Take the 2-minute tour ×
Graphic Design Stack Exchange is a question and answer site for Graphic Design professionals, students, and enthusiasts. It's 100% free, no registration required.

I want to check something I don't quite understand with the size/quality properties of an image.

OK, as much as I understand, the size of an image you create in Photoshop, for example, will determine what rectangular area of screen pixels it takes.

And the resolution determines what will be the area of the smallest editable element/block on this picture.

So, if I have an image 1024x1024 with 72 PPI:

  • it will take 1024x1024 pixels on screen;
  • but what area will an editable block will take - how do you calculate it?

Update:

What do I mean by the smallest editable block is the smallest area of the picture you can manipulate. I probably am deeply mistaken, but I think that if you set smaller PPI, you will not be able to edit every single pixel on the picture, but instead will be able to manipulate only bigger chunks of it. Is this correct?

And finally, the produced image, doesn't have a metadata (besides having single colour blocks with size x pixels^2) about the used resolution, right?

It will be awesome if someone answers this, thanks in advance!

share|improve this question
1  
Are you in fact asking how to determine the desired PPI value? Or are you asking how to determine size of something that has no PPI –  joojaa May 15 at 12:15
1  
Or are you asking how the pribter does it? oOr are you asking how photosop resamples the image? –  joojaa May 15 at 12:24
    
Please, check the update of my question. –  Nikolay Tsenkov May 15 at 14:41
1  
You can always edit the amount of pixels no matter what the PPI. The PPI is only a conversion value for translating the image to reality. Pixels dont have an area. Computers dont care, yes you could zoom the picture but the computer in general does not care of your PPI settings it shows pixels. If your image is 100 pixels wide its 100 pixles wide whatever that happens to be. –  joojaa May 15 at 14:45
    
+1 to joojaa for his comment. I just want to add that the editor will always let you edit everything. It will not "filter" the editable pixels based on DPI. –  Aᵂᴱ May 26 at 12:14

5 Answers 5

up vote 4 down vote accepted

I'll try a short-n-sweet answer:

What is the purpose of the resolution property of images (when creating/editing)?

The purpose of the PPI is to tell certain software what size the image should be when printed on a piece of paper. That is all. And most software doesn't care anyways.

A 100x100px image at 72dpi is the exact same image as 100x100px at 300dpi. The only difference is that certain software will print them at different sizes by default.

I believe you're getting confused because you are connecting PPI to 'interpolate' in PhotoShop's set up screens. In this situation, PPI is merely a calculation tool. And does confuse lots of people.

share|improve this answer
    
Thank you, Thank you, Thank you. Did I say ... Thank you! :) –  Nikolay Tsenkov May 15 at 17:50

PPI is a technical conversion factor from pixels to physical dimensions so a image pf 1024 x 1024 at 72 PPI would be 14.22 inches by 14.22 inches or 36.12 mm by 36.12 mm in size.

PPI has no bearing on how big the images are displayed on computer screens. Computer screens show pixels. It only has meaning in very special circumstances such as printing.

Simply divide the pixel quantity with PPI for physical units. You can also figure this out with dimension analysis. So the formula is:

Pixels/PPI = distance in inches

but you were asking the area of the pixel. Individual pixels are dimensionless so they have no area. But if you want to interpret the pitch as an area then

area of pixel = (1/PPI)^2

This is offcourse meaningless since pixels are not squares just samples. See A Pixel Is Not A Little Square.

However, in a image file PPI is a piece of non-authoritative metadata. That is to say its some sort of guide for some purpose or it might have no purpose. In fact 72 PPI and 75 PPI values are typical for systems where the PPI has no meaning whatsoever. 72 and 75 is quite often the default guess of systems as to what the resolution of a monitor is. This off course has not been true for a long time if ever. Its often filled in by the system if no PPI has ever been specified.

So in a image file the information is a technical tool for suggesting what the size of the image is going to be if printed, or for doing fast calculations for quality etc.. This information may or may not be used by the actual printing process. Even in cases where it is used its more of a guide than a actual statement of fact.

