>>601590When you open the image, you see that three in four pixels of a square are black (or in practice, nigh-black. The original image is a composite. It consists of three in four pixels taken from a first image we want to see in the thumbnail. The color space is then transformed thus that zero point stays at zero but the white point is moved down to somewhere near 0.8 or so.
The fourth pixel of every four pixel square is taken from the image we want to see when the image is opened. The color space of these pixels is transformed to the high upper fraction, meaning the things we would like to be black in are now something like 0.9 and the things we would like to be white are 1.0. Thus the image now consists of two interleaved images, the other of which is slightly darker than it should be and the other that a casual viewer would just see as nigh white pixels.
After this, the GAMA chunk of PNG is modified thus, that when displaying the image, the standard color transformation of PNG format takes the 0.9 (or so) interleved image black point into the real black point and leaves the interleaved image white point at white. This give us a normal image that has a 'black' grid of the 3/4 interleaved image the color space of which is transformed to 0 by the gamma correction.
When 4chan board software creates thumbnails, it (for the sake of speed) ignores the GAMA chunk and creates the thumbnail out of the not gamma corrected image, resulting in the original picture. As the picture is saved as-is, your web browser opening it will still apply the gamma correction and we see the magic happen.
