Question

How do you write out a 16×16 PNG file for a file type icon in Cocoa? I tried this before with code like the snippet below. The snippet worked on a PPC machine with Mac OS X 10.5, but no longer seems to work on Mac OS X 10.6: the PNG that’s written out has size 512×512, rather than 16×16.

NSImage * icon = [[NSWorkspace sharedWorkspace] iconForFileType: NSFileTypeForHFSTypeCode(kGenericFolderIcon)];
[icon setSize: NSMakeSize(16.0,16.0)];
NSBitmapImageRep * bitmapRep = [NSBitmapImageRep imageRepWithData: [icon TIFFRepresentation]];  
NSData * data = [bitmapRep representationUsingType: NSPNGFileType properties: nil];
[data writeToFile: @"/tmp/test.png" atomically: NO];

From what I understand from the documentation on NSImage and NSImageRep, the fact that the above code worked before was coincidence, as setSize: only sets the “drawing size” of the image which doesn’t necessarily match the “physical size” of its representation(s).

So what is the correct way to get the 16×16 size PNG file? Also, icons on Mac OS X can include a specially-drawn version for the 16×16 size, which is not just a scaled-down version of the “big” icon; how do you make sure this special version is written out to the PNG file when one is available?

Answer 1

If you are sure that there’s a 16×16 sized version of the icon present in the NSImage, you could look through the representations (-[NSImage representations]) and select the one you’re interested in by looking at the size (-[NSImageRep pixelsWide] and pixelsHigh). You could then write the representation to disk.

A better way would be to create a 16×16 bitmap context, draw the image into that context and then save the contexts contents. This way also works if the original icon does not contain a 16×16 representation.