Support quantization in restricted color space (e.g., to produce ARGB4444 images)

[annulen]

Use cases:

• Sometimes images are used on device which does not support full 32-bit color (e.g., can display only ARGB4444 or RGB565 palette). In this case indexed image with TrueColor, produced by pngquant, might look ugly because of lower color resolution of screen. (Obtaining indexed images is not really important in this case - it's about proper quantization into restricted color space, e.g. 4-bit per channel)
• Sometimes 8 bit palette is not enough to get desired image quality, but it is desirable to leverage quantization to reduce image size nevertheless.
• I have one use case where I MUST use indexed 8-bit image with palette color space restricted to 4 bit per channel (if you are wondering what the hell is it, such images are used for hardware cursor image on one obscure platform)

Right now I use Gimp script based on [1], adapted to batch processing of images, to quantize them into ARGB444 color space, than I use pngquant if I need to make them indexed (or simply reduce size further). However, [1] uses Floyd-Steinberg and you claim it is not the best dithering algorithm. It's also hard to set up on server (requires whole Gimp + custom script + custom palettes).

[1] http://registry.gimp.org/node/25275

[kornelski]

For ARGB444 use --posterize=4. For RGB565 --posterize=3 will work, but leave one bit of green unused.

[annulen]

Cool! Looks like this option is undocumented.

However, it would be great if pngquant could generate non-indexed images for sake of improved quality. I have an image [1] with color gradients and I'm wondering how much quality can I get from it in ARGB4444. [2] is what I get with Gimp-based solution, and I'm sure that's not the best possible conversion.

[1] http://higgs.rghost.ru/57363540/image.png
[2] http://higgs.rghost.ru/57363557/image.png

[kornelski]

The option is in the readme and the manpage. I've updated the built-in help. 554add4

[kornelski]

In this particular case I'm afraid you won't see improvement with pngquant, because after posterization that image has only 145 colors anyway, so there's nothing to improve in the quantization step.