How to do comparisons of bitmaps in iOS?

Question

I have three CGLayers who\'s data I\'d like to compare.

void *a = CGBitmapContextGetData(CGLayerGetContext(layerA));
void *b = CGBitmapContextGetData(CGLayerGetC         


        

Answer 1

What about the CGBlendModes? kCGBlendModeDestinationOver acts as OR for A and B, and then you can use kCGBlendModeDestinationIn to AND that result with C.

Answer 2

Here's a naive implementation, assuming all three are the same size:

unsigned char *a = CGBitmapContextGetData(CGLayerGetContext(layerA));
unsigned char *b = CGBitmapContextGetData(CGLayerGetContext(layerB));
CGContextRef context = CGLayerGetContext(layerC);
unsigned char *c = CGBitmapContextGetData(context);
size_t bytesPerRow = CGBitmapContextGetBytesPerRow(context);
size_t height = CGBitmapContextGetHeight(context);
size_t len = bytesPerRow * height;
BOOL bitsFound = NO;
for (int i = 0; i < len; i++) {
    if ((a[i] | b[i]) & c[i]) { bitsFound = YES; break; }
}

Since you're hankering for QuickDraw, I assume you could have written that yourself, and you know that will probably be slow.

If you can guarantee the bitmap sizes, you could use int instead of char and operate on four bytes at a time.

For more serious optimization, you should check out the Accelerate framework.