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The PNG specification contains a metadata attribute that allows you to specify the gamma to render the image with. This attribute is intended to be used to ensure that images look identical on all computers. This is a very normal image processing process called Gamma CorrectionThe PNG specification defines the gAMA chunk (the chunk that stores the gamma value) to change the image output like so: light_out = image_sample^(1 / gamma) This scales the image values exponentially based on the reciprocal of the gamma value. If the gamma value is around 1 like it normally is this function has little noticeable effect. During this process, the lowest brightness value for a pixel is 0 and the highest is 1.If we set the PNG gamma attribute to a very low value, making the exponent value very high (since it is the reciprocal), all darker pixels will be made black and all lighter pixels will be mapped to the normal spectrum.We can reverse this mapping for a very low value of the gamma attribute (I use 0.023) to get a PNG image where all the pixels of the image are mapped to very light colors. If we then set the gamma value of the PNG to 0.023 the image will look somewhat normal, except for the rounding errors introduced by crunching the image into high values. The thing is, not all renderers support the gamma attribute. We can abuse this to create a magic thumbnail by taking two images of the same size and creating a new image twice their dimensions. One image is run through the previously mentioned reverse gamma filter that makes all pixels very bright and the other is darkened so that it has no very bright pixels. The images are then spaced out in grids around each other (see image). The resulting image is saved as a PNG file with a gAMA of 0.023. When the image is displayed in a renderer that supports gamma the light pixels become dark but visible colors and the normal pixels become a grid of dark pixels.
