Cannot get the color of a pixel in this scenario

Question

I've tried using the category solution described in this post (How to get the RGB values for a pixel on an image on the iphone). It doesn't seem to work in my scenario.

In my scenario, I draw circles in response to a tap gesture:

//mainImageView is an IBOutlet of class UIImageView. So here we simply get a pointer to its image.
UIImage *mainImage = self.mainImageView.image;
//Start an image context.
UIGraphicsBeginImageContextWithOptions(self.mainImageView.bounds.size, NO, 0.0);
//Save the context in a handy variable for use later.
CGContextRef context = UIGraphicsGetCurrentContext();

for (Ant *ant in self.ants) { //Ant is a subclass of NSObject.
   //First draw the existing image into the context.
   [mainImage drawInRect:CGRectMake(0, 0, self.mainImageView.bounds.size.width, self.mainImageView.bounds.size.height)];

   // Now draw a circle with position and size specified by the Ant object properties.
   // Set the width of the line
   CGFloat lineWidth = ant.size.width / 20;
   CGContextSetLineWidth(context, lineWidth);
   CGContextSetFillColorWithColor(context, ant.color.CGColor);
   CGContextSetStrokeColorWithColor(context, [UIColor blackColor].CGColor);
   CGContextBeginPath(context);
   CGContextAddArc(context,
                   ant.location.x,                           //center x coordinate
                   ant.location.y,                           //center y coordinate
                   ant.size.width/2 - lineWidth/2,           //radius of circle
                   0.0, 2*M_PI, YES);
   CGContextDrawPath(context, kCGPathFillStroke);
}
self.mainImageView.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

So far so good. This draws the circle with the colors as expected into self.mainImageView.image and upon repeated calls, more copies are drawn at the new location. Responding to some event (could be a timer - could be a tap), the ant objects' locations are updated, the idea is to look at what's on the screen at the new location and do some action based upon the color at that location. Another method, moveAnt, includes the following code:

CGPoint newLocation = /*calculation of the location is not relevant to this discussion*/
UIColor *colorAtNewLocation = [self.mainImageView.image colorAtPosition:newLocation];

The values for red, green, and blue are always zero, and the value of alpha is apparently random each session but remains the same during a session.

So what's going on here? It seems that the colorAtPosition method in the category must be expecting a format of CGImageRef that is different from what is actually being created, above. Is that right? This surprises me, because it looks like that method was written specifically in order to have a known format, which is why I decided to try it out.

Answer 1

The problem is that the CGImageRef doesn't know about the scale of the image. The problem was fixed by changing the method in the category to include a scale argument. The position dimensions are then multiplied by the scale to get the correct image to query the color.