PNG doesn't truncate¹ the input data.
Right, because it doesn't need
to, since that had to have been done in the first place when you picked a width-and-height.
Think of it this way. From the original image, for a given output file size, there is an upper bound on the number of pixels times the absolute accuracy of each pixel times the overall accuracy of the image. Information theory tells us that. PNG choses to hold the overall accuracy and absolute accuracy constant, thus bounding the number of pixels. JPEG choses to hold only the overall accuracy of the picture constant, thus allowing the number of pixels to vary inversely with the individual accuracy.
Certainly, if individual accuracy is important, you can't use JPEG. I'm not arguing that. I'm just saying that at some point, you had to go from "real world continuous values" into "discrete values stored in a file". That's lossy. PNG just lets you pick certain things to hold sacred in that process, and JPEG lets you pick certain other things to hold sacred. I'm happy we have both.
-- Randal L. Schwartz, Perl hacker
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