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Forum: Digital Processing, Software, and Printing
11-30-2016, 11:45 AM
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take a very plain image - a flat white wall, or a simple text sign - something with sharp borders between large blocks of color. Compress that with JPG and you'll definitely see the compression happen. JPG is terrible at flat fields, it leaves weird square blocks of slightly different color floating around. That's why in the good old days of web, you'd use GIF for anything like that, and JPG for anything with lots of detail and smooth transitions between colors, JPG was cool with those.
You can see a bunch of info and examples of compression artifacts from jpg here: image quality - What are jpeg artifacts and what can be done about them? - Photography Stack Exchange
As for PPI, I don't think you understand what that is. It has nothing at all in any way shape or form to do with the image contents. When you say 300 PPI or 600 PPI you're saying "I want the default printed size for this to have 300/600 pixels per inch" - so if it's a 3000x6000 pixel image, you're saying you want it, by default, to print it at 10x20 inches (or 5x10 inches at 600). When you actually try to print the image, you can change that setting without modifying the image data at all.
---------- Post added 11-30-16 at 01:48 PM ----------
 Here's a sample of JPG compression at 50% on a flat color block
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