UIImage图片grey处理

- (UIImage *) convertToGreyscale:(UIImage *)i {
    int kRed = 1;
    int kGreen = 2;
    int kBlue = 4;
    int colors = kGreen;
    int m_width = i.size.width;
    int m_height = i.size.height;
    uint32_t *rgbImage = (uint32_t *) malloc(m_width * m_height * sizeof(uint32_t));
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(rgbImage, m_width, m_height, 8, m_width * 4, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast);
    CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
    CGContextSetShouldAntialias(context, NO);
    CGContextDrawImage(context, CGRectMake(0, 0, m_width, m_height), [i CGImage]);
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    // now convert to grayscale
    uint8_t *m_imageData = (uint8_t *) malloc(m_width * m_height);
    for(int y = 0; y < m_height; y++) {
        for(int x = 0; x < m_width; x++) {
                uint32_t rgbPixel=rgbImage[y*m_width+x];
                uint32_t sum=0,count=0;
                if (colors & kRed) {sum += (rgbPixel>>24)&255; count++;}
                if (colors & kGreen) {sum += (rgbPixel>>16)&255; count++;}
                if (colors & kBlue) {sum += (rgbPixel>>8)&255; count++;}
                m_imageData[y*m_width+x]=sum/count;
        }
    }
    free(rgbImage);
    // convert from a gray scale image back into a UIImage
    uint8_t *result = (uint8_t *) calloc(m_width * m_height *sizeof(uint32_t), 1);
    // process the image back to rgb
    for(int i = 0; i < m_height * m_width; i++) {
        result[i*4]=0;
        int val=m_imageData[i];
        result[i*4+1]=val;
        result[i*4+2]=val;
        result[i*4+3]=val;
    }
    // create a UIImage
    colorSpace = CGColorSpaceCreateDeviceRGB();
    context = CGBitmapContextCreate(result, m_width, m_height, 8, m_width * sizeof(uint32_t), colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast);
    CGImageRef image = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    UIImage *resultUIImage = [UIImage imageWithCGImage:image];
    CGImageRelease(image);
    // make sure the data will be released by giving it to an autoreleased NSData
    [NSData dataWithBytesNoCopy:result length:m_width * m_height];
    return resultUIImage;
}

网上看到的，特意留下