How do I create an 8-bit PNG with transparency from an NSBitmapImageRep?

Question

I have a 32-bit NSBitmapImageRep which has an alpha channel with essentially 1-bit values (the pixels are either on or off).

I want to save this bitmap to an 8-bit PNG file with transparency. If I use the -representationUsingType:properties: method of NSBitmapImageRep and pass in NSPNGFileType, a 32-bit PNG is created, which is not what I want.

I know that 8-bit PNGs can be read, they open in Preview with no problems, but is it possible to write this type of PNG file using any built-in Mac OS X APIs? I'm happy to drop down to Core Image or even QuickTime if necessary. A cursory examination of the CGImage docs didn't reveal anything obvious.

EDIT: I've started a bounty on this question, if someone can provide working source code that takes a 32-bit NSBitmapImageRep and writes a 256-color PNG with 1-bit transparency, it's yours.

Answer 1

How about pnglib? It's really lightweight and easy to use.

Answer 2

pngnq (and new pngquant which achieves higher quality) has BSD-style license, so you can just include it in your program. No need to spawn as separate task.