iphone - 检测具有透明背景的 UIImage 的裁剪矩形

Question

获取具有透明背景的 UIImage 并确定要裁剪到的最小矩形以便只留下可见图像数据(当然，如果图像数据不是矩形，则还有额外的透明背景)的策略是什么？

我找到了很多关于将 UIImage 裁剪为 CGRect 的信息，大量需要用户干预的裁剪 View Controller ，以及几个带有图像处理类和类别的开源库(包括 MGImageUtilities 和 NYXImagesKit)，但还没有解决问题我的特殊问题。

我当前的应用程序针对 iOS 5.0，因此与它的兼容性将是最佳的。

编辑:顺便说一句，我希望有一种比蛮力更好的方法，即在最坏的情况下，在行和列中的图像数据中寻找边缘边界的情况下查看每个像素。

Answer 1

你有没有机会看到https://gist.github.com/spinogrizz/3549921 ？

看起来这正是您所需要的。

只是为了不丢失，从那个页面复制粘贴:

- (UIImage *) imageByTrimmingTransparentPixels {
    int rows = self.size.height;
    int cols = self.size.width;
    int bytesPerRow = cols*sizeof(uint8_t);

    if ( rows < 2 || cols < 2 ) {
        return self;
    }

    //allocate array to hold alpha channel
    uint8_t *bitmapData = calloc(rows*cols, sizeof(uint8_t));

    //create alpha-only bitmap context
    CGContextRef contextRef = CGBitmapContextCreate(bitmapData, cols, rows, 8, bytesPerRow, NULL, kCGImageAlphaOnly);

    //draw our image on that context
    CGImageRef cgImage = self.CGImage;
    CGRect rect = CGRectMake(0, 0, cols, rows);
    CGContextDrawImage(contextRef, rect, cgImage);

    //summ all non-transparent pixels in every row and every column
    uint16_t *rowSum = calloc(rows, sizeof(uint16_t));
    uint16_t *colSum = calloc(cols, sizeof(uint16_t));

    //enumerate through all pixels
    for ( int row = 0; row < rows; row++) {
        for ( int col = 0; col < cols; col++)
        {
            if ( bitmapData[row*bytesPerRow + col] ) { //found non-transparent pixel
                rowSum[row]++;
                colSum[col]++;
            }
        }
    }

    //initialize crop insets and enumerate cols/rows arrays until we find non-empty columns or row
    UIEdgeInsets crop = UIEdgeInsetsMake(0, 0, 0, 0);

    for ( int i = 0; i<rows; i++ ) {        //top
        if ( rowSum[i] > 0 ) {
            crop.top = i; break;
        }
    }

    for ( int i = rows; i >= 0; i-- ) {     //bottom
        if ( rowSum[i] > 0 ) {
            crop.bottom = MAX(0, rows-i-1); break;
        }
    }

    for ( int i = 0; i<cols; i++ ) {        //left
        if ( colSum[i] > 0 ) {
            crop.left = i; break;
        }
    }

    for ( int i = cols; i >= 0; i-- ) {     //right
        if ( colSum[i] > 0 ) {
            crop.right = MAX(0, cols-i-1); break;
        }
    }

    free(bitmapData);
    free(colSum);
    free(rowSum);

    if ( crop.top == 0 && crop.bottom == 0 && crop.left == 0 && crop.right == 0 ) {
        //no cropping needed
        return self;
    }
    else {
        //calculate new crop bounds
        rect.origin.x += crop.left;
        rect.origin.y += crop.top;
        rect.size.width -= crop.left + crop.right;
        rect.size.height -= crop.top + crop.bottom;

        //crop it
        CGImageRef newImage = CGImageCreateWithImageInRect(cgImage, rect);

        //convert back to UIImage
        return [UIImage imageWithCGImage:newImage];
    }
}