Question

I’m using the following code to rotate images in opencv. However, afterwards the corners are colored in black. I thought when I set borderMode to BORDER_CONSTANT I can specify the color. But whatever color I set (the last parameter), the result is always black.

cv::Mat rotated; 
float rotation = 3.0f; 
cv::warpAffine(img, 
              rotated, 
              rotation, 
              img.size(),  
              cv::INTER_CUBIC, 
              cv::BORDER_CONSTANT, 
              cv::Scalar(1.0, 1.0, 1.0, 0.0));
cv::imshow("rotated", rotated); 

Could someone please explain me, what I am doing wrong here?

I know I could use cv::floodFill afterwards, but somehow it should be possible to do using only warpAffine.

Answer 1

Did you try some values bigger than 1? White should be

Scalar(255,255,255);

Blue:

Scalar(255,0,0);

And so on.

Values in OpenCV matrices are expressed as uchars (0..255), and not (0.0..1.0)