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remove alpha channel from bitmap

If I have an instance of NSImage, anImage, initialized from a PDF file, I know I can save that to a bitmap file in TIFF format via

[[anImage TIFFRepresentation] writeToFile: @"/tmp/foo.tiff" atomically: YES];

To get the bitmap in PNG format, I tried the following variant:

NSData *tiffData = [anImage TIFFRepresentation];
NSBitmapImageRep *bitmap = [NSBitmapImageRep imageRepWithData:tiffData];
[[bitmap representationUsingType:NSPNGFileType properties:nil] writeToFile: @"/tmp/foo.png" atomically: YES];

However, although Preview.app displays foo.png correctly, other apps (like GraphicConverter) don't -- all I see in GC is a black square, and an inverse of the bitmap if I choose "Show Alpha Channel."

How can I strip the alpha channel from the TIFF representation to get a "pure" PNG file (or JPEG, or BMP,...) that doesn't contain an alpha channel, so that other apps can import the bitmap?

Thanks,

Jason
--
J. McKenzie Alexander
Department of Philosophy, Logic and Scientific Method
London School of Economics and Political Science
Houghton Street, London WC2A 2AE
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