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in college 20 years ago, my media teacher tried to tell us how gif compression worked. her knowledge was really limited, and i think it was she, among various others too over the years, who told me "is it you or i who is the teacher?".

anyway, she never explained jpg compression, and ive never heard anyone else talks about its techniques and pitfalls.

its quite simple though.

jpg essentially takes every 8x8 pixels, and turn them into an equation. the higher quality settings you use, the more advanced equation.

for noise or highly detailed images, you get discernable block artifacts, as they are too unique to turn them mathematical, and the equation fails. just like how FLAC geeks say classical concerts dont work with mp3, because it cant keep up with all the instrument, even at 320kbps.

again, ive heard almost no one talk about jpg artifacts, and they happily oversharpen and downsize their images, upload as jpg online, only to have it get compressed again. like what happens on soundcloud, even if you upload a well-produced mp3, the site will still apply both a limiter and compressor to the sound, then downgrade it to 64kbps - 4% of the original information of a CD. its not much better in flac, unless you use a 96khz 24bit original.

it gets worse still, to conserve space, if an image has parts with low detail or smoothness, it shreds the detail. this comes from that jpg was intitially created for television, so they assume youre looking at an image that is in 5% of your vision.

and even further, most sites and programs, both image and video, further downgrade the signal by compressing the colour space further. JPG doesnt use RGB but rather LAB, which means theres 1 channel for luminance, and 2 channels for colour. and theres almost no colour detail at all.

this was intially not a problem. they trained the algorithm on normal, bland photos of people and things, but mostly nature scenes.

but as HDR has started reigning, the compression artifacts are even more abundant. with HDR you can crank up the saturation a lot, and still have perfect detail in it.

but JPG turns them all into a blocky mess. you see, instead of making an equation out of every 8x8 pixel square, they turn it into 1:2 or as far as 4:2, meaning, with high saturation shots, you only get 32x16 pixel equation of the detail.

this can be easily seen in red flowers, where the colour seems to bleed into other things, and you get no discernable detail of the flower at all.

PNG, a greater algorithm than GIF with no copyrights, fixed all this, as far back as 1997. it compresses images to 1:3 completely lossless. for dark or very smooth images, this can go down to 1:10 even. but for a JPG of the same, its more like 1:100, and you just get a grainy blocky mess. but for some reason, neither websites nor photo geeks have adopted this system. and ANG - animated PNG, also followed suit, but its even more ignored. you can find sites which convert your images to an ANG though, but barely any website or software supports it.

av Tsofmia Neptlith (ris och ros)

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