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Comment by enriquto

5 years ago

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.

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enriquto

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crazygringo  5 years 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?

  • enriquto  5 years 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

    • crazygringo  5 years 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.

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  • chromaton  5 years ago

    GIMP has it: Colors > Tone Mapping > Retinex

    • crazygringo  5 years 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

      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.

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