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It's worth noting that even if PhotoDNA didn't do any extra obfuscation, 144 bytes of grayscale isn't a lot to work with. If we assume each pixel takes up half

by KTibow 3y ago

It's worth noting that even if PhotoDNA didn't do any extra obfuscation, 144 bytes of grayscale isn't a lot to work with. If we assume each pixel takes up half a byte that's barely a 17x17 image.

kragen 3y ago
raw grayscale downscaling is not the best possible lossy image compression i have a high-quality 1280×720 color jpeg of macaulay culkin here which is 99191 bytes; that's 0.86 bits per pixel. recompressing it with the gimp with quality set to 15%, 4:2:0 subsampling, arithmetic coding, no color profile, and no progressive reduces it to 13269 bytes, the only visible effect being a subtle posterized blockiness in low-contrast areas of the photo; subtle details of the facial expression and hair texture survive. that's 0.115 bits per pixel the same compression ratio at 144 bytes would be a 100×100 full-color image. jpeg can't compress small images that well, but plausibly something could
- qayxc 3y ago
  The compression ratio depends on the entropy of the image more than anything else. Chroma subsampling isn't the reason for low bitrates - by your own logic, grayscale would be smaller since it's effectively removing any chroma information. What gets you from 24 bpp to 0.12 bpp is primarily the arithmetic coder and that only depends on entropy. Just for shits and giggles I took two images (resized and cropped to 1280x720) and saved them as jpeg (15%, 4:2:0). The high entropy image (a garden with lots of flowers [1]) resulted in 83.476 bytes. The low entropy image (a stylised drawing of hills with a sunrise [2]) resulted in 22.299 bytes using the same settings. To further confirm the hypothesis, I applied a size 4 average blur to the low entropy image to further reduce details and saved it using the same settings. The result was a 19.104 bytes file. Doing the same to the high entropy image resulted in a 34.646 bytes file - a whopping 60% reduction in size by just reducing the entropy. This shows that reducing entropy is the significant factor here, not colour. It also means that you can get some 100x100 images to compress down to 144 bytes, but for most that simply doesn't apply. [1] https://images.hdqwalls.com/wallpapers/sunrise-landscape-minimalism-5k-ex.jpg [2] https://homesfeed.com/wp-content/uploads/2015/08/wonderful-landscaping-ideas-for-front-of-house-and-backyard-with-cool-walkways-and-colorful-flower-plants-and-trees.jpg

by KTibow 3y ago

It's worth noting that even if PhotoDNA didn't do any extra obfuscation, 144 bytes of grayscale isn't a lot to work with. If we assume each pixel takes up half a byte that's barely a 17x17 image.

kragen 3y ago

raw grayscale downscaling is not the best possible lossy image compression i have a high-quality 1280×720 color jpeg of macaulay culkin here which is 99191 bytes; that's 0.86 bits per pixel. recompressing it with the gimp with quality set to 15%, 4:2:0 subsampling, arithmetic coding, no color profile, and no progressive reduces it to 13269 bytes, the only visible effect being a subtle posterized blockiness in low-contrast areas of the photo; subtle details of the facial expression and hair texture survive. that's 0.115 bits per pixel the same compression ratio at 144 bytes would be a 100×100 full-color image. jpeg can't compress small images that well, but plausibly something could

qayxc 3y ago
The compression ratio depends on the entropy of the image more than anything else. Chroma subsampling isn't the reason for low bitrates - by your own logic, grayscale would be smaller since it's effectively removing any chroma information. What gets you from 24 bpp to 0.12 bpp is primarily the arithmetic coder and that only depends on entropy. Just for shits and giggles I took two images (resized and cropped to 1280x720) and saved them as jpeg (15%, 4:2:0). The high entropy image (a garden with lots of flowers [1]) resulted in 83.476 bytes. The low entropy image (a stylised drawing of hills with a sunrise [2]) resulted in 22.299 bytes using the same settings. To further confirm the hypothesis, I applied a size 4 average blur to the low entropy image to further reduce details and saved it using the same settings. The result was a 19.104 bytes file. Doing the same to the high entropy image resulted in a 34.646 bytes file - a whopping 60% reduction in size by just reducing the entropy. This shows that reducing entropy is the significant factor here, not colour. It also means that you can get some 100x100 images to compress down to 144 bytes, but for most that simply doesn't apply. [1] https://images.hdqwalls.com/wallpapers/sunrise-landscape-minimalism-5k-ex.jpg [2] https://homesfeed.com/wp-content/uploads/2015/08/wonderful-landscaping-ideas-for-front-of-house-and-backyard-with-cool-walkways-and-colorful-flower-plants-and-trees.jpg