Efficient color sampling for UIImage. http://stackoverflow.com/questions/144250/how-to-get-the-rgb-values-for-a-pixel-on-an-image-on-the-iphone/

UIImage+Picker.h

#import <UIKit/UIKit.h>


@interface UIImage (Picker)

- (UIColor *)colorAtPosition:(CGPoint)position;

@end

UIImage+Picker.m

#import "UIImage+Picker.h"


@implementation UIImage (Picker)

- (UIColor *)colorAtPosition:(CGPoint)position {
	
	CGPoint transformedPoint = [self transformPoint:position];
	CGRect sourceRect = CGRectMake(transformedPoint.x, transformedPoint.y, 1.f, 1.f);
	CGImageRef imageRef = CGImageCreateWithImageInRect(self.CGImage, sourceRect);

	CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
	unsigned char *buffer = malloc(4);
	CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big;
	CGContextRef context = CGBitmapContextCreate(buffer, 1, 1, 8, 4, colorSpace, bitmapInfo);
	CGColorSpaceRelease(colorSpace);
	CGContextDrawImage(context, CGRectMake(0.f, 0.f, 1.f, 1.f), imageRef);
	CGImageRelease(imageRef);
	CGContextRelease(context);
	
	CGFloat r = buffer[0] / 255.f;
	CGFloat g = buffer[1] / 255.f;
	CGFloat b = buffer[2] / 255.f;
	CGFloat a = buffer[3] / 255.f;
	
	free(buffer);
	
	return [UIColor colorWithRed:r green:g blue:b alpha:a];
}

- (CGPoint)transformPoint:(CGPoint)point {
	
	CGImageRef imageRef = self.CGImage;
	CGFloat width = CGImageGetWidth(imageRef);
	CGFloat height = CGImageGetHeight(imageRef);
	
    CGAffineTransform transform = CGAffineTransformIdentity;
	
    switch (self.imageOrientation) {
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, width, height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;
			
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
            transform = CGAffineTransformTranslate(transform, width, 0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;
			
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, 0, height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
        case UIImageOrientationUp:
        case UIImageOrientationUpMirrored:
            break;
    }
	
    switch (self.imageOrientation) {
        case UIImageOrientationUpMirrored:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, width, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
			
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, height, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
        case UIImageOrientationUp:
        case UIImageOrientationDown:
        case UIImageOrientationLeft:
        case UIImageOrientationRight:
            break;
    }
	
	CGPoint transformed = CGPointApplyAffineTransform(point, transform);
	
	if (transformed.x >= width) {
		transformed.x = width - 1;
	}
	if (transformed.y >= height) {
		transformed.y = height -1;
	}
	
	return transformed;
}

@end