How to get pixel data from a UIImage (Cocoa Touch) or CGImage (Core Graphics)?

Question

I have a UIImage (Cocoa Touch). From that, I'm happy to get a CGImage or anything else you'd like that's available. I'd like to write this function:

- (int)getRGBAFromImage:(UIImage *)image atX:(int)xx andY:(int)yy {
  // [...]
  // What do I want to read about to help
  // me fill in this bit, here?
  // [...]

  int result = (red << 24) | (green << 16) | (blue << 8) | alpha;
  return result;
}

Thanks!

Answer 1

One way of doing it is to draw the image to a bitmap context that is backed by a given buffer for a given colorspace (in this case it is RGB): (note that this will copy the image data to that buffer, so you do want to cache it instead of doing this operation every time you need to get pixel values)

See below as a sample:

// First get the image into your data buffer
CGImageRef image = [UIImage CGImage];
NSUInteger width = CGImageGetWidth(image);
NSUInteger height = CGImageGetHeight(image);
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);
CGColorSpaceRelease(colorSpace);

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

// Now your rawData contains the image data in the RGBA8888 pixel format.
int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
red = rawData[byteIndex];
green = rawData[byteIndex + 1];
blue = rawData[byteIndex + 2];
alpha = rawData[byteIndex + 3];

Answer 2

i understand the above code but i don't know what does xx and yy means. can anybody tell me about that two variables. i don't know where are these variables defined. in the above code i havn't seen those variables defined.

Answer 3

Apple's Technical Q&A QA1509 shows the following simple approach:

CFDataRef CopyImagePixels(CGImageRef inImage)
{
    return CGDataProviderCopyData(CGImageGetDataProvider(inImage));
}

Use CFDataGetBytePtr to get to the actual bytes (and various CGImageGet* methods to understand how to interpret them).

Answer 4

FYI, I combined Keremk's answer with my original outline, cleaned-up the typos, generalized it to return an array of colors and got the whole thing to compile. Here is the result:

+ (NSArray*)getRGBAsFromImage:(UIImage*)image atX:(int)xx andY:(int)yy count:(int)count
{
    NSMutableArray *result = [NSMutableArray arrayWithCapacity:count];

    // First get the image into your data buffer
    CGImageRef imageRef = [image CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    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);
    CGColorSpaceRelease(colorSpace);

    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);

    // Now your rawData contains the image data in the RGBA8888 pixel format.
    int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
    for (int ii = 0 ; ii < count ; ++ii)
    {
        CGFloat red   = (rawData[byteIndex]     * 1.0) / 255.0;
        CGFloat green = (rawData[byteIndex + 1] * 1.0) / 255.0;
        CGFloat blue  = (rawData[byteIndex + 2] * 1.0) / 255.0;
        CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;
        byteIndex += 4;

        UIColor *acolor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
        [result addObject:acolor];
    }

  free(rawData);

  return result;
}

Answer 5

how do i make this find the average pixel color from the image and use that for the color?