expose indexed color profile for PNG

[Hugo-Trentesaux]

It was discussed in #389 that exposing indexed color profiles of PNG crate could be done quite easily. A use case is given here for light thumbnail generation in Zola ssg. It can significantly decrease media size and then produce much lighter web pages.

[HeroicKatora]

The one thing that's not clear to me is interface. We could add a method of PNGEncoder which does the conversion internally. An alternative is to add a method that switches the mode to prefer indexing, which would only expose the high-level goal of producing small images without giving specifically that this is done via indexing. This would have the advantage of providing some parameter for the acceptable loss of image fidelity that might occur as part of color mapping. Yet another alternative would be to add an interaction with ColorMap in some capacity.

[Hugo-Trentesaux]

@fintelia is this "not planned" as in "not planned yet" (which I would understand) or "not planned" as in "planned not to", for which I would like to know more about.

[fintelia]

It is more "not planning to add an API for this to the image crate". I'd support adding the logic to the png crate to automatically write indexed PNGs when it improved the compression ratio

[Hugo-Trentesaux]

Well, it always improves the compression ratio, the problem is that depending on the image it can lead to visible posterization artifacts. That's why using a color palette is a user choice more than a technical decision that could be automated.

[fintelia]

I guess there's two different features here:

1. Automatically detect if an image uses <=256 colors during encoding. If so, write a paletted PNG.
2. Do lossy encoding by reducing the image to only use 256 colors.

I'm supportive of implementing (1) in the png crate. (2) feels much more specialized though, so probably would require more discussion. And in any case, that's probably not a common enough feature that it needs to be exposed in this crate rather than just being available in the format specific crate.

[Hugo-Trentesaux]

Ok, I understand better now. I'll file an issue in the png crate and see their thoughts about it. But guessing if an image uses less than 256 colors can fail due to edge anti-aliasing which introduces a lot of intermediate colors.