2

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?

  • 3
    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
  • 4
    The difference is 228ppi – John Jun 20 '13 at 9:37
  • PNG file format does not even have a (standard) PPI field if I remember correctly – Yorik Sep 18 '18 at 14:30
3

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.

5

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.

2

Short version:

The only difference is the number stored in the metadata.


Extended version:

This number only recommends the "scale" when you are using a program that actually makes sense of it. These programs are used normally to actually print your document. Think of it as a Post-it saying: "Use this number, please".

It only "recommends" because you can choose to simply ignore it.

Let's think that you open the image on an image viewer and you want to print it. You could have several options when printing:

  • Fit to page
  • Fit to the page with the paper rotated
  • Declared size
  • Scaled version

The first two options will simply ignore the PPI embedded in the metadata, the real printed size will depend on the actual paper size, orientation, margins, etc.

But the other two options will use the declared number in the metadata as a reference to calculate the print size.

Some other programs will use it to define an initial size when working with it, for example, a layout program like InDesign, but again you can simply ignore it and resize the image inside the program.


The window of DOOM!

All this confusion comes from what I call "The Window of Doom" when exporting from Illustrator. Even if you originally defined your document to a specific pixel dimension (as your example 1280x768) this window can ruin your export.

The text is in Spanish but the idea is clear

This "default value" will give you an image of 2667x1600px instead of your original size of 1280x768px.

enter image description here

And this "default value" will give you a file 5333x3200px

enter image description here

But look how the original file was defined when created!:

enter image description here

The selection box defines this as an option for the raster effects, but the program actually it considers all the file to be that PPI!

Add that the archaic, obsolete, useless and misleading number 72 PPI as a resolution for web... How this windows handle the information and the export settings have no sense.


Some other programs have a different approach, simply let you know all the parameters of the export. Here is Corel Draw:

enter image description here

0

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.

  • 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
-1

There is an awesome article on Wikipedia about this subject. https://en.m.wikipedia.org/wiki/Pixel_density But, as it has been said before, if your document is to be printed, choose 300ppi, but for the web and game development, 72ppi is suitable.

  • 1
    Hi Archie1, we like long answers that don't solely rely on links because they can go dead. Can you elaborate what relevant information lies behind your link? – Ovaryraptor Sep 18 '18 at 14:18
  • OK, I will take your remarks into account. – Achie1 Sep 19 '18 at 8:35
-1

The best way to understand this is to suppose you have a photo to "blow up" or enlarge, and then when you enlarge it, you either have broad fields of .jpeg trash artifacts and it doesn't look very precise, or the human brain is saying "hey that's so simplified and smooth, where's all the detail?" So to sharpen it up for printing it as a giant poster, you would have to go in and manually add detail where you find it lossy. It's /kind/ of like a resolution, but 72 ppi would be a lossy version of 300 ppi.

Folks are saying it isn't necessarily lossy, and that's kind of sort of true because if you don't add any more detail to a 'blown-up' image, it will just look like a scaled-up image, but that can look either like blotch-y .jpeg trash, or at best "cartoon-ish," owing to the lack of complexity in the interstitial areas which previously had no data. So complexity always increases with scale, just remember that.

So a relatively decent 72 ppi, while fine when looking at your < 2'x2' monitor, will look more simplistic and primitive in print than a 300 and printed ppi. So either begin the work in 300 ppi and what you see is what you get, or transition it there and then go in and add the arm hairs. Does that make sense?

I'm not trying to say that either print or digital are more precise or complex, only that dpi (a printing term) and ppi (a CG term) are both relative to the actual physical size of the final product.

-2

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.

  • 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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