# DPI for canvas print



## SilkeLeenen (Jun 5, 2016)

For my mom's birthday we had a photoshoot with the grandchildren. The photo we would like to print on a canvas is 3530 x 2353 and the desired size is 140 cm x 933 cm. Now the software says that the DPI is too low. Can somebody please help? Is there a way to change the DPI or is the quality of the photo just too low for the size? That would be a pity, because we payed a lot for these professional photographs.

Greetings, Silke


----------



## KmH (Jun 5, 2016)

Is 140 cm x 933 cm a typo?
That works out to 55 inches by 367 inches when converted from cm..
Is the print to be 140 cm by 93 cm - 55 inches by 36.6 inches?

At any rate print resolution (pixels per inch PPI, not dots per inch DPI) is a function of image resolution (3530 x 2353 pixels) and the print size.
If you change the PPI and the pixel dimensions stay the same you also change the print size.

3530 pixels / 367 inches = 9.6 ppi.
3530 pixels divided by 55 inches = 64.18 ppi - still a pretty low print resolution number.

The pixel dimensions are a 3:4 aspect ratio wherein the long side of the photo is 1.5x longer than the short side.

A standard print size with that aspect ratio is a 30 x 20 (76.2 cm x 50.8 cm)
Printing the long side 30 inches would result in a print resolution of 117.7 ppi 3530 px / 30 inches = 117.7 ppi) -  maybe sufficient ppi for an acceptable print.

Larger prints tend to be viewed from further away than smaller prints. The increased viewing distance reduces the need for a higher ppi value.
The texture of a canvas print _increases_ the need for print resolution to offset the resolution lost from the texture.


----------



## SilkeLeenen (Jun 6, 2016)

KmH said:


> Is 140 cm x 933 cm a typo?
> That works out to 55 inches by 367 inches when converted from cm..
> Is the print to be 140 cm by 93 cm - 55 inches by 36.6 inches?



No I meant 140 cm x 93,3 cm, sorry!


----------



## john.margetts (Jun 6, 2016)

SilkeLeenen said:


> For my mom's birthday we had a photoshoot with the grandchildren. The photo we would like to print on a canvas is 3530 x 2353 and the desired size is 140 cm x 933 cm. Now the software says that the DPI is too low. Can somebody please help? Is there a way to change the DPI or is the quality of the photo just too low for the size? That would be a pity, because we payed a lot for these professional photographs.
> 
> Greetings, Silke


You can change the PPI of the picture in Photoshop or Gimp to whatever you want using the Image Size option in one of the menus. Doing this will slightly reduce picture quality but will prevent individual pixels showing in the print.


----------



## SilkeLeenen (Jun 6, 2016)

john.margetts said:


> You can change the PPI of the picture in Photoshop or Gimp to whatever you want using the Image Size option in one of the menus. Doing this will slightly reduce picture quality but will prevent individual pixels showing in the print.



I just did that, now my photo is 8309 x5539 and 12,7 MB, so now when they print it on a 140 x 93 cm canvas, the quality must be sufficient?


----------



## Vtec44 (Jun 6, 2016)

Is the software provided by the print lab?


----------



## SilkeLeenen (Jun 6, 2016)

Vtec44 said:


> Is the software provided by the print lab?


Yes it is


----------



## KmH (Jun 6, 2016)

Did you wonder how your photo now has 2.35x more pixels?
You increased the image resolution (pixel dimensions) to 8309 x 5539 and those new pixels are virtual pixels that had be be created by a math algorithm.
The algorithm considers the original pixels and then creates pixels based on the color and luminosity original pixels near where the virtual pixels will go.
Expect a reduction in image quality because the photo file now contains 2.35x more made up pixels than original pixels.

Changing the PPI cannot reduce the quality of the image as long as the images pixel dimensions are not changed, but it can affect how good a print looks because changing the PPI changes the size of a print if only the original pixels are used to make the print.

If you did not start from a Raw file or didn't edit a copy of the original JPEG or TFF file, you cannot now go back and undo the massive 2.35x image resolution increase you apparently did.


----------



## john.margetts (Jun 6, 2016)

Photoshop creating new pixels is only an extension of the demosaicing process when converting the original RAW file. If you pixel peep, the image will look worse. If you print the picture large, the image will look better. As the required end result is looking at a large print, the look of the large print is the only aspect that matters and the new 'virtual' pixels will improve things.


----------



## KmH (Jun 6, 2016)

john.margetts said:


> Photoshop creating new pixels is only an extension of the demosaicing process when converting the original RAW file.


I'm always up for learning more about how Photoshop works.
Where does Adobe say that?


----------



## john.margetts (Jun 7, 2016)

KmH said:


> john.margetts said:
> 
> 
> > Photoshop creating new pixels is only an extension of the demosaicing process when converting the original RAW file.
> ...


As far as I know, Adobe doesn't say that. I based my comment on understanding how demosaicing works.


----------



## KmH (Jun 7, 2016)

john.margetts said:


> As far as I know, Adobe doesn't say that. I based my comment on understanding how demosaicing works.


What the OP did didn't apply demosaicing.

The photo sites (photo diodes/pixels) do not record color. They only record luminosity.
Demosacing compares real adjacent pixels to each other to interpolate (educated guess) what color of light generated the analog voltage signal (luminosity) a pixel developed.
The demosaicing algorithm has to interpolate color based on the green, red, and blue filter pattern in the camera's Bayer Array. Each pixel can only have 1 of the 3 colors filtering the light that the pixel accumulates during an exposure.

What the OP did, increasing the image resolution, has to use a different type of algorithm.


----------



## john.margetts (Jun 7, 2016)

KmH said:


> john.margetts said:
> 
> 
> > As far as I know, Adobe doesn't say that. I based my comment on understanding how demosaicing works.
> ...


then he would end up with a green picture!

Adobe Raw and other RAW converters automatically demosaic the RAW file as the camera does if you save the picture as a JPEG. Not demosaicing is just not an option.


----------



## john.margetts (Jun 7, 2016)

KmH said:


> john.margetts said:
> 
> 
> > As far as I know, Adobe doesn't say that. I based my comment on understanding how demosaicing works.
> ...


Each pixel in the RAW file has one colour channel (as you mention above). The RAW converter has to best-guess the other two colour channels based on the pixels around the relevant pixel.

When upscaling, Photoshop (or Gimp et al) has to best-guess three colour channels for the new pixel, again based on the surrounding pixels. The difference between this and demosaicing is one of scale - two new colour channels or three new colour channels. The algorithm for both will not be much different.


----------



## KmH (Jun 7, 2016)

For changing the resolution of a photo, Photoshop CC 2015 offers several different algorithms:
• Automatic
• Preserve Details
• Bicubic Smoother
• Bicubic Sharper
• Bicubic
• Nearest Neighbor
• and Bilinear.

Photoshop image size and resolution


----------



## john.margetts (Jun 8, 2016)

KmH said:


> For changing the resolution of a photo, Photoshop CC 2015 offers several different algorithms:
> • Automatic
> • Preserve Details
> • Bicubic Smoother
> ...


I use Gimp which offers a similar range of algorithms. For demosaicing I use UFRaw which offers m five different demosaicing algorithms. What is the relevance of having choice?


----------

