How do I release a CGImageRef in iOS

Question

I am writing this method to calculate the average R,G,B values of an image. The following method takes a UIImage as an input and returns an array containing the R,G,B values of the input image. I have one question though: How/Where do I properly release the CGImageRef?

-(NSArray *)getAverageRGBValuesFromImage:(UIImage *)image
{
    CGImageRef rawImageRef = [image CGImage];

    //This function returns the raw pixel values
    const UInt8 *rawPixelData = CFDataGetBytePtr(CGDataProviderCopyData(CGImageGetDataProvider(rawImageRef)));

    NSUInteger imageHeight = CGImageGetHeight(rawImageRef);
    NSUInteger imageWidth = CGImageGetWidth(rawImageRef);

    //Here I sort the R,G,B, values and get the average over the whole image
    int i = 0;
    unsigned int red = 0;
    unsigned int green = 0;
    unsigned int blue = 0;

    for (int column = 0; column< imageWidth; column++)
    {
        int r_temp = 0;
        int g_temp = 0;
        int b_temp = 0;

        for (int row = 0; row < imageHeight; row++) {
            i = (row * imageWidth + column)*4;
            r_temp += (unsigned int)rawPixelData[i];
            g_temp += (unsigned int)rawPixelData[i+1];
            b_temp += (unsigned int)rawPixelData[i+2];

        }

        red += r_temp;
        green += g_temp;
        blue += b_temp;

    }

    NSNumber *averageRed = [NSNumber numberWithFloat:(1.0*red)/(imageHeight*imageWidth)];
    NSNumber *averageGreen = [NSNumber numberWithFloat:(1.0*green)/(imageHeight*imageWidth)];
    NSNumber *averageBlue = [NSNumber numberWithFloat:(1.0*blue)/(imageHeight*imageWidth)];


    //Then I store the result in an array
    NSArray *result = [NSArray arrayWithObjects:averageRed,averageGreen,averageBlue, nil];


    return result;
}

I tried two things: Option 1: I leave it as it is, but then after a few cycles (5+) the program crashes and I get the "low memory warning error"

Option 2: I add one line CGImageRelease(rawImageRef) before the method returns. Now it crashes after the second cycle, I get the EXC_BAD_ACCESS error for the UIImage that I pass to the method. When I try to analyze (instead of RUN) in Xcode I get the following warning at this line "Incorrect decrement of the reference count of an object that is not owned at this point by the caller"

Where and how should I release the CGImageRef?

Thanks!

Answer 1

Your mergedColor works great on an image loaded from a file, but not for an image capture by the camera. Because CGBitmapContextGetData() on the context created from a captured sample buffer doesn't return it bitmap. I changed your code to as following. It works on any image and it is as fast as your code.

- (UIColor *)mergedColor
{
     CGImageRef rawImageRef = [self CGImage];

    // scale image to an one pixel image

    uint8_t  bitmapData[4];
    int bitmapByteCount;
    int bitmapBytesPerRow;
    int width = 1;
    int height = 1;

    bitmapBytesPerRow = (width * 4);
    bitmapByteCount = (bitmapBytesPerRow * height);
    memset(bitmapData, 0, bitmapByteCount);
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate (bitmapData,width,height,8,bitmapBytesPerRow,
             colorspace,kCGBitmapByteOrder32Little|kCGImageAlphaPremultipliedFirst);
    CGColorSpaceRelease(colorspace);
    CGContextSetBlendMode(context, kCGBlendModeCopy);
    CGContextSetInterpolationQuality(context, kCGInterpolationMedium);
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), rawImageRef);
    CGContextRelease(context);
    return [UIColor colorWithRed:bitmapData[2] / 255.0f
                    green:bitmapData[1] / 255.0f
                             blue:bitmapData[0] / 255.0f
                            alpha:1];
}

Answer 2

CFDataRef abgrData = CGDataProviderCopyData(CGImageGetDataProvider(rawImageRef));

const UInt8 *rawPixelData = CFDataGetBytePtr(abgrData);

...

CFRelease(abgrData);