Draw NSImage onto another NSImage (Better)

gistfile1.txt

@interface NSImage (NSBitmapImageRep)
- (NSBitmapImageRep *)bitmapImageRepresentation;
@end

@implementation NSImage (NSBitmapImageRep)
- (NSBitmapImageRep *)bitmapImageRepresentation
{
        int width = [self size].width;
    int height = [self size].height;
    
    if(width < 1 || height < 1)
        return nil;
    
    NSBitmapImageRep *rep = [[NSBitmapImageRep alloc]
                             initWithBitmapDataPlanes: NULL
                             pixelsWide: width
                             pixelsHigh: height
                             bitsPerSample: 8
                             samplesPerPixel: 4
                             hasAlpha: YES
                             isPlanar: NO
                             colorSpaceName: NSDeviceRGBColorSpace
                             bytesPerRow: width * 4
                             bitsPerPixel: 32];
    
    NSGraphicsContext *ctx = [NSGraphicsContext graphicsContextWithBitmapImageRep: rep];
    [NSGraphicsContext saveGraphicsState];
    [NSGraphicsContext setCurrentContext: ctx];
    [self drawAtPoint: NSZeroPoint fromRect: NSZeroRect operation: NSCompositeCopy fraction: 1.0];
    [ctx flushGraphics];
    [NSGraphicsContext restoreGraphicsState];
    
    return rep;
    
    //CGImageRef CGImage = [self CGImageForProposedRect:nil context:nil hints:nil];
    //return [[NSBitmapImageRep alloc] initWithCGImage:CGImage];
}
@end

// Draw NSImage onto NSImage at position p
// Ugly but works, will use it and improve it in next project
// by using objc category or something like that.
// Why this?
//   This code is better than [NSImage lock] and [NSImage unlock], because it will not produce x2 large png 
//   bug while some retina external displays connected. 

void drawImageToImage(NSImage *srcImage, NSImage *destImage, NSPoint p) {
    NSBitmapImageRep *rep = [destImage bitmapImageRepresentation];
    
    if (rep == nil) {
        NSLog(@"Rep is nil");
    }
    
    [NSGraphicsContext saveGraphicsState];
    NSGraphicsContext *ctx = [NSGraphicsContext graphicsContextWithBitmapImageRep: rep];
    [NSGraphicsContext setCurrentContext:ctx];
    
    // Drawing
    [srcImage drawAtPoint:p
                    fromRect:NSZeroRect
                   operation:NSCompositeSourceOver
                    fraction:1.0];
    // End drawing
    [ctx flushGraphics];
    [NSGraphicsContext restoreGraphicsState];
    
    // For test purpose
    //NSData *data = [rep representationUsingType:NSPNGFileType properties:Nil];
    //[data writeToFile: @"/Users/aaron/Desktop/test.png" atomically: NO];
    return [[NSImage alloc] initWithCGImage:[rep CGImage] size:NSZeroSize];
}