Create and write paletted RGBA PNG using NSImage

I'm trying to create paletted PNG image (8-bit per pixel) that uses RGBA palette (32-bit per palette entry) using Cocoa framework*.

I've tried few combinations for [NSBitmapImageRep initWithBitmapDataPlanes:…] method. It seems to create appropriate bitmap for bitsPerSample:2 bitsPerPixel:8.

However, when I try to write such bitmap with [NSBitmapImageRep representationUsingType:NSPNGFileType…] I get:

libpng error: Invalid bit depth for RGBA image
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If I try other bit depths, then I get 32-bit per pixel (non-paletted) image.

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*) I know I could just use libpng, but that's not an answer I'm looking for.

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2 bits per sample, 8 per pixel will not get you an indexed PNG--it will, in theory, create an RGBA PNG file with 2 bits per sample, just as it suggests. Now, such an image has 256 possible colour values per pixel (including alpha channel) but it's not indexed in the sense of having a colour lookup table.

To my knowledge, there is no way to specify a colour palette when using NSBitmapImageRep. You will probably have to use libpng directly to get the effect you want. (By the way, it doesn't matter if you aren't looking for this answer. It's still the correct answer to this particular problem and saying "no!" isn't going to change the universe around you.)

However, before you do that, if you tell us why you think/know you need an indexed PNG, we may be able to point you toward a better or simpler solution.