templateImageByMappingWhiteToClear:

gistfile1.m

- (NSImage *)templateImageByMappingWhiteToClear:(NSImage *)image {
    NSImage *output = [NSImage imageWithSize:[image size] flipped:NO drawingHandler:^BOOL(NSRect dstRect) {
        CGImageRef srcCGImage = [image CGImageForProposedRect:&dstRect context:[NSGraphicsContext currentContext] hints:nil];
        CGRect bounds = CGRectMake(0, 0, CGImageGetWidth(srcCGImage), CGImageGetHeight(srcCGImage));
        CGContextRef maskCtx = CGBitmapContextCreate(NULL, bounds.size.width, bounds.size.height, 8, 0, CGColorSpaceCreateDeviceGray(), 0);
        CGContextDrawImage(maskCtx, bounds, srcCGImage);
        CGImageRef mask = CGBitmapContextCreateImage(maskCtx);
        CFRelease(maskCtx);

        CGContextRef ctx = [[NSGraphicsContext currentContext] graphicsPort];
        CGContextDrawImage(ctx, dstRect, srcCGImage);
        [[NSColor blackColor] set];
        NSRectFillUsingOperation(dstRect, NSCompositeSourceAtop);
        
        CGContextClipToMask([[NSGraphicsContext currentContext] graphicsPort], dstRect, mask);
        NSRectFillUsingOperation(dstRect, NSCompositeClear);
        
        CFRelease(mask);
        return YES;
    }];
    [output setTemplate:YES];
    
    return output;
}