How to efficiently and fast blur an image on the iPhone?

Question

If I have a UIImage or CGContextRef or the pure bitmap data (direct access to decompressed ARGB-8 pixels), what's my best option to blur an image with radius 10 pixels as fast as possible?

Answer 1

Either use a stack blur, a box blur or use the OpenGL texture blur (google the first two, and check the Apple dev samples for the latter).

Answer 2

I've implemented a stackBlur algorithm for iOS, which is close to GaussianBlur but very fast:

https://github.com/tomsoft1/StackBluriOS

Check for instance here:

Blur an UIImage on change of slider

Answer 3

https://github.com/rnystrom/RNBlurModalView

-(UIImage *)boxblurImageWithBlur:(CGFloat)blur bluringImage : (UIImage *) image
{

    int boxSize = (int)(blur * 40);
    boxSize = boxSize - (boxSize % 2) + 1;

    CGImageRef img = image.CGImage;
    enter code here
    vImage_Buffer inBuffer, outBuffer;

    vImage_Error error;

    void *pixelBuffer;


    //create vImage_Buffer with data from CGImageRef

    CGDataProviderRef inProvider = CGImageGetDataProvider(img);
    CFDataRef inBitmapData = CGDataProviderCopyData(inProvider);


    inBuffer.width = CGImageGetWidth(img);
    inBuffer.height = CGImageGetHeight(img);
    inBuffer.rowBytes = CGImageGetBytesPerRow(img);

    inBuffer.data = (void*)CFDataGetBytePtr(inBitmapData);

    //create vImage_Buffer for output

    pixelBuffer = malloc(CGImageGetBytesPerRow(img) * CGImageGetHeight(img));

    if(pixelBuffer == NULL)
        NSLog(@"No pixelbuffer");

    outBuffer.data = pixelBuffer;
    outBuffer.width = CGImageGetWidth(img);
    outBuffer.height = CGImageGetHeight(img);
    outBuffer.rowBytes = CGImageGetBytesPerRow(img);

    // Create a third buffer for intermediate processing
    void *pixelBuffer2 = malloc(CGImageGetBytesPerRow(img) * CGImageGetHeight(img));
    vImage_Buffer outBuffer2;
    outBuffer2.data = pixelBuffer2;
    outBuffer2.width = CGImageGetWidth(img);
    outBuffer2.height = CGImageGetHeight(img);
    outBuffer2.rowBytes = CGImageGetBytesPerRow(img);

    //perform convolution
    error = vImageBoxConvolve_ARGB8888(&inBuffer, &outBuffer2, NULL, 0, 0, boxSize, boxSize, NULL, kvImageEdgeExtend);
    error = vImageBoxConvolve_ARGB8888(&outBuffer2, &inBuffer, NULL, 0, 0, boxSize, boxSize, NULL, kvImageEdgeExtend);
    error = vImageBoxConvolve_ARGB8888(&inBuffer, &outBuffer, NULL, 0, 0, boxSize, boxSize, NULL, kvImageEdgeExtend);

    if (error) {
        NSLog(@"error from convolution %ld", error);
    }

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef ctx = CGBitmapContextCreate(outBuffer.data,
                                            outBuffer.width,
                                            outBuffer.height,
                                            8,
                                            outBuffer.rowBytes,
                                            colorSpace,
                                            kCGImageAlphaNoneSkipLast);
    CGImageRef imageRef = CGBitmapContextCreateImage (ctx);
    UIImage *returnImage = [UIImage imageWithCGImage:imageRef];

    //clean up
    CGContextRelease(ctx);
    CGColorSpaceRelease(colorSpace);

    free(pixelBuffer);
    CFRelease(inBitmapData);

    CGImageRelease(imageRef);

    return returnImage;
}