inconsistencies in colors when drawing - ios

Question

I have a UIImageView and I draw to it using UIColor orangeColor. Now, I have a function that is supposed to detect the pixelColor of a pixel tapped on.
R: 1.000000 G: 0.501961 B: 0.000000
That's the RGB value I receive when attempting to detect the pixelColor for UIOrange
It should be.
R: 1.000000 G: 0.5 B: 0.000000
Here's my function
- (UIColor *)colorAtPixel:(CGPoint)point {
// Cancel if point is outside image coordinates
if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, _overlay_imageView.frame.size.width, _overlay_imageView.frame.size.height), point)) {
return nil;
}
// Create a 1x1 pixel byte array and bitmap context to draw the pixel into.
// Reference: http://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage
NSInteger pointX = trunc(point.x);
NSInteger pointY = trunc(point.y);
CGImageRef cgImage = _overlay_imageView.image.CGImage;
NSUInteger width = CGImageGetWidth(cgImage);
NSUInteger height = CGImageGetHeight(cgImage);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
int bytesPerPixel = 4;
int bytesPerRow = bytesPerPixel * 1;
NSUInteger bitsPerComponent = 8;
unsigned char pixelData[4] = { 0, 0, 0, 0 };
CGContextRef context = CGBitmapContextCreate(pixelData,
1,
1,
bitsPerComponent,
bytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextSetBlendMode(context, kCGBlendModeCopy);
// Draw the pixel we are interested in onto the bitmap context
CGContextTranslateCTM(context, -pointX, -pointY);
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
CGContextRelease(context);
// Convert color values [0..255] to floats [0.0..1.0]
CGFloat red = (CGFloat)pixelData[0] / 255.0f;
CGFloat green = (CGFloat)pixelData[1] / 255.0f;
CGFloat blue = (CGFloat)pixelData[2] / 255.0f;
CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;
return [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
}
Any ideas?
I must mention, my UIImageView has a clearBackground, and its ontop of a black canvas. Is that maybe the issue?

Answer 1

There's nothing wrong with your function. This is a a result of floating point math. Half of an integer 255 (the max value of an unsigned byte) is either 127/255.0 or 128/255.0 depending on how you round. Neither one of those is 0.5. They are 0.498039215686275 and 0.501960784313725 respectively.
EDIT: I guess I should add that the colors in the CGImage are stored as bytes, not floats. So when you create your orange with a float in UIColor its getting truncated to R:255, G:128, B:0 A:255. When you read this back as a float you get 1.0 0.501961 B: 0.0 A: 1.0