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This article is missing a crucial pre-processing step to dithering algorithms: apply a Retinex-like filter to enhance the local contrast before doing the dither

This article is missing a crucial pre-processing step to dithering algorithms: apply a Retinex-like filter to enhance the local contrast before doing the dithering. This gives a dramatic improvement of the final result. In fact, by exploring the scale parameter of the pre-processing step, you find a continuous family of binarisations that interpolates between global tresholding and local dithering.

crazygringo 6y ago
That's fascinating -- do you have any links to examples? I'm searching online but can't find anything at all. I've never heard of using Retinex in the context of dithering, and wondering what specifically you mean by Retinex-"like"? I'm also really curious what contexts this has been most successful in. E.g. was it used for dithering in images or games back in the 1990's when we were limited to 16-bit or 256-bit color? Or is this something more recently explored in academia or in some niche imaging applications?
- chromaton 6y ago
  GIMP has it: Colors > Tone Mapping > Retinex
  - crazygringo 6y ago
    Ah thanks, just tried it out and it indeed produces quite a different result using that filter (default settings) before dithering. Here's a side-by-side comparison using an image from the front page of nytimes.com (be sure to click to zoom in for the full effect): https://imgur.com/a/mrHl7FW https://imgur.com/a/mrHl7FW Without it (left), a photo remains "accurate" in terms of brightness levels. But with it (right), it becomes far more high-contrast to feel closer to an illustration or painting. Which certainly makes it clearer. But while it brings out details in middle levels, it totally blows out shadows and highlights. E.g. the texture of his mask, shirt, and her hand are much clearer. But on the other hand, their hair (and a background object) just turn solid black and lose all detail. But certainly, the vastly higher contrast makes for a much more compelling image IMO.
    - toast0 6y ago
      Here's a link to the color image https://static01.nyt.com/images/2020/12/21/well/21well-klass/21well-klass-threeByTwoSmallAt2X.jpg?quality=75&auto=webp&disable=upscale&width=400 https://static01.nyt.com/images/2020/12/21/well/21well-klass... I agree; the filtered image (right) is more aesthetically pleasing; but it feels much less accurate. It would depend on the intent of the image I think. If you're creating art, and using photographs in the creation, it's not a problem. If you're reporting on the world and just want to reduce the data size (or use a monochrome output medium), I wouldn't do it like this.
    - anotheryou 6y ago
      Black and white images need more contrast to be pleasing. I think that's most of the effect you see here. Maybe you'd want to start with a decent black and white photograph to get a better comparison. No matter what you do you probably also don't want to end up with large patches of solid white or black in your source image (unless it's the background). The hair already feels like drowning in black. But to take care of that you need to use photoshop and be careful with the gradient curves :)
- enriquto 6y ago
  > I'm also really curious what contexts this has been most successful in. E.g. was it used for dithering in images or games back in the 1990's when we were limited to 16-bit or 256-bit color? Or is this something more recently explored in academia or in some niche imaging applications? No need to speak in the past tense! It is not a "niche" application, either. Think about it: gray ink is almost never used. All printing into paper is done by dithering black ink into white paper. This includes bank notes, passports, product labels, etc. Besides dithering being used everywhere, it is a very active area of research, both in academia and in industry. In my lab we have seen a few industrial projects concerning dithering. It's a vast and very beautiful subject. > do you have any links to examples? Take a look here for a couple of examples: http://gabarro.org/ccn/linear_dithering.html http://gabarro.org/ccn/linear_dithering.html
  - crazygringo 6y ago
    Huh, to be honest I feel like I've only ever seen halftoning when printing onto paper -- I've never associated dithering with printing at all. And the "linear dithering" you're describing, when I think of images in certain banknotes and passports or quality seals that I'd call "engraved", I've always assumed were hand-drawn by an artist. But I like what you're describing and linking to, as a way to achieve that hand-drawn effect algorithmically, to include a directional texture element! Thanks for sharing.
    - enriquto 6y ago
      > Huh, to be honest I feel like I've only ever seen halftoning when printing onto paper -- I've never associated dithering with printing at all. Yep, sorry about my sloppy terminology. I always use "halftoning" and "dithering" interchangeably. Yet, notice that today's printers are often matrix-based, i.e., like a high-resolution binary screen, with a bit of ink smearing depending on the type of paper/plastic. > I've always assumed were hand-drawn by an artist. Maybe some are still drawn by hand, but most printed stuff is at some point dithered automatically (and a lot of critical information can be embedded on the dithering patterns).
JKCalhoun 6y ago
I wonder how "Retinex" relates to levels, contrast, white and black point cutoff....

