However, for my application, I need access to the raw pixels, I'm fine with the format of the CGImage, and can't do processing with drawing. I need to bundle up the pixels I get and pass them to another library (non-Macintosh). Maybe I could make the performance better but I'm not that concerned about it right now, over just getting this to work. I also don't want/need to use Core Image.
So, my question still remains, why does my code crash? Is the bitmap Rep object somehow unsafe to access the raw bitmap data? If so, is there a safe way to do this?
What jumped out at me is that you're looking at the pixel format of the CGImage but working with the NSBitmapImageRep data. The pixel format is usually the same, but isn't always.
What I'd suggest and is safe is the go the third way suggested in the relnotes:
The solution is _not_ to try to handle the complete range of formats that NSBitmapImageRep's data might be in, that's way too hard. Instead, draw the bitmap into something whose format you _know_, then look at that.
That looks like this:
NSBItmapImageRep *bitmapIGotFromAPIThatDidNotSpecifyFormat;
NSBitmapImageRep *bitmapWhoseFormatIKnow = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:NULL pixelsWide:width pixelsHigh:height
bitsPerSample:bps samplesPerPixel:spp hasAlpha:alpha isPlanar:isPlanar
colorSpaceName:colorSpaceName bitmapFormat:bitmapFormat bytesPerRow:rowBytes
bitsPerPixel:pixelBits];
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setContext:[NSGraphicsContext graphicsContextWithBitmapImageRep:bitmapWhoseFormatIKnow]];
[bitmapIGotFromAPIThatDidNotSpecifyFormat draw];
[NSGraphicsContext restoreGraphicsState];
unsigned char *bitmapDataIUnderstand = [bitmapWhoseFormatIKnow bitmapData];
This produces no more copies of the data than just accessing bitmapData of bitmapIGotFromAPIThatDidNotSpecifyFormat, since that data would need to be copied out of a backing CGImage anyway. Also note that this doesn't depend on the source drawing being a bitmap. This is a way to get pixels in a known format for any drawing, or just to get a bitmap. This is a much better way to get a bitmap than calling -TIFFRepresentation, for example. It's also better than locking focus on an NSImage and using -[NSBitmapImageRep initWithFocusedViewRect:].
So, to sum up:
(1) Drawing is fast. Playing with pixels is not.
(2) If you think you need to play with pixels, (a) consider if there's a way to do it with drawing or (b) look into CoreImage.
(3) If you still want to get at the pixels, draw into a bitmap whose format you know and look at those pixels.
That looks like this:
NSBItmapImageRep *bitmapIGotFromAPIThatDidNotSpecifyFormat;
NSBitmapImageRep *bitmapWhoseFormatIKnow = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:NULL pixelsWide:width pixelsHigh:height
bitsPerSample:bps samplesPerPixel:spp hasAlpha:alpha isPlanar:isPlanar
colorSpaceName:colorSpaceName bitmapFormat:bitmapFormat bytesPerRow:rowBytes
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setContext:[NSGraphicsContext graphicsContextWithBitmapImageRep:bitmapWhoseFormatIKnow]];
[bitmapIGotFromAPIThatDidNotSpecifyFormat draw];
[NSGraphicsContext restoreGraphicsState];
unsigned char *bitmapDataIUnderstand = [bitmapWhoseFormatIKnow bitmapData];
This produces no more copies of the data than just accessing bitmapData of bitmapIGotFromAPIThatDidNotSpecifyFormat, since that data would need to be copied out of a backing CGImage anyway. Also note that this doesn't depend on the source drawing being a bitmap. This is a way to get pixels in a known format for any drawing, or just to get a bitmap. This is a much better way to get a bitmap than calling -TIFFRepresentation, for example. It's also better than locking focus on an NSImage and using -[NSBitmapImageRep initWithFocusedViewRect:].
So, to sum up:
(1) Drawing is fast. Playing with pixels is not.
(2) If you think you need to play with pixels, (a) consider if there's a way to do it with drawing or (b) look into CoreImage.
(3) If you still want to get at the pixels, draw into a bitmap whose format you know and look at those pixels.
