Software for creating PNG 8bit transparent images?

Question

I\'m looking for software to create PNG8 format transparent images as per this article.

NOTE: I need a linux solution myself, but please submit answ

Answer 1

Ah, if I remember correctly, when I have read this article some months ago, pngquant hadn't a Windows version. I see it has one now. So I tried it, and pngnq too.
The latter seems to do a slightly better job on the IceAlpha.png test image (from libpng.org), at the cost of a slightly bigger image (can be post-processed with pngcrush or pngout anyway). The dithering algorithms (the two of pngquant, the only one of pngnq) are different, it might be worth having both tools, converting images with all algorithms and see what looks the best.

For the record, on the Windows side, IrfanView (4.10) display very well these images (using the transparency level on each palette entry) while XnView (1.85.1) and Gimp (2.4) apply only a full transparency/opaque display, à la Gif: the light bulb given as example in the linked article has a transparent background around it, but the orange part is fully opaque.

And the excellent utility TweakPNG shows we have a PLTE (palette, 222 entries) chunk and a tRNS (alpha values for palette colors, 222 entries) chunk. Even more, it allows to edit each palette entry, color and alpha level. Might be an interesting complementary tool for this format.

Note on IrfanView support: if it handles correctly PNG8 for transparency, it doesn't handle gamma information in PNG files: on the toucan image or the ping-pong image, I had to apply a gamma of 2.4 to get similar (lighter) colors.
Note also that IrfanView does an awful job of converting 32bit PNG image to 256, allowing only one transparent color, which looks bad if full color was dithered!

I see that the GIMP manual states: "his “PNG8” format, like GIF, uses only one bit for transparency; only two transparency levels are possible, transparent or opaque. " while the ISO/W3C standard states: "The tRNS chunk specifies either alpha values that are associated with palette entries (for indexed-colour images) or a single transparent colour (for greyscale and truecolour images).". The PNG spec. 1.2 added: "Although simple transparency is not as elegant as the full alpha channel, it requires less storage space and is sufficient for many common cases."
It looks like the unique transparent color is more implemented than the full transparency palette, alas. At least browsers get it right.