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

Posted by Rob Keniger on Stack Overflow See other posts from Stack Overflow or by Rob Keniger
Published on 2010-03-03T04:11:02Z Indexed on 2010/03/08 1:49 UTC
Read the original article Hit count: 353

Filed under:

png

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cocoa

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images

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nsbitmapimagerep

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mac

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.

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