iOS 获取图片上某一像素点的颜色

   //获取图片某一点的颜色
    - (UIColor *)colorAtPixel:(CGPoint)point {
        if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, self.image.size.width, self.image.size.height), point)) {
            return nil;
        }
        
        NSInteger pointX = trunc(point.x);
        NSInteger pointY = trunc(point.y);
        CGImageRef cgImage = self.image.CGImage;
        NSUInteger width = self.image.size.width;
        NSUInteger height = self.image.size.height;
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        int bytesPerPixel = 4;
        int bytesPerRow = bytesPerPixel * 1;
        NSUInteger bitsPerComponent = 8;
        unsigned char pixelData[4] = { 0, 0, 0, 0 };
        CGContextRef context = CGBitmapContextCreate(pixelData,
                                                     1,
                                                     1,
                                                     bitsPerComponent,
                                                     bytesPerRow,
                                                     colorSpace,
                                                     kCGImageAlphaPremultipliedLast |     kCGBitmapByteOrder32Big);
        CGColorSpaceRelease(colorSpace);
        CGContextSetBlendMode(context, kCGBlendModeCopy);
        
        CGContextTranslateCTM(context, -pointX, pointY-(CGFloat)height);
        CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
        CGContextRelease(context);
        
        CGFloat red   = (CGFloat)pixelData[0] / 255.0f;
        CGFloat green = (CGFloat)pixelData[1] / 255.0f;
        CGFloat blue  = (CGFloat)pixelData[2] / 255.0f;
        CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;

        return [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
    }