获取透明图像的边界框？

Question

我将透明的 .png 加载到 UIImage 中。

如何计算真正的边界框。例如。如果真实图像小于 .png 尺寸。

感谢您的帮助

Answer 1

假设“图像的边界框”只是图像中的一个矩形，以像素坐标指定。

您希望image 的矩形包含所有 alpha 大于 threshold 的像素（这相当于说所有不在此矩形中的像素的 alpha 都低于 threshold）。之后，您可以在屏幕坐标（或任何您想要的）中转换此矩形。

基本算法是从包含整个图像的矩形开始，然后水平收缩矩形，然后垂直（或垂直然后水平）。

我不懂Objective-C，所以我把代码放在纯C里（有些函数只是为了让代码更清晰）：

typedef struct Rectangle
{
    unsigned int x1, y1, x2, y2;
} Rectangle;

typedef struct Image
{
    unsigned int height,width;
    unsigned int* data;
} Image;

unsigned char getPixelAlpha(Image* img, unsigned int x, unsigned int y)
{
    unsigned int pixel = 0; // default = fully transparent

    if(x >= img->width || y >= img->height)
        return pixel; // Consider everything not in the image fully transparent

    pixel = img->data[x + y * img->width];
    return (unsigned char)((pixel & 0xFF000000) >> 24);
}

void shrinkHorizontally(Image* img, unsigned char threshold, Rectangle* rect)
{
    int x, y;

    // Shrink from left
    for(x = 0; x < (int)img->width; x++)
    {
        // Find the maximum alpha of the vertical line at x
        unsigned char lineAlphaMax = 0;
        for(y = 0; y < (int)img->height; y++)
        {
            unsigned char alpha = getPixelAlpha(img,x,y);
            if(alpha > lineAlphaMax)
                lineAlphaMax = alpha;
        }

        // If at least on pixel of the line if more opaque than 'threshold'
        // then we found the left limit of the rectangle
        if(lineAlphaMax >= threshold)
        {
            rect->x1 = x;
            break;
        }
    }


    // Shrink from right
    for(x = img->width - 1; x >= 0; x--)
    {
        // Find the maximum alpha of the vertical line at x
        unsigned char lineAlphaMax = 0;
        for(y = 0; y < (int)img->height; y++)
        {
            unsigned char alpha = getPixelAlpha(img,x,y);
            if(alpha > lineAlphaMax)
                lineAlphaMax = alpha;
        }

        // If at least on pixel of the line if more opaque than 'threshold'
        // then we found the right limit of the rectangle
        if(lineAlphaMax >= threshold)
        {
            rect->x2 = x;
            break;
        }
    }
}

// Almost the same than shrinkHorizontally.
void shrinkVertically(Image* img, unsigned char threshold, Rectangle* rect)
{
    int x, y;

    // Shrink from up
    for(y = 0; y < (int)img->height; y++)
    {
        // Find the maximum alpha of the horizontal line at y
        unsigned char lineAlphaMax = 0;
        for(x = 0; x < (int)img->width; x++)
        {
            unsigned char alpha = getPixelAlpha(img,x,y);
            if(alpha > lineAlphaMax)
                lineAlphaMax = alpha;
        }

        // If at least on pixel of the line if more opaque than 'threshold'
        // then we found the up limit of the rectangle
        if(lineAlphaMax >= threshold)
        {
            rect->y1 = x;
            break;
        }
    }


    // Shrink from bottom
    for(y = img->height- 1; y >= 0; y--)
    {
        // Find the maximum alpha of the horizontal line at y
        unsigned char lineAlphaMax = 0;
        for(x = 0; x < (int)img->width; x++)
        {
            unsigned char alpha = getPixelAlpha(img,x,y);
            if(alpha > lineAlphaMax)
                lineAlphaMax = alpha;
        }

        // If at least on pixel of the line if more opaque than 'threshold'
        // then we found the bottom limit of the rectangle
        if(lineAlphaMax >= threshold)
        {
            rect->y2 = x;
            break;
        }
    }
}

// Find the 'real' bounding box
Rectangle findRealBoundingBox(Image* img, unsigned char threshold)
{
    Rectangle r = { 0, 0, img->width, img->height };
    shrinkHorizontally(img,threshold,&r);
    shrinkVertically(img,threshold,&r);
    return r;
}

现在您已经获得了图像中边界框的像素坐标，您应该能够将其转换为设备坐标。

Answer 2

CGRect myImageViewRect = [myImageView frame];
CGSize myImageSize = [[myImageView image]size];

if(myImageSize.width < myImageViewRect.size.width){
   NSLog(@"it's width smaller!");
}
if(myImageSize.height < myImageViewRect.size.height){
   NSLog(@"it's height smaller!");
}

如果您希望图像调整为图像视图的大小，您可以调用

[myImageView sizeToFit];