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It's possible to get slightly better compression by losslessly rearranging data in a JPEG (DC components are compressed row by row, and prefer fewer horizontal

by pornel 2y ago

It's possible to get slightly better compression by losslessly rearranging data in a JPEG (DC components are compressed row by row, and prefer fewer horizontal color changes). However, here the author seems to accidentally fully recompress the images, and falls into the classic trap of "looks almost the same but the file is much smaller!" That's what every lossy format does every time. They're designed to lose data that is hardest to see with the naked eye. At the top end of the quality range, file sizes grow exponentially in proportion to quality (it's natural consequence of allowing less and less data to be lost, and approaching lossless compression that has a hard limit on how effective it can be). But conversely this means that going from very high quality to still-pretty-high quality moves the file size along the exponential curve and seems to give dramatic reduction in file size. This isn't any trick, that's how it works. File size change is easy to measure, but visual quality change is difficult to quantify, so people disregard the visual change. In reality they're just moving a point along a curve, and recompression gets a worse curve (less quality for the same file size) than the quality-to-file-size ratio of compressing the file at the lower quality from the start.

vanderZwan 2y ago
> However, here the author seems to accidentally fully recompress the images, and falls into the classic trap of "looks almost the same but the file is much smaller!" Except they didn't quite do that: yes, they recompressed the image instead of using the lossles rotation that JPEG is capable of. However, they then compared a recompressed rotated image to a recompressed image that wasn't rotated, and noted there was still a significant size differnce. He also claims to have verified in GIMP that the two recompressed files were visually identical after rotating (I'm a little suspicious of that bit, since you wouldn't notice a tiny difference unless you use the "difference" layer mode and theo manually amplify the miniscule differences with something like the curves tool)
- sotix 2y ago
  Yes this is the crux and the fun of my discovery! I was surprised how using sips to rotate the image resulted in a lower size than using sips or ImageMagick to directly compress the image. I’d encourage you to download the image from the link in the article and try yourself if you have a Mac and then compare them with GIMP because it’s very possible I didn’t do a perfect job with that.
  - lifthrasiir 2y ago
    See my other comment for the result from ImageMagick, which shows little difference regardless of the orientation. For sips, there is a possibility that chroma subsampling impacted the result (because there are two different scaling factors for each axis) and you are technically comparing different images.

by pornel 2y ago

vanderZwan 2y ago

> However, here the author seems to accidentally fully recompress the images, and falls into the classic trap of "looks almost the same but the file is much smaller!" Except they didn't quite do that: yes, they recompressed the image instead of using the lossles rotation that JPEG is capable of. However, they then compared a recompressed rotated image to a recompressed image that wasn't rotated, and noted there was still a significant size differnce. He also claims to have verified in GIMP that the two recompressed files were visually identical after rotating (I'm a little suspicious of that bit, since you wouldn't notice a tiny difference unless you use the "difference" layer mode and theo manually amplify the miniscule differences with something like the curves tool)

sotix 2y ago
Yes this is the crux and the fun of my discovery! I was surprised how using sips to rotate the image resulted in a lower size than using sips or ImageMagick to directly compress the image. I’d encourage you to download the image from the link in the article and try yourself if you have a Mac and then compare them with GIMP because it’s very possible I didn’t do a perfect job with that.
- lifthrasiir 2y ago
  See my other comment for the result from ImageMagick, which shows little difference regardless of the orientation. For sips, there is a possibility that chroma subsampling impacted the result (because there are two different scaling factors for each axis) and you are technically comparing different images.