Get the RGBA values for a pixel from an image

get_pixel_values.m

CGImageRef image = uiimage.CGImage;
NSUInteger width = CGImageGetWidth(image);
NSUInteger height = CGImageGetHeight(image);

// Setup 1x1 pixel context to draw into
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char rawData[4];
int bytesPerPixel = 4;
int bytesPerRow = bytesPerPixel;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(rawData, 
                                             1, 
                                             1, 
                                             bitsPerComponent,
                                             bytesPerRow,
                                             colorSpace,
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextSetBlendMode(context, kCGBlendModeCopy);

// Draw the image
CGContextDrawImage(context,
                   CGRectMake(-offset.x, offset.y-height, width, height),
                   image);

// Done
CGContextRelease(context);

// Get the pixel information     
unsigned char red   = rawData[0];
unsigned char green = rawData[1];
unsigned char blue  = rawData[2];
unsigned char alpha = rawData[3];