UIImage彩色置灰

第一种（问题：透明部分转换成了黑色）

+ (UIImage *)grayImage:(UIImage *)sourceImage {
    size_t width = sourceImage.size.width;
    size_t height = sourceImage.size.height;
    uint32_t bitmapInfo = kCGBitmapByteOrderDefault; // kCGImageAlphaNone;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    /// 第四个参数：8     bits per component
    CGContextRef context = CGBitmapContextCreate (nil, width, height, 8, 0,
                                                  colorSpace, bitmapInfo);
    CGColorSpaceRelease(colorSpace);

    if (context == NULL) {
        return nil;
    }

    CGContextDrawImage(context, CGRectMake(0, 0, width, height),
                       sourceImage.CGImage);
    UIImage *grayImage = [UIImage imageWithCGImage:CGBitmapContextCreateImage(context)];
    CGContextRelease(context);

    return grayImage;
}

第二种（问题：性能上可能不是很好）

// 实现 1
+ (UIImage *)grayscaleImage:(UIImage *)image {
    CIImage *ciImage = [[CIImage alloc] initWithImage:image];
    CIImage *grayscale = [ciImage imageByApplyingFilter:@"CIColorControls"
                                    withInputParameters: @{kCIInputSaturationKey : @0.0}];
    return [UIImage imageWithCIImage:grayscale];
}

// 实现 2
+ (UIImage *)convertImageToGrayScale:(UIImage *)image {
    CIImage *inputImage = [CIImage imageWithCGImage:image.CGImage];
    CIContext *context = [CIContext contextWithOptions:nil];

    CIFilter *filter = [CIFilter filterWithName:@"CIColorControls"];
    [filter setValue:inputImage forKey:kCIInputImageKey];
    [filter setValue:@(0.0) forKey:kCIInputSaturationKey];
    CIImage *outputImage = filter.outputImage;

    CGImageRef cgImageRef = [context createCGImage:outputImage fromRect:outputImage.extent];

    UIImage *result = [UIImage imageWithCGImage:cgImageRef];
    CGImageRelease(cgImageRef);

    return result;
}

第三种（符合需求）

注意：// 如果不乘以 source.scale，图片会模糊

• size_t width = sourceImg.size.width * sourceImg.scale;
• size_t height = sourceImg.size.height * sourceImg.scale;

typedef enum {
    ALPHA   = 0,
    BLUE    = 1,
    GREEN   = 2,
    RED     = 3
} PIXELS;

+ (UIImage *)convertToGrayscale:(UIImage *)sourceImg {
    // 如果不乘以 source.scale，图片会模糊
    size_t width = sourceImg.size.width * sourceImg.scale;
    size_t height = sourceImg.size.height * sourceImg.scale;
    // the pixels will be painted to this array
    uint32_t *pixels = (uint32_t *)malloc(width * height * sizeof(uint32_t));
    // clear the pixels so any transparency is preserved
    memset(pixels, 0, width * height * sizeof(uint32_t));

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    CGContextRef context = CGBitmapContextCreate(pixels, width, height, 8,
                                                 width * sizeof(uint32_t), colorSpace,
                                                 kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast);
    // paint the bitmap to our context which will fill in the pixels array
    CGContextDrawImage(context, CGRectMake(0,0, width, height), sourceImg.CGImage);
    for(int y = 0; y < height; y++) {
        for(int x = 0; x < width; x++) {
            uint8_t *rgbaPixel = (uint8_t *) & pixels[y * width + x];
            // 图片转灰度公式：http://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale
            uint32_t gray = 0.299 * rgbaPixel[RED] + 0.587 * rgbaPixel[GREEN] + 0.114 * rgbaPixel[BLUE];

            rgbaPixel[RED] = gray;
            rgbaPixel[GREEN] = gray;
            rgbaPixel[BLUE] = gray;
        }
    }

    CGImageRef image = CGBitmapContextCreateImage(context);

    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    free(pixels);

    UIImage *resultImg = [UIImage imageWithCGImage:image];
    CGImageRelease(image);

    return resultImg;
}

图像像素颜色反转（OpenCV）

- (void)viewDidLoad {
    [super viewDidLoad];

    UIImage *mImage =  [UIImage imageNamed:@"001.jpg"];
    self.imageView.image = mImage;

    IplImage *image = [self convertToIplImage:mImage];
    CvScalar cs;
    for (int i=0; i< image->height; i++) {
        for (int j=0; j < image->width; j++) {
            cs = cvGet2D(image, i, j);
            cs.val[0] = 255 - cs.val[0];
            cs.val[1] = 255 - cs.val[1];
            cs.val[2] = 255 - cs.val[2];
            cvSet2D(image,i, j, cs);
        }
    }
    self.imageView1.image = [self convertToUIImage:image];
}

/// UIImage类型转换为IPlImage类型
- (IplImage *)convertToIplImage:(UIImage *)image {  
    CGImageRef imageRef = image.CGImage;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    IplImage *iplImage = cvCreateImage(cvSize(image.size.width, image.size.height), IPL_DEPTH_8U, 4);
    CGContextRef contextRef = CGBitmapContextCreate(iplImage->imageData, iplImage->width, iplImage->height, iplImage->depth, iplImage->widthStep, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault);
    CGContextDrawImage(contextRef, CGRectMake(0, 0, image.size.width, image.size.height), imageRef);
    CGContextRelease(contextRef);
    CGColorSpaceRelease(colorSpace);

    IplImage *ret = cvCreateImage(cvGetSize(iplImage), IPL_DEPTH_8U, 3);
    cvCvtColor(iplImage, ret, CV_RGB2BGR);
    cvReleaseImage(&iplImage);
    return ret;
}  

/// IplImage类型转换为UIImage类型
- (UIImage *)convertToUIImage:(IplImage *)image {  
    cvCvtColor(image, image, CV_BGR2RGB);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    NSData *data = [NSData dataWithBytes:image->imageData length:image->imageSize];
    CGDataProviderRef provider = CGDataProviderCreateWithCFData((CFDataRef)data);
    CGImageRef imageRef = CGImageCreate(image->width, image->height, image->depth, image->depth * image->nChannels, image->widthStep, colorSpace, kCGImageAlphaNone | kCGBitmapByteOrderDefault, provider, NULL, false, kCGRenderingIntentDefault);
    UIImage *ret = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    CGDataProviderRelease(provider);
    CGColorSpaceRelease(colorSpace);

    return ret;  
}