crazygringo 6y ago

That's fascinating -- do you have any links to examples? I'm searching online but can't find anything at all. I've never heard of using Retinex in the context of dithering, and wondering what specifically you mean by Retinex-"like"? I'm also really curious what contexts this has been most successful in. E.g. was it used for dithering in images or games back in the 1990's when we were limited to 16-bit or 256-bit color? Or is this something more recently explored in academia or in some niche imaging applications?

chromaton 6y ago
GIMP has it: Colors > Tone Mapping > Retinex
- crazygringo 6y ago
  Ah thanks, just tried it out and it indeed produces quite a different result using that filter (default settings) before dithering. Here's a side-by-side comparison using an image from the front page of nytimes.com (be sure to click to zoom in for the full effect): https://imgur.com/a/mrHl7FW https://imgur.com/a/mrHl7FW Without it (left), a photo remains "accurate" in terms of brightness levels. But with it (right), it becomes far more high-contrast to feel closer to an illustration or painting. Which certainly makes it clearer. But while it brings out details in middle levels, it totally blows out shadows and highlights. E.g. the texture of his mask, shirt, and her hand are much clearer. But on the other hand, their hair (and a background object) just turn solid black and lose all detail. But certainly, the vastly higher contrast makes for a much more compelling image IMO.
  - toast0 6y ago
    Here's a link to the color image https://static01.nyt.com/images/2020/12/21/well/21well-klass/21well-klass-threeByTwoSmallAt2X.jpg?quality=75&auto=webp&disable=upscale&width=400 https://static01.nyt.com/images/2020/12/21/well/21well-klass... I agree; the filtered image (right) is more aesthetically pleasing; but it feels much less accurate. It would depend on the intent of the image I think. If you're creating art, and using photographs in the creation, it's not a problem. If you're reporting on the world and just want to reduce the data size (or use a monochrome output medium), I wouldn't do it like this.
  - anotheryou 6y ago
    Black and white images need more contrast to be pleasing. I think that's most of the effect you see here. Maybe you'd want to start with a decent black and white photograph to get a better comparison. No matter what you do you probably also don't want to end up with large patches of solid white or black in your source image (unless it's the background). The hair already feels like drowning in black. But to take care of that you need to use photoshop and be careful with the gradient curves :)
enriquto 6y ago
> I'm also really curious what contexts this has been most successful in. E.g. was it used for dithering in images or games back in the 1990's when we were limited to 16-bit or 256-bit color? Or is this something more recently explored in academia or in some niche imaging applications? No need to speak in the past tense! It is not a "niche" application, either. Think about it: gray ink is almost never used. All printing into paper is done by dithering black ink into white paper. This includes bank notes, passports, product labels, etc. Besides dithering being used everywhere, it is a very active area of research, both in academia and in industry. In my lab we have seen a few industrial projects concerning dithering. It's a vast and very beautiful subject. > do you have any links to examples? Take a look here for a couple of examples: http://gabarro.org/ccn/linear_dithering.html http://gabarro.org/ccn/linear_dithering.html
- crazygringo 6y ago
  Huh, to be honest I feel like I've only ever seen halftoning when printing onto paper -- I've never associated dithering with printing at all. And the "linear dithering" you're describing, when I think of images in certain banknotes and passports or quality seals that I'd call "engraved", I've always assumed were hand-drawn by an artist. But I like what you're describing and linking to, as a way to achieve that hand-drawn effect algorithmically, to include a directional texture element! Thanks for sharing.
  - enriquto 6y ago
    > Huh, to be honest I feel like I've only ever seen halftoning when printing onto paper -- I've never associated dithering with printing at all. Yep, sorry about my sloppy terminology. I always use "halftoning" and "dithering" interchangeably. Yet, notice that today's printers are often matrix-based, i.e., like a high-resolution binary screen, with a bit of ink smearing depending on the type of paper/plastic. > I've always assumed were hand-drawn by an artist. Maybe some are still drawn by hand, but most printed stuff is at some point dithered automatically (and a lot of critical information can be embedded on the dithering patterns).

JKCalhoun 6y ago

I wonder how "Retinex" relates to levels, contrast, white and black point cutoff....