I've not explored Velocity Engine programming under the PowerPC or its
equivalent under the Intel processor family, but your needs seem like
they might be solved by using that as a solution to further improve the
performance.
Graham Cox wrote:
On 25 May 2008, at 5:13 pm, Jens Alfke wrote:
There are geometric techniques for doing this, that wouldn't require
working with pixels. Computing the bounding box of a Bézier curve is
very easy, so you can usually trivially reject most of them by
checking the rects for intersection. For the rest, you have to flatten
them into polygons and test each segment. (Doesn't Quartz 2D have
functions to do this?)
I already do basic geometric testing to weed out obvious non-hits, but
beyond that I have to test pixels because I'm not just dealing with a
simple stroke and/or fill of the path (for example distributing
arbitrary images with varying scales and angles along a path).
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.
If you do test against pixels, work with the raw pixmap instead of
making an Obj-C call to get every pixel. Get the pointer to its pixel
data, and use the rowbytes and the pixel size to iterate over pixels.
If the pixmap is always in a known format, it's pretty easy. (Sounds
like it's 8-bit grayscale, which is as easy as it gets, with one byte
per pixel.)
Yep, this is a good idea (and also thanks to Ken who suggested the same
thing). I see a ~10x speed-up doing this:
NSBitmapImageRep* bits = [self pathBitmapInRect:ir];
// if any pixels in this bitmap are set, we have a hit.
// fast method tests 4 pixels at a time in the raw data:
unsigned dataLength = ([bits bytesPerRow] * [bits
pixelsHigh]) >> 2;
unsigned* data = (unsigned*)[bits bitmapData];
unsigned k = 0;
// scan until we hit a non-white value or reach the end
while(( k < dataLength ) && ( data[k++] == 0xFFFFFFFF ));
hit = ( k < dataLength );
a good improvement for now - thanks. If it needs more the vImage stuff
looks like a good option.
G._______________________________________________
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