Also, PPI is the better descriptive sibling of DPI. And these two are sometimes synonyms to each other. Also used PPM and DPM which is the metric equivalent of PPI an DPI respectively.

TL;DR PPI is a technical conversion factor from pixels to physical units

share|improve this answer
1  
Thanks, but this doesn't answer my question. Pixels/PPI determines the physical dimensions of image on a display with such PPI, not what I asked. –  Nikolay Tsenkov May 15 at 8:31
    
Lets say I make an image with size = NxN with resolution=y, I use only vector graphics, then export a png with the same size. Then I create a project with same size but resolution=2*y, copy all my vector work from image1 and export a new png. image2 will have a better quality, right? The resolution property tipped Photoshop of the desired quality. How does Photoshop figure out what is the size of the minimal editable block (or better yet the minimal block with the same colour & alpha in the future export)? –  Nikolay Tsenkov May 15 at 8:32
    
I have never seen 75 PPI as a typical value. Mac use 72, and Windows use 96. See Wikipedia for historic details on this. –  Aᵂᴱ May 15 at 8:36
    
@NikolayTsenkov no im not talking bout displays in talking avout conversion to physical values has nothing to do with displays. –  joojaa May 15 at 12:01

For typical images, a pixel is a single byte. This is the smallest atomic unit. It is a number from 0 to 255 and is interpreted as a grey value, 0 being black, 255 being white, and 254 values between. For a greyscale image, there is only one channel called grey, for RGB color, there are three bytes stored "per pixel": one value per color channel per pixel. (CMYK has 4 greyscale channels; any alpha channels adds another greyscale channel etc.)

For your 1024 x 1024 example, the smallest unit is a pixel without any regard to the ppi value, which is not relevant. The smallest editable block will always be a pixel. In your example, there are no inches. Therefore, Pixels per Inch cannot be calculated and has no meaning, neither philosophically nor existentially.

You may have seen people say that screens have 72ppi etc. This is a "lie for children." It is obvious that two 1024x768 screens, one which is 20 inches, one which is 13 inches will have a different ppi.

PPI is the same sort of number as MPH or KPH.

360 miles, 60mph means 360/60 = 6 hours
360 miles, 100mph means 360/100 = 3.6 hours

360 pixels, 60ppi means 360/60 = 6 inches
360 pixels, 100ppi means 360/100 = 3.6 inches

It is important to notice that, just like "60mph" tells you nothing about the time or the distance to any individual destination, 60ppi tells you nothing about the physical dimensions of any individual image.

Resolution has multiple definitions.

In the case of making images it simply means the number if pixels. When referring to the resolution of a screen or image, this is the typical definition. This is the actual pixel data. Altering this alters data. Given the huge variety in screen sizes and native resolutions, for screen and web, inches generally do not exist and so ppi is irrelevant. (It is probably better to say "inches are undefined.")

In the more general case, it means the number of physical units represented by an individual pixel. When referring to ppi, this is the meaning intended. For image manipulation and storage on disk, this is merely a tag. Altering this does not alter pixel data. (Notice in the pixel example above that the inches change without any alteration in the pixel count).

In many programs, the user interface will ASSUME you want to alter the pixel data when altering the ppi flag. I think this is a source of confusion for many people. In photoshop it can be turned off, but in the image resize dialog, if you change the ppi field, it will alter the pixel size automatically. If you reset the pixel dimensions back to what they were before hitting "ok" you will see that the file size estimate shows no change.

Note that if I set an image to some ginormous ppi value in photoshop, and set it to 3 inches in DTP software, the RIP software which makes the plate is probably going to resample it to something else based upon the line resolution of the CMYK halftone screens.

