can a PNG image be indexed, with alpha components

[Jim Higson]

For example, choosing from a palete of 256 rbga colours?

 

[Andrew Crowe]

For example, choosing from a palete of 256 rbga colours?

Yes. The only program I've seen that will let you create these is
Macromedia Fireworks (GIMP might be able to but I can't confirm that).

However indexed PNGs with alpha channels will not work in Internet
Explorer under any circumstances (the PNG workaround only works with
32bit PNGs)


Andrew

 

[Jim Higson]

Yes. The only program I've seen that will let you create these is
Macromedia Fireworks (GIMP might be able to but I can't confirm that).

As far as I can tell the GIMP does not.
Under Image>Mode>Indexed there is nothing to choose the alpha mode.

However indexed PNGs with alpha channels will not work in Internet
Explorer under any circumstances (the PNG workaround only works with
32bit PNGs)

*sigh* I suppose that rules it out then, unless I have two versions so at
least the images load quicker for users of decent browsers.

 

[Glenn Randers-Pehrson]

For example, choosing from a palete of 256 rbga colours?

Yes. R,G,B go in the PLTE chunk and A in the tRNS chunk. The "toucan.png"
image that's somewhere on the libpng.org web site is such an image.

Glenn

 

[Jim Higson]

Yes. R,G,B go in the PLTE chunk and A in the tRNS chunk. The
"toucan.png" image that's somewhere on the libpng.org web site is such an
image.

Hmm... looks promising, know of any command line program on Unix(Linux) to
convert 32 bit with alpha to indexed?

Specifically, I have images like this:

http://www.masmodels.com/images/port-frame-norm.png

which need alpha transparency but not many colours.

(yes, 2.4k isn't a great burden on the server, but this is more out of a
vague interest in seeing how small the PNG format can get things than real
bandwidth savings)

Jim

 

[Glenn Randers-Pehrson]

Hmm... looks promising, know of any command line program on Unix(Linux) to
convert 32 bit with alpha to indexed?

Try pngquant from www.libpng.org

Glenn

 

[Jim Higson]

Try pngquant from www.libpng.org

Glenn

Thanks.
Got the image from 2.4 to 1.5k without loss of quality. I'll be using this
in future, on a larger image a 62% size reduction would really help.

Of course, IE users get the larger one

new image:
http://www.masmodels.com/images/port-frame-norm-fs8.png