# resize images without losing accuracy ?



## kmool (May 29, 2007)

Hi guys 

I am new here , so I really dono if this is the right place to post .

I need to resize some images so that I can add them for a book .

For example 

Original 








resized






The problem is that the resized image is not readable ? any help so that I can resize and read the text ?

Thank you


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## Big Mike (May 29, 2007)

What software are  you using to resize the image?


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## fmw (May 30, 2007)

Some software will allow you to do what you want.  Some will not.  If yours will not, then you can resize a copy of the image and save the original for another time and another type of software.


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## The_Traveler (Jun 1, 2007)

You need to remember that you are viewing at 72 DPI and printing is done at 250 so it is crucial that you size the image so that it has enough pixels to print legibly at the actual dimensions you want.

If the original is fully sized as we see it at 1254 - it will flow off the screen when viewed but will print at about 5 inches wide (at 250/bpi).


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## EOS_JD (Jun 1, 2007)

When you resized I think you just selected the image and dragged in the corners.....right?  It's better to resize by editing the number of pixels in the image file.

Are you resizing to print or for viewing?  If to view on the web, resize to 800 pixels on the longest edge. If for printing what size do you need the image to be?


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## skieur (Jun 1, 2007)

If you look at image size in Photoshop, PaintShop Pro or whatever editor you are using, it generally shows a large inch size as in 30 inches by 20 inches or whatever and 72 pixels per inch in another section.

The approach is to up the pixels per inch as in change 72 to 300 ppi or whatever number that reduces the image size to about your printed page size as in 8 by 10 or perhaps smaller.

The result is better printing quality and an appropriate image size.

skieur


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## EOS_JD (Jun 1, 2007)

skieur said:


> If you look at image size in Photoshop, PaintShop Pro or whatever editor you are using, it generally shows a large inch size as in 30 inches by 20 inches or whatever and 72 pixels per inch in another section.
> 
> The approach is to up the pixels per inch as in change 72 to 300 ppi or whatever number that reduces the image size to about your printed page size as in 8 by 10 or perhaps smaller.
> 
> ...


 
You do not need to change the ppi at all.  In a case above, just switch off resampling and change the image size to the size you want to print. The ppi will amend itself and so long as it is around 200ppi or higher it'll print perfectly fine.  In fact the above image will probably print fine at less than 200ppi. Perhaps if it was a photo you may want higher if you were only printing a 10x8.

What we need to know is the use you are putting the image to.


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## skieur (Jun 3, 2007)

EOS_JD said:


> You do not need to change the ppi at all. In a case above, just switch off resampling and change the image size to the size you want to print. The ppi will amend itself and so long as it is around 200ppi or higher it'll print perfectly fine. In fact the above image will probably print fine at less than 200ppi. Perhaps if it was a photo you may want higher if you were only printing a 10x8.
> 
> What we need to know is the use you are putting the image to.


 
When you switch off resampling you lock the aspect ratio or constrain proportions which does necessarily give you the print size you want, so this does not necessarily work.    Adjusting the ppi, gets you closer to the size you want and with 8 by 10 when the image involves print 300ppi or greater would be best.

skieur


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## EOS_JD (Jun 4, 2007)

skieur said:


> When you switch off resampling you lock the aspect ratio or constrain proportions which does necessarily give you the print size you want, so this does not necessarily work. Adjusting the ppi, gets you closer to the size you want and with 8 by 10 when the image involves print 300ppi or greater would be best.
> 
> skieur


 
Best thing to do then is use the crop tool to adjust the aspect ratio. Set the image size you want (in inches, mm, pixels whatever) in the boxes at the top of the screen. Leave the resolution box blank to avoid interpolation.

If you want a 10x8 just insert the figures into the boxes as 10in x 8in and drag the crop tool over your image. If you need a particular resolution (like 300ppi), you can also add this into the resolution box but be aware this will interpolate (up or down) your image.

I prefer to leave the resolution blank because for small images up to 10x8 even with cropping I generally get more than the 240ppi-300ppi I require. For larger images, because they are viewed from slightly further away, you can print at much lower resolutions without any noticable drop in quality.

I was sort of guessing the ratio of the above image wasn't going to change so that's why I didn't mention this earlier but using the crop tool is the easiest and quickest way to resize any image.

