c++ - 在具有透明度的图像中创建边界框

Question

我想用一种特定颜色裁剪图像，就像透明蒙版一样，我想创建最小的框来包围图像。

我的意思的图片肯定会更好地解释它:

RGB(255,0,255) 是透明蒙版，绿色框代表所需的边界框。

我已经弄清楚了边界框的顶线:

    int nAlloc = (128 * 128) * 4;
    unsigned char* buf = new unsigned char[nAlloc];
    GetBitmapBits(hBMP, nAlloc, buf);
    for(int i = 0; i < nAlloc; i += 4)
    {
        if(buf[i] == 255 && buf[i + 1] == 0 && buf[i + 2] == 255)
        {
            continue;
        }
        // If I hit a first non-transparent pixel, 
        // I can then calculate the row where is that pixel located.
        else if(set_pixel == false)
        {
            set_pixel = true;
            index = ceil((float)(i / 4.0 / 128.0));
        }
        ... // Converting non-transparent pixels to Black&White
    }

    //I'm then drawing the bitmap to window like so:
    TransparentBlt(hdc, 5, 305 - index, 128, 128, hDC, 0, 0, 128, 128, RGB(255, 0, 255));

我想，我也知道如何确定最后一行，但我不确定，也不知道如何找出边界框的边。

Answer 1

要找到图像的边界框，您只需按行和按列检查图像，直到找到不透明的像素。通过这样做，您可以获得边界框的最小值和最大值。

RECT BoundingBox = { 0,0,0,0 };
const int nAlloc = (128 * 128) * 4;
unsigned char* buf = new unsigned char[nAlloc];
GetBitmapBits(hBMP, nAlloc, buf);

bool found;

//search upper bound
found = false;
for (int row = 0; row<128 && !found; row++) //row
{
    for (int col = 0; col<128 && !found; col++) //column
    {
        int idx = (row * 128 + col) * 4;
        if (!(buf[idx] == 255 && buf[idx + 1] == 0 && buf[idx + 2] == 255)) //not transparent
        {
            BoundingBox.top = row;
            found = true;
        }
    }
}

//search lower bound
found = false;
for (int row = 127; row >= 0 && !found; row--) //row
{
    for (int col = 127; col >= 0 && !found; col--) //column
    {
        int idx = (row * 128 + col) * 4;
        if (!(buf[idx] == 255 && buf[idx + 1] == 0 && buf[idx + 2] == 255)) //not transparent           {
            BoundingBox.bottom = row;
            found = true;
        }
    }
}


//search left bound
found = false;
for (int col = 0; col<128 && !found; col++) //row
{
    for (int row = 0; row<128 && !found; row++) //column
    {
        int idx = (row * 128 + col) * 4;
        if (!(buf[idx] == 255 && buf[idx + 1] == 0 && buf[idx + 2] == 255)) //not transparent           {
            BoundingBox.left = col;
            found = true;
        }
    }
}


//search right bound
found = false;
for (int col = 127; col >= 0 && !found; col--) //row
{
    for (int row = 127; row >= 0 && !found; row--) //column
    {
        int idx = (row * 128 + col) * 4;
        if (!(buf[idx] == 255 && buf[idx + 1] == 0 && buf[idx + 2] == 255)) //not transparent           {
            BoundingBox.right = col;
            found = true;
        }
    }
}

//now the variable "BoundingBox" contains your BoundingBox

希望我的回答对您有所帮助。