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I created two documents with same pixel dimensions (1280x768).

  • Document 1: 72ppi

  • Document 2: 300ppi

When I saved both in .png, they have the same size on the hard disk. Why do I need PPI? Why can't I just use pixels?

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This may be helpful: graphicdesign.stackexchange.com/questions/13777/… –  Scott Jun 16 '13 at 17:25
    
In the context of image meta-data, PPI is only relevant for when you're going to print your images (at which point # pixels / PPI setting = physical dimensions of the printed image. –  DA01 Jun 17 '13 at 20:57
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The difference is 228ppi –  John Jun 20 '13 at 9:37

4 Answers 4

up vote 1 down vote accepted

Because in print, you need other dimensions as well as the information of the density to be able to judge if the result will print ok. Yes, as long as you stick to web you can mostly ignore it. But in print it makes all the difference.

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Related answer about the pixel density: graphicdesign.stackexchange.com/questions/9105/… –  KMSTR Jun 16 '13 at 11:54
    
Retina display too –  Ryan Jun 18 '13 at 0:09

Answering the specific points in the question:

...two documents with same pixel dimensions (1280x768)...

When I saved both in .png, they have the same size on the hard disk

Those aren't normal low and high res variants - what you have there, is two images that are identical, except that the low PPI thinks it's going to spread those 1280x768 pixels thinly across a large area, while the high PPI one thinks it's going to concentrate those 1280x768 pixels in a high quality print on a small area. But the important stuff - the data - is the same.

I'm guessing what happened here is, you changed the resolution in [something like] Photoshop leaving [something like] the 'Resample Image' box unchecked.

This means your image didn't actually change - it just changed how many inches of paper it thinks it can stretch those pixels over.

If you go back to the 300ppi original and scale it down again with "resample image" (or equivalent) checked, it should give you the more lightweight low-res file you expected.


If you'd had the 'Resample image' box or equivalent checked when you changed the resolution down from 300 to 72 to make the low-res image, it would have kept the size in inches or centimeters the same, and simplified the image so that each inch had only 72 pixels instead of 300. For example, If you had a business card design, 3.5 inches by 2 inches, 300 PPI, and you did this, it would still be a 3.5 inch by 2 inch business card, but it would have fewer pixels. It would be a smaller file and would print lower quality, on paper of the same size.

If the 'Resample image' box was unchecked, it would keep the 1250 by 600 pixels, and just calculate how many inches this could cover at 72 pixels per inch. It would be exactly the same image, just thinking it could print bigger because it thinks you're less fussy about print quality, so it thinks it can stretch its pixels further.

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ppi just means 'when you display it, how many pixels are you cramming into an inch of real world space'. Higher ppi (density) means smaller area to stretch your pixels across.

As computer screens have a fixed set of pixels, based on your monitor, it will always display the image at the same size (1 pixel = 1 pixel) regardless of how big or small the ppi (density) is.

Typically screen is considered 72ppi (or 96ppi), but it really depends on your monitor, so ppi is only really relevant to print.

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What about, why pictures looks different for example on a modern TV, i know the TV is 720 x somethin, but why the picture looks more clear and larger –  Ideal Designs Jun 17 '13 at 23:10
    
@IdealDesigns more clear and larger than what? –  DA01 Jun 18 '13 at 3:45
    
@IdealDesigns the TV "720" or "1080" or "480" refer to the width of the image being broadcast. The larger the image, the better the picture appears. It has NOTHING to do with ppi. –  Scott Jun 19 '13 at 18:28
    
@IdealDesigns It is a similar concept to ppi where the image will be sharper the more density of pixels to physical perceivable area, but don't confuse the two. A 42 inch screen @ 720 is stretching 720 pixels across the real world 42 inches, while a 42 inch 1920 full HD screen stretches many more pixels over that same area, creating a denser, sharper image. –  John Jun 19 '13 at 18:34
    
Technically a 42 inch screen is a diagonal measurement, but hopefully you get the idea. –  John Jun 19 '13 at 18:52

ppi is short form of pixel per inch, also called as dpi, dot per inch. Changing resolution, in this case 72 ppi or 300 ppi will change only document size. Document size is changed when we need some changes in image size going for printing. Total number of pixels remains unchanged and hence despite change in document size image size remains same. It is to noted that image size and document size are two different things.

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PPI is not DPI. And there is no direct pixel-for-pixel correlation between two different ppi settings. The total number of pixels does not remain unchanged. –  Scott Jun 19 '13 at 18:37

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