Finding RGB values at position inside UIImageView after scaling

I have a png image, dimensions 1214x1214 (and double that for retina) on the iPad and have it set up as a UIImageView located at screen co-ordinates (0,-20). In order that it fits the screen during a device rotation/orientation change I've set it to be of type Aspect Fit.

What I'd like to do is to be able to touch the screen and read off the RGB values of the pixel beneath my touch. I've implemented a UIGestureRecognizer and tied it to the UIImage and am getting touch co-ordinates back successfully.

What's giving me trouble is that I've tried implementing several of the ways of retrieving RGB values (for example [How to get the RGB values for a pixel on an image on the iphone])1 but my RGB values seem as if the image is skewed and mapped in a different place on the UIView.

My question is that, how can I cater for the fact that I've set the UIImageView to be Aspect Fit and the fact that the device may be in either landscape or portrait (upside down or rightway up) ?


Ok, so I worked it out, this may be helpful to someone who's trying to do something similar.

I calculated the scaled size of the Image using this function from another answer

-(CGRect)frameForImage:(UIImage*)image inImageViewAspectFit:(UIImageView*)imageView
    float imageRatio = image.size.width / image.size.height;

    float viewRatio = imageView.frame.size.width / imageView.frame.size.height;

    if(imageRatio < viewRatio)
        float scale = imageView.frame.size.height / image.size.height;

        float width = scale * image.size.width;

        float topLeftX = (imageView.frame.size.width - width) * 0.5;

        return CGRectMake(topLeftX, 0, width, imageView.frame.size.height);
        float scale = imageView.frame.size.width / image.size.width;

        float height = scale * image.size.height;

        float topLeftY = (imageView.frame.size.height - height) * 0.5;

        return CGRectMake(0, topLeftY, imageView.frame.size.width, height);

took the touch points from registering a function as a listener

CGPoint tapPoint = [sender locationInView:imageMap];

altered the touch points based on where my image was moved to by rotation of the iPad

if([UIApplication sharedApplication].statusBarOrientation == UIInterfaceOrientationPortrait ||
   [UIApplication sharedApplication].statusBarOrientation == UIInterfaceOrientationPortraitUpsideDown )
  // portrait (y has increased, x has stayed the same)
  tapPoint.y -= rectScaleSize.origin.y;
  // landscape (x has increased, y has stayed the same)
  tapPoint.x -= rectScaleSize.origin.x;

Then rescaled based upon the original size of the image and its Aspect Fit size

tapPoint.x = (tapPoint.x * imageMap.image.size.width) / rectScaleSize.size.width;
tapPoint.y = (tapPoint.y * imageMap.image.size.height) / rectScaleSize.size.height;

where imageMap.image was my original image and rectScaleSize was the return from the frameForImage function

and, finally got the RGB values

CGImageRef image  = [imageMap.image CGImage];
NSUInteger width  = CGImageGetWidth(image);
NSUInteger height = CGImageGetHeight(image);
// NSLog(@"RGB Image is %d x %d",width,height);

CGColorSpaceRef colorSpace  = CGColorSpaceCreateDeviceRGB();
unsigned char *rawData      = malloc(height * width * 4);
NSUInteger bytesPerPixel    = 4;
NSUInteger bytesPerRow      = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;

CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace,
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

CGContextDrawImage(context, CGRectMake(0, 0, width, height),image);

int byteIndex = (bytesPerRow * (int)tapPoint.y) + (int)tapPoint.x * bytesPerPixel;
int red = rawData[byteIndex];
int green = rawData[byteIndex + 1];
int blue = rawData[byteIndex + 2];
//int alpha = rawData[byteIndex + 3];

NSLog(@"RGB is %d,%d,%d",red,green,blue);

Seems to work just fine, hope it comes in useful. Comments welcome if I did something terribly wrong!

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