[TCH]

Originally Posted by pandy71

are You using inverse gamma before processing

Nope. My aim was to convert "normal" pictures, not tricky ones, which screws up conversion algorithms or deceive the human eyes.

Originally Posted by pandy71

did You considered non RGB color space

I did before, but the Wu quantizer only supports RGB colourspace, so it would be pointless. Maybe another quantizer could do the trick in YCgCo, but all of the other quantizers produced much worse results. (With the exception of IrfanView, but Irfan is not a helpful person, he did not told me, what algorithm he uses.)

Originally Posted by pandy71

Ok, my point with half level was that in EHB only bright colors can be selected and HB colors are always half of the bright one - it may happen that HB colors will be same i.e. non optimal CLUT values halved will be equal thus focusing on selecting lower half should provide better CLUT usage IMHO.

That's what i said too. I just said, that changing the brightness could cause colours to go white or black and the quantizer would select bad colours, so a gamma intensify would be better.

Originally Posted by pandy71

spatial color seem to be hard to beat

Public spatial quantizer algorithms produced much worse results, than the Wu quantizer. Maybe something is missing from the picture.

Originally Posted by pandy71

Ok, alpha 1 bit is supported by hardware ECS and maybe AGA(?) - You can set transparency for genlock http://amiga-dev.wikidot.com/hardware:colorx .

This is applying for colour registers, not for pixels, thus it's impossible to convert most alpha layered PNG image to use this technique, with the only exception are the ones which contains palette assigned alpha values and uses only 32 colours, those could be converted by using their topmost bit, but the ILBM have a "pure" 24-bit RGB palette and not supports this.

Originally Posted by Thorham

It's true that there's absolutely no point in storing alpha channel in an IFF file, however, it may be useful to add some BMP out features. You could add an option to output the PNG as two BMP files: One 24bit for the RGB data, and one 8bit for the alpha data.

And what good it would do for us on the Amiga?

Originally Posted by Thorham

It's probably mostly useful for programmers, and if that PNG library you were talking about earlier is easy to use and capable enough, then such features are probably not worth the effort.

LibPNG is the industry standard, it supports everything in PNG. Also it's part of most of systems, so you can use it through library calls, or - since it's opensource and (thankfully) it's not GPL - you can just copy the sources from it and integrate into your program. Suits most needs of most projects.
I only did not used it, because i found it not enough lightweight for a classic Amiga; so i just wrote my own PNG handler, it also can load and render any PNG, but it only weighs 24 kB (as in compiled AMD64 binary).