4 ms·

TLDR: There are much better compression algorithms than dithering

magicalhippo 5y ago
Dithering isn't a compression algorithm[1]. It's an algorithm that adds noise to effectively increase bitdepth when downsampling. In the case of images, it makes color-reduced versions not look terrible. Adding noise is almost always a bad thing for compression. The undithered images would compress much better. [1]: https://en.wikipedia.org/wiki/Dither https://en.wikipedia.org/wiki/Dither
- rjmunro 5y ago
  If added noise is the aesthetic effect you are going for, it's probably better to ship a low quality (<50) JPEG of the original image and add the "dithering" in the client by overlaying noise there.
  - marginalia_nu 5y ago
    Dithering is fairly expensive operation. Please don't do it client-side each time a resource is fetched.
    - WithinReason 5y ago
      It's much cheaper than decompressing a JPEG
      - marginalia_nu 5y ago
        Compressing, perhaps. But decompressing? Not really.
        WithinReason 5y ago
        Floyd-steinberg dithering is at least an order of magnitude cheaper than JPEG decompression
        marginalia_nu 5y ago
        Maybe if all else is equal, but let's not forget you'd be doing the dithering in javascript, and the JPEG decompression is implemented in a low level language, in practice probably hand written assembly (as performance-critical compression code often is).
  - hunter2_ 5y ago
    You can't add the noise after quantization, it needs to be done at the same time as quantization, because otherwise you are losing the information needed to intelligently feather the edges. Think of it this way. You start out with a series of numbers between 0 and 100. Your job is to represent this series as best as possible within a range of just 0 to 10. Without dither, you would just round each original number to its closest multiple of 10; all your 31's become 3 and all your 34's become 3. With dither, nearly all of your 31's become 3 and many of your 34's become 3 but nearly half of them become 4. Without dither: 31, 31, 34, 34 becomes 3 3 3 3. With dither: 31, 31, 34, 34 might become 3 3 3 4 on a typical run. You absolutely cannot calculate 3 3 3 4 based on 3 3 3 3. You need the original full set of information in order to calculate 3 3 3 4. Now add on the fact that it's not just random noise that makes this work. Neighboring pixels influence whether to round up or round down. You want that influence to come from original high-depth data, not already-rounded data.
- WithinReason 5y ago
  It is when combined with quantisation, and the article specifically refers to it as such.

Dithering isn't a compression algorithm[1]. It's an algorithm that adds noise to effectively increase bitdepth when downsampling. In the case of images, it makes color-reduced versions not look terrible. Adding noise is almost always a bad thing for compression. The undithered images would compress much better. [1]: https://en.wikipedia.org/wiki/Dither https://en.wikipedia.org/wiki/Dither

rjmunro 5y ago
If added noise is the aesthetic effect you are going for, it's probably better to ship a low quality (<50) JPEG of the original image and add the "dithering" in the client by overlaying noise there.
- marginalia_nu 5y ago
  Dithering is fairly expensive operation. Please don't do it client-side each time a resource is fetched.
  - WithinReason 5y ago
    It's much cheaper than decompressing a JPEG
    - marginalia_nu 5y ago
      Compressing, perhaps. But decompressing? Not really.
      - WithinReason 5y ago
        Floyd-steinberg dithering is at least an order of magnitude cheaper than JPEG decompression
        marginalia_nu 5y ago
        Maybe if all else is equal, but let's not forget you'd be doing the dithering in javascript, and the JPEG decompression is implemented in a low level language, in practice probably hand written assembly (as performance-critical compression code often is).
- hunter2_ 5y ago
  You can't add the noise after quantization, it needs to be done at the same time as quantization, because otherwise you are losing the information needed to intelligently feather the edges. Think of it this way. You start out with a series of numbers between 0 and 100. Your job is to represent this series as best as possible within a range of just 0 to 10. Without dither, you would just round each original number to its closest multiple of 10; all your 31's become 3 and all your 34's become 3. With dither, nearly all of your 31's become 3 and many of your 34's become 3 but nearly half of them become 4. Without dither: 31, 31, 34, 34 becomes 3 3 3 3. With dither: 31, 31, 34, 34 might become 3 3 3 4 on a typical run. You absolutely cannot calculate 3 3 3 4 based on 3 3 3 3. You need the original full set of information in order to calculate 3 3 3 4. Now add on the fact that it's not just random noise that makes this work. Neighboring pixels influence whether to round up or round down. You want that influence to come from original high-depth data, not already-rounded data.
WithinReason 5y ago
It is when combined with quantisation, and the article specifically refers to it as such.