I need a 50×50 mm, 600×600 pixel (digital) picture for my passport. What the hell does that mean? If I’m looking at it on my pc I can make it 50×50. But if I open it in my phone it’ll be 5×5, if I open it in a bigger monitor it’ll be 100×100. How do I know what display they’ll be looking at it on?
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I think you’re mixing up 2 different meanings of pixel. When we talk about a screen we talk about pixel density, how many points of color a screen can render in an inch. This changes from screen to screen and I believe this is why you think the number of pixels of an image is arbitrary. But when we talk about pixels in an image we are talking about something different. A pixel based image format (jpeg, png, gif …) is a file that contains a list of colors. Literally inside a 600×600 png is a list that’s like red, red, red, yellow, yellow, pink and so on for 360,000 entries. When you open this file on a screen, how many screen pixels are allocated for each image pixel is dependent on the screen and the scale of the image. No matter how big or small you display it, the file is the same, which is why if you scale it up too far it will no longer render correctly and will look grainy or “pixelated”
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