The figure of 300ppi is described as the highest quality you need because your eyes can't resolve any higher than this figure (I'm led to believe) but as I noted above you can still print at lower resolutions and you'd be hard pushed to see any difference between 240ppi and 300ppi. the larger the print the further away it will be viewed and therefore the less the resolution can be. My 19"x13" prints are printed at 180ppi and look amazing. I can hardly see much difference even close up.

Hope this helps.

Regards
Jim


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## PatriK-b (Jun 4, 2007)

What you are showing here is a resize vs resample issue.

There are many ways to change the size of a picture, we can categorize them in two categories:
The first one, known as resize, used for exemple if you change the img size constraints in html, it just takes n pixels out of m if you want to resize from m to n.

The second, will compute every pixels in the new image, as a mathematical result from several pixels in the source image. And here you will find different algorithms (splines, lanczos, hermite,...)

First method should never be used, as you will get poor (but fast) results.

Let's take an example:
if you have a image with a b&w pattern of pixels: bwbwbwbw
and you resize it to 50%
you will get bbbb or wwww
if you resample it to 50%
you will get gggg where g=gray

Independently of the algorithm you use, you will get a blur effect on resized text and lines because of edge smoothing by resampling algorithms.

Ihih.


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## skieur (Jun 4, 2007)

EOS_JD said:


> Best thing to do then is use the crop tool to adjust the aspect ratio. Set the image size you want (in inches, mm, pixels whatever) in the boxes at the top of the screen. Leave the resolution box blank to avoid interpolation.
> 
> Regards
> Jim


 
Whether you can use the crop tool or not, depends on how close you can come to the correct aspect ratio without using it.  Images with text such as the example at the beginning are not very flexible to being cropped.  

skieur


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## EOS_JD (Jun 4, 2007)

PatriK-b said:


> What you are showing here is a resize vs resample issue.
> 
> There are many ways to change the size of a picture, we can categorize them in two categories:
> The first one, known as resize, used for exemple if you change the img size constraints in html, it just takes n pixels out of m if you want to resize from m to n.
> ...


 
Not sure what you are saying here but if I'm right it's the second one (resampling) that is the one you should avoid in your scenario.  Resizing does not affect the original image (other than cropping some away).


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## EOS_JD (Jun 4, 2007)

skieur said:


> Whether you can use the crop tool or not, depends on how close you can come to the correct aspect ratio without using it. Images with text such as the example at the beginning are not very flexible to being cropped.
> 
> skieur


 
The crop tool does not actually need to "crop" the image. Select the crop tool and open an image (any image).

To resize it, I just enter the size I want the image to be printed at. Say i have a 20D file that is 72ppi and is 48.667 x 32.444. I know the 20D image is a 3:2 ratio so if I want to print this I can enter any 3:2 ratio to get a print. Say I want a 6x4, take the crop tool, insert the figures 6in x 4in in the boxes at the top and *select the whole image* with the crop tool. Press enter and now look at the image size. It'll be 6x4 @ 584ppi.

So the crop tool does not actually need to "crop" anything. It's being used here to quickly resize your image. It can also quickly resample if you insert a number in the ppi box but I try not to do this too much. Sometimes I'll downsample 6x4s to 300ppi.

It's also a quick way of creating a web sized image. If you want a 600x 400 pixel image just enter the size as 600px x 400px and select the whole image and voila. A small web sized image in seconds (without actually cropping any of the image).

The crop tool has more uses.


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## Omer_Media (Jun 4, 2007)

Hi,

The important thing to remember before resizing the image. you should adjust the dpi to 300 for the minimum print quality, before you resize the image area.

If you start with 72dpi and resize the image in scale and then change to 300dpi you will loose quality.

First dpi then image scale size.


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## EOS_JD (Jun 5, 2007)

Omer_Media said:


> Hi,
> 
> The important thing to remember before resizing the image. you should adjust the dpi to 300 for the minimum print quality, before you resize the image area.
> 
> ...


 
Sorry but this is all totally wrong information. I would strongly suggest reading about resolution before making posts like this as it's obvious you do not understand resolution.

