获取透明图像的BoundingBox？

Question

我将一个透明的.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维度.

谢谢你的帮助

Answer 1

假设"图像的边界框"只是图像中的矩形,以像素坐标指定.

您希望矩形image包含alpha大于的所有像素threshold(相当于表示此矩形中不包含的所有像素的alpha 都小于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,然后水平缩小矩形,然后垂直(或垂直然后水平)缩小矩形.

我不知道Objective-C,所以我把代码放在纯C中(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;
}

现在您已经在图像中以像素为单位设置了边界框的坐标,您应该能够在设备坐标中对其进行变换.

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!");
}

如果您希望将图像大小调整为图像视图的大小，您可以调用

[myImageView sizeToFit];