One final point is that if you keep the ppi (or MPH) fixed (say 100) and you want to a specific inches (time) factor (say 4), then the only number left to manipulate is the pixels (miles):

400 miles, 100mph means 400/100 = 4 hours
400 pixels, 100ppi means 400/100 = 4 inches

P.S. For printing, there is a rule of thumb that you want to provide 300ppi. All this means is that you want to provide 300 pixels for every inch in size you want it printed, the dpi flag means nothing only the number of pixels matters.

share|improve this answer
    
Exactly where confusion comes from. You have an image project with size 100x100 and PPI of 72 (already created). If you edit the PPI, Photoshop changes the size. This, at least to me, suggests there is a variable size of the min editable block in the image. OMG, I am even more confused... –  Nikolay Tsenkov May 15 at 15:10
    
Don't edit the PPI. It is not relevant. How big in inches is the final printed size? –  horatio May 15 at 15:16
2  
@NikolayTsenkov Theres 2 ways to change PPI you can change it and nothing happens but if you have interpolate checked then Photoshop will assume you wanted to change the pixels but were too lazy to calculate the pixel dimensions. –  joojaa May 15 at 15:18
    
Like jooja says, you can uncheck interpolate, but if you change the ppi of your 100 x 100 image and then go back an reset the pixel size to 100 x 100 before hitting OK, that is the same as "no interpolation" –  horatio May 15 at 15:20
1  
My personal opinion is that we should abolish the entire notion of PPI and DPI they do more harm than good. Or at least lock it tightly to room for just inner circle ininitates –  joojaa May 15 at 16:10

PPI does not have any meaning for an image without the concept of a physical media (like printing on paper). Some raster formats can have metadata for DPI/PPI that is used by printer software when printing the image. Vector formats have no concept of resolution because it has no concept of pixels or dots. The resolution you set, only have meaning when exporting to a raster format that supports resolution metadata.

From Wikipedia:

A digitally stored image has no inherent physical dimensions, measured in inches or centimeters. Some digital file formats record a DPI value, or more commonly a PPI (pixels per inch) value, which is to be used when printing the image. This number lets the printer or software know the intended size of the image, or in the case of scanned images, the size of the original scanned object.
(...)
For vector images, there is no equivalent of resampling an image when it is resized, and there is no PPI in the file because it is resolution independent (prints equally well at all sizes). However there is still a target printing size. Some image formats, such as Photoshop format, can contain both bitmap and vector data in the same file. Adjusting the PPI in a Photoshop file will change the intended printing size of the bitmap portion of the data and also change the intended printing size of the vector data to match. This way the vector and bitmap data maintain a consistent size relationship when the target printing size is changed.

72 PPI is often used as default resolution if you don't need a specific resolution to match a specific size when printed.

share|improve this answer
    
Thanks, I agree it's used (resolution) when exporting, but how? How is "virtual" (the min area/block of pixels with equal properties) pixel size (area) calculated? –  Nikolay Tsenkov May 15 at 10:21
1  
+1 Unmodding negative comment –  joojaa May 15 at 12:25
    
@Aᵂᴱ Please, check the update in my question. –  Nikolay Tsenkov May 15 at 14:43
    
@NikolayTsenkov: See joojaa's comment to your update on the question. –  Aᵂᴱ May 26 at 12:11

Здрасти! Hello. :D

It's not that hard to understand these measures. You want to know how big the area is and why? Let's take a picture of 1000px by 1000px. The area for these pixels is calculated by multiplying (number of pixels) * (pixel pitch of the display). So if we assume a pixel pitch of 0.25 mm, the area for this square would be 250mm by 250mm. This area would vary depending on screen density. Higher density -> smaller pitch -> smaller area. What is the smallest area? The smallest area you can control is 1/3 of a pixel. If we keep the pixel pitch example of 0.25mm / 3 = 0.083mm. PPI is pixels/points per inch, so 1inch = 25.4mm. When we divide by the pixel pitch of 0.25mm, we get 101.4ppi. So if we draw 101px by 101px square it would fill one square inch on the display.

share|improve this answer
    
Please, check the update of my question. –  Nikolay Tsenkov May 15 at 14:41
    
I meant area in pixels, not physical size. I though I could not edit a pixel in an image with smaller PPI. But as @joojaa pointed out - I can edit every pixel in the image, no matter of the PPI metadata. Thanks for taking the time to answer me. –  Nikolay Tsenkov May 15 at 15:01

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.