First digital images are made up of pixels not dots so the term is ppi (not dpi). The ppi information means nothing until you come to print your image. What is important in an image is the NUMBER of total pixels. With a 20D I have 3504 x 2336 pixels. This allows me to print almost any size of image I want. resolution and image print size are intrisically linked with the number of pixels. 

The 72ppi (or 300ppi for that matter) means nothing without knowing how many pixels you have in your image.

By default Photoshop shows my image to be 48.667 x 32.444 @ 72ppi

Now I can resize the image to 300ppi without changing the number of pixels in my image to get a print size of 

3504/300 = 11.68 x 7.7867

Both images will look exactly the same on your screen. Both images have the same number of pixels. Nothing has changed.

Now lets do what Omer suggests!

Again by default Photoshop shows my image to be 48.667 x 32.444 @ 72ppi. if I resample to 300ppi I get a 48.667 x 32.444 @ 300ppi which means my image is now 14600 x 9733.333 pixels in size and will be a HUGE file to work with and the image is likely to be very soft as the software is adding all those extra pixels with where it thinks they should go.

Maybe I'm not explaining too well but there's a zillion links in google relating to this.

I try mostly to print at native resolution (I resize with resampling switched off). I can print 19" x 13" prints @ 180ppi that look as good as a 10 x 8 @ 300ppi or a 6 x 4 ' 584ppi!

The human eye cannot interpolate above 300ppi so that is generally deemed to be the highest quality required for a print however as you print larger, the further away you will most likely view the image and therefore the lower the resolution you can print at.

I use a small equation to work out resolution for various print sizes or the number of pixels I require for a specific print size/resolution. 

pixels = ppi x print size
ppi = pixels/print size
Print size = pixels/ppi

Pixels = the number of pixels along one axis of your image
ppi = Image Resolution
Print Size = the resultant print size

Here's a couple of useful links.

http://www.creativepro.com/story/howto/25527.html

http://www.techsoup.org/learningcenter/software/page6045.cfm

http://www.digitalgalleryplus.com/fr_default.htm?/printing__image_resolution.htm


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## Omer_Media (Jun 5, 2007)

I understand that you wish to print for a book, If this is the case we have to think in dpi.

The reason why I set the image to 300dpi first is a pc, will calculate its algoratham from a higher dpi to give better data for the print.

Starting on a lower algoritham may look the same but the image will print with dots slightly further apart reducing quality.

Ask any other printer and they will tell you the same.

You will also need to consider what type of printing technology and print stock you will be using as this also effects quality and bleed when colours are over laid onto each other.


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## Omer_Media (Jun 5, 2007)

In photoshop open 2 files.

one at 300 dpi and one at 72dpi.

Draw a circle filled in black both the same size.

now change the one with 72dpi to 300dpi.

zoom into both of these.

one which started as a 300dpi has the better quality and no fuzz round the sides.

This is because the algoritham works diffrently on lower dpi.

Now imagine these dots printed onto paper, no fuzz better quality at they are packed together. 

Remeber how newspapers use to look years ago all with lots of dots and white space ? In todays age they have better quality due to printing process able to use more dots tightly packed.


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## PatriK-b (Jun 5, 2007)

EOS_JD said:


> Not sure what you are saying here but if I'm right it's the second one (resampling) that is the one you should avoid in your scenario.  Resizing does not affect the original image (other than cropping some away).


First, thank for you previous reply which clarify the ppi, because resolution often confuses people. ppi is useful only to calculate physical printing area of a picture, if you give number of pixels and resolution instead of forcing print area. Changing this value for photo print has no meaning, has you constraint the print size.

I just opened ps and understand why my words are confusing you.

PS do not use 'resample' term as other softwares (and I) do.

With resample checkbox unchecked you only change the ppi information field (and again, yes, it's only a field, it does not affect image data)
With resample checkbox checked, the number of pixels is changed, a filter is applied to get the new picture.

I try to sum up here my minds (and what can be found in a non-ps world  )
- resize just remove (when reducing) or clone (when enlarging) pixels to get the new image. This is very fast, generally used for preview only and this does not exist in ps.
- resample create new pixels, calculated by applying a filter to the original image pixels, this is in ps.

I hope I clarified. :???: not easy to do it in english.


