一般来讲，我们会使用 CoreImage 自带滤镜进行模糊。

- (UIImage *)blurryImage:(UIImage *)image withBlurLevel:(CGFloat)blur
{
    CIImage *inputImage = [CIImage imageWithCGImage:image.CGImage];
    CIFilter *filter    = [CIFilter filterWithName:@"CIGaussianBlur"
                                     keysAndValues:kCIInputImageKey, inputImage,
                           @"inputRadius", @(blur),
                           nil];

    CIImage *outputImage = filter.outputImage;

    CGImageRef outImage = [[CIContext contextWithOptions:nil] createCGImage:outputImage fromRect:[outputImage extent]];
    return [UIImage imageWithCGImage:outImage];
}

更高效的方法是，使用 Accelerate.framework 进行高效计算。（参考 Sugar）

#import <Accelerate/Accelerate.h>
- (UIImage *)blurryImage:(UIImage *)image withBlurLevel:(CGFloat)blur
{
    if (blur < 0.f || blur > 1.f)
    {
        blur = 0.5f;
    }
    int boxSize = (int)(blur * 100);
    boxSize = boxSize - (boxSize % 2) + 1;

    CGImageRef img = image.CGImage;

    vImage_Buffer inBuffer, outBuffer;
    vImage_Error error;

    void *pixelBuffer;

    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);

    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);

    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);

    CGColorSpaceRelease(colorSpace);
    CGImageRelease(imageRef);

    return returnImage;
}