technology which is why I say you are deluding yourself if you think you can ascertain whom [sic] is correct about an aspect of sensor technology by looking at photographs.


I would no more assume that a photograph was indicative of someone's knowledge of camera sensor theory than I would assume that taxi-driver's skill behind the wheel is indicative his knowledge of the workings of the internal combustion engine, however, since your thread title is: "*Understanding HDR from a photographer's perspective*." I have to take from that you are a knowledgeable photographer, and in particular well versed in the "making" of HDRs, since you couldn't apply theoretical knowledge without practical experience. Therefore, to assess the extent of said practical background, I would like to seem some of your work. 

Oh, and BTW, it's "ascertain *who* is correct".


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## Moglex (Sep 1, 2008)

tirediron said:


> You in fact only answered the former; to the latter you simply said, "The resolution of the sensor". Does term as you intend it refer to the number of pixels on the sensor? Does it refer to the sensors ability to distinguish between different wavelengths of light, or? Again, there was no sarcasm intended, but the term 'resolution' when applied to the sensor of a digital camera can be interpreted in several different ways.



Oh dear. I had hoped that your questions were genuine but it is clear from the above that you are just playing silly beggers.

You claim that your questions were genuine but you make no comment about the answering of the first - did it enlighten you?

About the second you are obviously not really interested as you are merely nit picking. When talking about the dynamic range of a sensor could any intelligent person *really* believe that a reference to resolution referred to the number of pixels? Even if by some stretch they could, could they really be so stupid as to omit the one resolution that actually relates to the subject in hand?

The answer, to keep playing your game, is the number of bits of luminance information per channel.

Now, do you understand why what Helen said about the dynamic range showed a fundamental lack of understanding of the subject or do you want to keep nit-picking?



> I would no more assume that a photograph was indicative of someone's knowledge of camera sensor theory than I would assume that taxi-driver's skill behind the wheel is indicative his knowledge of the workings of the internal combustion engine, however, since your thread title is: "*Understanding HDR from a photographer's perspective*." I have to take from that you are a knowledgeable photographer, and in particular well versed in the "making" of HDRs, since you couldn't apply theoretical knowledge without practical experience. Therefore, to assess the extent of said practical background, I would like to seem some of your work.



You've still got it completely wrong, though, haven't you? The article as you quoted is:  "Understanding HDR from a photographer's perspective", not: "How to actually produce an HDR image". Had it been the latter your desire to see some evidence would have made sense. As it is it makes none.




> Oh, and BTW, it's "ascertain *who* is correct".



You are quite correct. A silly slip, but I trust you realise that by descending to contravening netiquette and picking up on spelling, typos or grammatical errors you have clearly shown that your pretence at a legitimate desire to understand is just that: a pretence?


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## tirediron (Sep 1, 2008)

Have a nice day!


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## table1349 (Sep 1, 2008)

As a member of the "Photographic Community," mind you I am only speaking for myself as a member, I have but one thing to say.  *"Yawn...."*  ( I must have missed the election of spokesman for the photographic community or was it on the Florida ballot in 2004?)

This whole discussion reminds me of the workings of our sun.

*Fusion in the Sun step by step*

  The overall fusion reaction  4 1H + 2 e -->  4He + 2 neutrinos + 6 photons  happens in several steps. (This is not different from methane burning, which also happens in steps.)  
*Step 1*

  1H + 1H -->  2H + antielectron + neutrino  
	

	
	
		
		

		
			




  This step is nearly impossible. 

 The protons repel each other because they both have + 1 charge.
 They have to get within 10-15 m of each other for the strong interactions to hold them together.
 One of the protons has to change into a neutron (emitting an anti-electron and neutrino).
 This requires a _weak_ interaction.
 But the chance of this weak interaction happening just when the protons are together is almost zero.
 
 This near impossiblity has two consequences: 

 The gas must be very hot, so that the protons hit each other with high speed.
 This lets them get near to each other, even though they repel each other.
 
