Identify a percentage of transparent pixels in an area of UIImageView - iphone

Question

I'm trying to set up a collision type hit test for a defined of pixels within a UIImageView. I'm only wish to cycle through pixels in a defined area.
Here's what I have so far:
- (BOOL)cgHitTestForArea:(CGRect)area {
BOOL hit = FALSE;
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
float areaFloat = ((area.size.width * 4) * area.size.height);
unsigned char *bitmapData = malloc(areaFloat);
CGContextRef context = CGBitmapContextCreate(bitmapData,
area.size.width,
area.size.height,
8,
4*area.size.width,
colorspace,
kCGImageAlphaPremultipliedLast);
CGContextTranslateCTM(context, -area.origin.x, -area.origin.y);
[self.layer renderInContext:context];
//Seek through all pixels.
float transparentPixels = 0;
for (int i = 0; i < (int)areaFloat ; i += 4) {
//Count each transparent pixel.
if (((bitmapData[i + 3] * 1.0) / 255.0) == 0) {
transparentPixels += 1;
}
}
free(bitmapData);
//Calculate the percentage of transparent pixels.
float hitTolerance = [[self.layer valueForKey:#"hitTolerance"]floatValue];
NSLog(#"Apixels: %f hitPercent: %f",transparentPixels,(transparentPixels/areaFloat));
if ((transparentPixels/(areaFloat/4)) < hitTolerance) {
hit = TRUE;
}
CGColorSpaceRelease(colorspace);
CGContextRelease(context);
return hit;
}
Is someone able to offer any reason why it isn't working?

Answer 1

I would suggest using ANImageBitmapRep. It allows for easy pixel-level manipulation of images without the hassle of context, linking against other libraries, or raw memory allocation. To create an ANImgaeBitmapRep with the contents of a view, you could do something like this:
BMPoint sizePt = BMPointMake((int)self.frame.size.width,
(int)self.frame.size.height);
ANImageBitmapRep * irep = [[ANImageBitmapRep alloc] initWithSize:sizePt];
CGContextRef ctx = [irep context];
[self.layer renderInContext:context];
[irep setNeedsUpdate:YES];
Then, you can crop out your desired rectangle. Note that coordinates are relative to the bottom left corner of the view:
// assuming aFrame is our frame
CGRect cFrame = CGRectMake(aFrame.origin.x,
self.frame.size.height - (aFrame.origin.y + aFrame.size.height),
aFrame.size.width, aFrame.size.height);
[irep cropFrame:];
Finally, you can find the percentage of alpha in the image using the following:
double totalAlpha;
double totalPixels;
for (int x = 0; x < [irep bitmapSize].x; x++) {
for (int y = 0; y < [irep bitmapSize].y; y++) {
totalAlpha += [irep getPixelAtPoint:BMPointMake(x, y)].alpha;
totalPixels += 1;
}
}
double alphaPct = totalAlpha / totalPixels;
You can then use the alphaPct variable as a percentage from 0 to 1. Note that, to prevent leaks, you must release the ANImageBitmapRep object using release: [irep release].
Hope that I helped. Image data is a fun and interesting field when it comes to iOS development.