Optimized alternative to CGContextDrawImage

Question

I'm currently working a lot with CoreGraphics on OSX.

I've run Time Profiler over my code and found the biggest hang-up is in CGContextDrawImage. It's part of a loop that gets called many times per second.

I don't have any way of optimizing this code per se (since it's in the Apple libraries) - but I am wondering if there's a speedier alternative or way to improve the speed.

I'm using CGContextDraw image after some blend-mode code such as: CGContextSetBlendMode(context, kCGBlendModeDifference); so alternative implementations would need to be able to support blending.

Time profiler results:

3658.0ms   15.0%    0.0               CGContextDrawImage
3658.0ms   15.0%    0.0                ripc_DrawImage
3539.0ms   14.5%    0.0                 ripc_AcquireImage
3539.0ms   14.5%    0.0                  CGSImageDataLock
3539.0ms   14.5%    1.0                   img_data_lock
3465.0ms   14.2%    0.0                    img_interpolate_read
2308.0ms    9.4%    7.0                     resample_band
1932.0ms    7.9%    1932.0                   resample_byte_h_3cpp_vector
369.0ms     1.5%    369.0                    resample_byte_v_Ncpp_vector
1157.0ms    4.7%    2.0                     img_decode_read
1150.0ms    4.7%    8.0                      decode_data
863.0ms     3.5%    863.0                     decode_swap
267.0ms     1.0%    267.0                     decode_byte_8bpc_3

Update:

The actual source is something along the lines of the following:

/////////////////////////////////////////////////////////////////////////////////////////
- (CGImageRef)createBlendedImage:(CGImageRef)image
                     secondImage:(CGImageRef)secondImage
                       blendMode:(CGBlendMode)blendMode
{
    // Get the image width and height
    size_t width = CGImageGetWidth(image);
    size_t height = CGImageGetHeight(image);

    // Set the frame
    CGRect frame = CGRectMake(0, 0, width, height);

    // Create context with alpha channel
    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 width,
                                                 height,
                                                 CGImageGetBitsPerComponent(image),
                                                 CGImageGetBytesPerRow(image),
                                                 CGImageGetColorSpace(image),
                                                 kCGImageAlphaPremultipliedLast);

    if (!context) {
        return nil;
    }

    // Draw the image inside the context
    CGContextSetBlendMode(context, kCGBlendModeCopy);
    CGContextDrawImage(context, frame, image);

    // Set the blend mode and draw the second image
    CGContextSetBlendMode(context, blendMode);
    CGContextDrawImage(context, frame, secondImage);

    // Get the masked image from the context
    CGImageRef blendedImage = CGBitmapContextCreateImage(context);
    CGContextRelease(context);

    return blendedImage;
}

/////////////////////////////////////////////////////////////////////////////////////////
- (CGImageRef)createImageTick
{
    // `self.image` and `self.previousImage` are two instance properties (CGImageRefs)

    // Create blended image (stage one)
    CGImageRef stageOne = [self createBlendedImage:self.image
                                       secondImage:self.previousImage
                                         blendMode:kCGBlendModeXOR];

    // Create blended image (stage two) if stage one image is 50% red
    CGImageRef stageTwo = nil;

    if ([self isImageRed:stageOne]) {
        stageTwo = [self createBlendedImage:self.image
                                secondImage:stageOne
                                  blendMode:kCGBlendModeSourceAtop];
    }

    // Release intermediate image
    CGImageRelease(stageOne);

    return stageTwo;
}

Answer 1

@JeremyRoman et al: Thank you so much for your comments. I am drawing the same image a couple of times per loop, onto different contexts with different filters, and combining with new images. Does resampling include switching from RGB to RGBA? What could I try to speed up or eliminate resampling? – Chris Nolet

This is what Core Image is for. See the Core Image Programming Guide for details. CGContext is designed for rendering final images to the screen, which it sounds like is not your goal with every image you're creating.