 Even so, the reaction is very rare.  That is why the Sun is still burning after 4.6 billion years!
 *Step 2*

  electron + antielectron --> photon + photon 
	

	
	
		
		

		
		
	


	



 The antielectron soon hits an electron. They annihilate to make two photons. (These high energy photons will be absorbed in the gas, eventually giving lots of low energy photons.)  *Step 3*

  2H + 1H -->  3He + photon  
	

	
	
		
		

		
		
	


	



 The 2H hits a proton and sticks to it, making a 3He and emitting a photon in the process. (This high energy photon will be absorbed in the gas, eventually giving lots of low energy photons.)  *Step 4*

  3He + 3He -->  4He  + 1H+ 1H  
	

	
	
		
		

		
		
	


	



 Since these reactions make 3He nuclei, there are some of these bouncing around in the gas. When two of them hit each other, the 2 neutrons and 4 protons of  rearrange themselves into one 4He nucleus and two free protons.  Note that steps 1,2 and 3 each happen twice for each time step 4 happens.
  The net result is 
 1H + 1H -->  2H + antielectron + neutrino
1H + 1H -->  2H + antielectron + neutrino
electron + antielectron --> photon + photon
electron + antielectron --> photon + photon
2H + 1H -->  3He + photon
2H + 1H -->  3He + photon
3He + 3He -->  4He  + 1H+ 1H  
  That is 
 6 1H + 2 e   -->  4He + 2 1H  + 2 neutrinos + 6 photons 
 or 
 4 1H + 2 e   -->  4He   + 2 neutrinos + 6 photons 
 The amount of energy released in each separate reaction has been measured. The net energy release is 26 MeV.   There are some other reactions also. They are less significant because they don't produce much energy. [For this reason, please don't try to memorize them.] But they have a special significance because they produce neutrinos, which we will study. 

 3He + 4He -->  7Be + photon
   7Be = (4 p + 3 n)
 
 7Be + electron -->  7Li + neutrino
   7Li = (3 p + 4 n)
 
 7Li + 1H --> 2  4He
 alternatively
 7Be + 1H -->  8B + photon
   8B = (5 p + 3 n)
 
 8B --> 2  4He + antielectron + neutrino
 
   There is another energy producing cycle that is important in hot stars (temperatures bigger than 16 x 106K). It is called the CNO cycle. In it, carbon is a catalyst. It is needed to make the reaction work, but is not used up. [Again, I would suggest that you not memorize the cycle, but _do_ understand the concept of a catalyst, which is important in practical chemical reactions.] 

 start with 12C
   12C = (6 p + 6 n)
 
 12C + 1H -->  13N + photon
   13N = (7 p + 6 n)
 
 13N --> 13C  + antielectron + neutrino
   13C = (6 p + 7 n)
 
 13C + 1H -->  14N + photon
   14N = (7 p + 7 n)
 
 14N + 1H -->  15O + photon
   15O = (8 p + 7 n)
 
 15O --> 15N  + antielectron + neutrino
   15N = (7 p + 8 n)
 
 15N + 1H -->  12C + 4He
 The net result is that you get the 12C back and you have used up 4 protons to make one 4He, along with two antielectrons and three photons. The antielectrons will annihilate with electrons to make photons.  So hydrogen is burned to make helium with carbon as a catalyst. 



The bottom line is, sun make light, sun produce heat, sun warm earth and sustain life.  Sun good.  



But I have printed this out for those nights when I am experiencing insomnia.  It will help.


P.S.  Helen, I still love you.


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## roadkill (Sep 1, 2008)

Helen and Moglex are in love


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## LaFoto (Sep 1, 2008)

You want this thread closed, methinks. 
Your wish is my demand.


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## LaFoto (Sep 2, 2008)

Well, hoping that everyone has calmed down some over the issues between yourselves at hand, this thread is being reopened, but also moved to its appropriate forum, i.e. the Graphic Programmes Forum. Please keep it a discussion free from personal insults.


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## Flash Harry (Sep 5, 2008)

cant you sleep


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## onedayillknowbetter (Sep 5, 2008)

I wonder if that dude's pictures get better the more he insults people on the internet.  Sheesh.

Close this back up again, and suspend Moglex for starting internet fights like a child.  I was hoping to actually learn something from this thread, but it was just a bunch of nit-picking ego-centric people bashing each other, and I actually feel dumber after reading it.


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