On May 25, 2008, at 12:55 AM, Graham Cox wrote:
Trying to squeeze a bit more speed out of certain bottlenecks in my
code. Here's one of them:
NSBitmapImageRep* bits = [self pathBitmapInRect:ir];
// if any pixels in this bitmap are set, we have a hit.
int x, y;
unsigned pixel;
for( y = 0; y < ir.size.height; ++y )
{
for( x = 0; x < ir.size.width; ++x )
{
[bits getPixel:&pixel atX:x y:y];
if ( pixel < 255 )
{
hit = YES;
goto endOfLoop;
}
}
}
endOfLoop:
The <bits> object is a grayscale bitmap where the background is
painted white and any set pixels are painted black. I just need to
know if there are *any* black pixels in the image. <ir> is an NSRect
which can be any size. The imaging method (not shown) accounts for
the origin of this rect, so the test scans from 0,0. The purpose is
to test whether a given rect intersects a rendered object (path),
not just test a point against it.
Is there any faster way of doing this than just iterating over the
pixels like this? I can't think of anything but someone might have a
bright idea.
Well, one approach would be to optimize away the method call overhead
using its IMP. <http://developer.apple.com/documentation/Performance/Conceptual/CodeSpeed/Articles/CriticalCode.html#//apple_ref/doc/uid/20001871-98344
>
Another approach would be to ask "bits" for direct access to its
bitmap data (using -bitmapData) and scanning that directly, avoiding
even a function call per pixel.
Lastly, you should consider using the vImage component of the
Accelerate framework for the most highly optimized implementation you
could hope for. <http://developer.apple.com/documentation/Performance/Conceptual/vImage/index.html
> I think you want the vImageHistogramCalculation_Planar8 function <http://developer.apple.com/documentation/Performance/Reference/vImage_histogram/Reference/reference.html#//apple_ref/doc/uid/TP40005491-CH210-vImageHistogramCalculation_Planar8
>.
Cheers,
Ken
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