• 利用kCGBlendModeLuminosit混合模式

- (UIImage *)grayishImage:(UIImage *)inputImage {
    
    CGFloat scale = [UIScreen mainScreen].scale;
    UIGraphicsBeginImageContextWithOptions(inputImage.size, YES, scale);
    
    CGRect rect = CGRectMake(0, 0, inputImage.size.width, inputImage.size.height);
    
    [inputImage drawInRect:rect blendMode:kCGBlendModeLuminosity alpha:1.0];
    
    UIImage *filterImage = UIGraphicsGetImageFromCurrentImageContext();
    
    UIGraphicsEndImageContext();
    
    return filterImage;
}

• 利用灰度公式將每個像素的顏色轉(zhuǎn)成灰色

- (UIImage *)convertToGrayImage:(UIImage *)inputImage {
    
    CGImageRef imageRef = [inputImage CGImage];
    
    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);
    
    size_t bitsPerComponent = 8;
    size_t bytesPerRow = width * 4;
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    
    uint32_t *pixels = (uint32_t *) calloc(bytesPerRow * height, sizeof(uint8_t));
    
    // kCGImageAlphaPremultipliedFirst      ARGB
    // kCGImageAlphaPremultipliedLast       RGBA
    
    // kCGBitmapByteOrder32Little   小端 (低位字節(jié)在前)
    // kCGBitmapByteOrder32Big      大端 (高位字節(jié)在前)
    CGContextRef context = CGBitmapContextCreate(pixels, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    
    for (int y = 0; y < height; y++) {
        for (int x = 0; x < width; x++) {
            uint8_t *rgbaPixel = (uint8_t *) &pixels[y * width + x];
            
            uint32_t gray = 0.3 * rgbaPixel[0] + 0.59 * rgbaPixel[1] + 0.11 * rgbaPixel[2];
            
            rgbaPixel[0] = gray;
            rgbaPixel[1] = gray;
            rgbaPixel[2] = gray;
        }
    }
    
    CGImageRef newImageRef = CGBitmapContextCreateImage(context);
    
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    free(pixels);
    
    UIImage *resultImage = [UIImage imageWithCGImage:newImageRef];
    
    CGImageRelease(newImageRef);
    
    return resultImage;
}

• 利用CGColorSpaceCreateDeviceGray()

- (UIImage *)changeToGrayImage:(UIImage *)inputImage {
    
    CGImageRef imageRef = [inputImage CGImage];
    
    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);
    
    size_t bitsPerComponent = 8;
    size_t bytesPerRow = width * 4;
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    
    uint32_t *pixels = (uint32_t *) calloc(bytesPerRow * height, sizeof(uint8_t));
    
    CGContextRef contextRef = CGBitmapContextCreate(pixels, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast);
    
    CGContextDrawImage(contextRef, CGRectMake(0, 0, width, height), imageRef);
    
    CGImageRef newImageRef = CGBitmapContextCreateImage(contextRef);
    
    CGContextRelease(contextRef);
    CGColorSpaceRelease(colorSpace);
    free(pixels);
    
    UIImage *resultImage = [UIImage imageWithCGImage:newImageRef];
    
    CGImageRelease(newImageRef);
    
    return resultImage;
    
}