View difference between Paste ID: ZWpUbNRy and ymCTLrA2

 #!/bin/bash
 # Create a png with a false thumbnail;
 # looks different when you view it full res.
 #
 # PNG Gamma trick (by @marcan42 / [email protected])
 #
 # This script implements an improved version of the gamma trick used to make
 # thumbnail images on reddit/4chan look different from the full-size image.
 #
 # Sample output (SFW; images by @Miluda):
 #  https://mrcn.st/t/homura_gamma_trick.png
 #  https://www.reddit.com/r/test/comments/6edthw/ (click for fullsize)
 #  https://twitter.com/marcan42/status/869855956842143744
 #
 # Some backstory, explanation and example (slightly NSFW):
 #  https://www.reddit.com/r/touhou/comments/6e6lga/a/di83t02/
 #
 # No idea who came up with the concept; this is an old trick, but past
 # implementations I've seen were less correct than this one :-)
 #
 # This trick works on at least Reddit, 4chan and similar imageboards, Google
 # Drive, Slack, and probably many others. It does not work on Twitter, as
 # Twitter always preprocesses PNG images and strips the gAMA chunk. It does,
 # however, work with e.g. imgur embed previews on Twitter.
 #
 # *Different* one-liner trick that works on Twitter (for grayscale images):
 #  https://twitter.com/marcan42/status/869858577116086272
 #
 # License: public domain
 
 high="$1" # High image (full-size original view)
 low="$2" # Low image (thumbnail) (should be the same size)
 
 output="output.png"
 [ ! -z "$3" ] && output="$3" # Output image
 
 size=$(convert "$high" -format "%wx%h" info:)
 
 # Give a slight brightness boost to the high source, then apply the gamma.
 # This ensures that the pixels look mostly white.
 convert "$high" -alpha off +level 3.5%,100% -gamma 20 high_gamma.png
 
 # Since the low image will be washed out, use gamma to darken it a bit, then
 # reduce its brightness to ensure that its pixels become black after PNG gamma.
 # 77% brightness gets crushed down to 0x00 or 0x01, enough for our purposes.
 low_gamma="-alpha off -gamma 0.8 +level 0%,77%"
 
 # To get rid of the slight "halo" of the high image, we're going to cancel it
 # out from the low image. The equation that we need is:
 #   output = ¾low + ¼                (what we want, for high = white)
 #   output = ¾output_low + ¼high     (what we get)
 # Solve for output_low:
 #   ¾output_low + ¼high = ¾low + ¼
 #   ¾output_low = ¾low + ¼ - ¼high
 #   output_low = low + ⅓ - ⅓high
 # This assumes "dumb" resizing (not gamma-correct). For gamma-correct resizing,
 # or for viewing at 1:1 (which is equivalent to gamma-correct resizing, because
 # physics, assuming your monitor isn't mangling things), this operation would
 # have to be done in a linear colorspace. In practice, the vast majority of
 # resizing implementations are not gamma-correct, so this works.
 convert \( "$low" $low_gamma \) high_gamma.png \
            -compose Mathematics -define compose:args='0,-0.33,1,0.33' \
            -composite low_adjusted.png
 
 # Now compose both images together using the mask, then set the gamma metadata.
 # Note that the typical display gamma is 2.2 and image gamma is the reciprocal
 # 1/2.2. Since we're adding a gamma of 20, we need 1/2.2/20 = 0.022727.
 # We also force the PNG encoder to include the gAMA chunk (and no other
 # spurious metadata).
 convert low_adjusted.png high_gamma.png -size $size pattern:gray25 -composite \
         -set gamma 0.022727 -define png:include-chunk=none,gAMA "$output"
 
 rm high_gamma.png low_adjusted.png