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.