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## EOS_JD (Jun 5, 2007)

Omer_Media said:


> I understand that you wish to print for a book, If this is the case we have to think in dpi.
> 
> The reason why I set the image to 300dpi first is a pc, will calculate its algoratham from a higher dpi to give better data for the print.
> 
> ...


 
Not sure where you are getting your info from but you are 100% wrong....  Fine if you switch off resampling as you are then not changing the document.  It's down to ONLY THE NUMBER OF PXELS IN YOUR OIGINAL. You may only require to resize - not resample.


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## EOS_JD (Jun 5, 2007)

Omer_Media said:


> In photoshop open 2 files.
> 
> one at 300 dpi and one at 72dpi.
> 
> ...


 
THis is not what you shopuld be doing. What you don't seem to understand is that you can change the image to 300ppi without resampling (adding pixels).....

You are actually changing the image by adding these pixels. You can change to 300ppi without adding pixels (honest you can).

Here try this - What is different about these two image sizes

1. 10"x8" @ 72ppi
2. 2.4" x 1.92" @ 300ppi

Both images are the same..... 720px x 576px



Omer_Media said:


> one which started as a 300dpi has the better quality and no fuzz round the sides.


Because you changed the number of pixels in the image (resampled). You can resize without resampling.



Omer_Media said:


> This is because the algoritham works diffrently on lower dpi.


 
No it's because you resampled. I'm saying don't resample because it will reduce the quality of your image (as you are indicating).



Omer_Media said:


> Now imagine these dots printed onto paper, no fuzz better quality at they are packed together.


 
You pack them closer together by reducing only the print size. Not by adding more pixels though.


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## EOS_JD (Jun 5, 2007)

PatriK-b said:


> First, thank for you previous reply which clarify the ppi, because resolution often confuses people. ppi is useful only to calculate physical printing area of a picture, if you give number of pixels and resolution instead of forcing print area. Changing this value for photo print has no meaning, has you constraint the print size.


 
You don't need to constrain the print size though.




PatriK-b said:


> I just opened ps and understand why my words are confusing you.
> 
> PS do not use 'resample' term as other softwares (and I) do.


 
Ps uses the correct terms



PatriK-b said:


> With resample checkbox unchecked you only change the ppi information field (and again, yes, it's only a field, it does not affect image data)


You can change print size and ppi (as you amend one the other changes automatically.



PatriK-b said:


> With resample checkbox checked, the number of pixels is changed, a filter is applied to get the new picture.


 
Yes that's correct. It's also called interpolation.




PatriK-b said:


> I try to sum up here my minds (and what can be found in a non-ps world  )
> - resize just remove (when reducing) or clone (when enlarging) pixels to get the new image. This is very fast, generally used for preview only and this does not exist in ps.


 
Not sure that is correct? You can certainly do anything in Ps regards adding canvas or deleting pixels (cropping) without resampling. But I'm not 100% sure what you mean.



PatriK-b said:


> - resample create new pixels, calculated by applying a filter to the original image pixels, this is in ps.


 
You can do this in Ps yes.



PatriK-b said:


> I hope I clarified. :???: not easy to do it in english.


 
I'm struggling to understand your thoughts exactly. You can increase the print size by reducing the ppi of an image and conversely you reduce the print size by increasing the ppi.

By adding or subtracting pixels by you are changing the number of pixels in the image therefore in my mind you are resampling. Although I do see that if you crop off pixels or add pixels by increasing the canvas size, you'll not resample the image - but I am not talking about this above (or below)

Use the equation I provided

Pixels (P)= Resolution (PPI) / Print Size (PS)
So lets try a couple of things.

I'll use my 20D image files here which are 3504 pixels x 2336 pixels.

When the come into Photoshop, generally they are displayer as 48.667" x 32.444" @ 72ppi. This is exactly the same as 35.04 x 23.36" @100ppi or 11.68" x 7.7867 @300ppi.

Multiply the PS x PPI and you'll get the number of pixels. So from this you can edit to almost any size without changing the number of pixels in your image. Obviously if you want to print around say 24 x 16 (which prints at 146ppi) or larger, then you may want to consider resampling a bit but even at 146ppi you'll get a quality image that will look great on a wall.

Does this help?


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