将 UIImage 裁剪为 alpha

我有一个相当大的、几乎全屏的图像，我将在 iPad 上显示它。图像的透明度约为 80%。我需要在客户端确定不透明像素的边界框，然后裁剪到该边界框。

扫描 StackOverflow 上的其他问题并阅读一些 CoreGraphics 文档，我想我可以通过以下方式完成此任务：

CGBitmapContextCreate(...) // Use this to render the image to a byte array

 ..
   - iterate through this byte array to find the bounding box
 ..

CGImageCreateWithImageInRect(image, boundingRect);

这似乎非常低效且笨拙。我可以用 CGImage 蒙版做一些聪明的事情，或者利用设备的图形加速来做到这一点吗？

感谢用户404709 的辛勤工作。 下面的代码也处理视网膜图像并释放 CFDataRef。

- (UIImage *)trimmedImage {

    CGImageRef inImage = self.CGImage;
    CFDataRef m_DataRef;
    m_DataRef = CGDataProviderCopyData(CGImageGetDataProvider(inImage));

    UInt8 * m_PixelBuf = (UInt8 *) CFDataGetBytePtr(m_DataRef);

    size_t width = CGImageGetWidth(inImage);
    size_t height = CGImageGetHeight(inImage);

    CGPoint top,left,right,bottom;

    BOOL breakOut = NO;
    for (int x = 0;breakOut==NO && x < width; x++) {
        for (int y = 0; y < height; y++) {
            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                left = CGPointMake(x, y);
                breakOut = YES;
                break;
            }
        }
    }

    breakOut = NO;
    for (int y = 0;breakOut==NO && y < height; y++) {

        for (int x = 0; x < width; x++) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                top = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }

    breakOut = NO;
    for (int y = height-1;breakOut==NO && y >= 0; y--) {

        for (int x = width-1; x >= 0; x--) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                bottom = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }

    breakOut = NO;
    for (int x = width-1;breakOut==NO && x >= 0; x--) {

        for (int y = height-1; y >= 0; y--) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                right = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }


    CGFloat scale = self.scale;

    CGRect cropRect = CGRectMake(left.x / scale, top.y/scale, (right.x - left.x)/scale, (bottom.y - top.y) / scale);
    UIGraphicsBeginImageContextWithOptions( cropRect.size,
                                           NO,
                                           scale);
    [self drawAtPoint:CGPointMake(-cropRect.origin.x, -cropRect.origin.y)
           blendMode:kCGBlendModeCopy
               alpha:1.];
    UIImage *croppedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CFRelease(m_DataRef);
    return croppedImage;
}