cocos2d content.size, boundingBox and size - iphone

Question

im writing a game to find the differences between 2 images. i created a subclass of CCSprite, Spot. firstly i tried to create small images and add itself according to it's position, but later i found the position is hard to determine, since it's hard to avoid offset of 1 or 2 pixels.
then i tried to make the Spot the same size as the image, with the other part transparent. but I still need to find out the 'hotspot' of finger tap. but when i use CGRectContainsPoint([self boundingBox], touchLocation), it's actually the whole image.
so is there any other way to do this? like content.size or self.size, and make a CGRect out of it's non-transparent part?
Thank you.

Answer 1

I figured it out now. here is my code: (it's actually quite simple
-(void) findRect:(NSString*) fn {
//the origin of mTex is top left
//the origin of CGRect is top left, in the coordinate system inside the image
int topLeftX = 0;
int topLeftY = 0;
for (int i = 0; i < image_width; i += 10) {
for (int j = 0; j < image_height; j += 10) {
if (([mTex pixelAt:ccp(i, j)].a & 0xFF) != 0) {
topLeftX = i;
topLeftY = j;
goto outer;
}
}
}
outer:;
int topRightX = 0;
for (int i = topLeftX; i < image_width; i += 10) {
if (([mTex pixelAt:ccp(i, topLeftY)].a & 0xFF) == 0) {
topRightX = i;
break;
}
}
if (topRightX == 0) {
topRightX = image_width - 1;
}
int bottomLeftY = 0;
for (int i = topLeftY; i < image_height; i += 10) {
if (([mTex pixelAt:ccp(topLeftX, i)].a & 0xFF) == 0) {
bottomLeftY = i;
break;
}
}
if (bottomLeftY == 0) {
bottomLeftY = image_height - 1;
}
areaRect = CGRectMake(topLeftX, topLeftY, topRightX - topLeftX, bottomLeftY - topLeftY);
}