Converting YUV into BGR or RGB in OpenCV

Question

I have a TV capture card that has a feed coming in as a YUV format. I\'ve seen other posts here similar to this question and attempted to try every possible method stated, b

Answer 1

Maybe someone is confused by color models YCbCr and YUV. Opencv does not handle YCbCr. Instead it has YCrCb, and it implemented the same way as YUV in opencv.

From the opencv sources https://github.com/Itseez/opencv/blob/2.4/modules/imgproc/src/color.cpp#L3830:

case CV_BGR2YCrCb: case CV_RGB2YCrCb:
case CV_BGR2YUV: case CV_RGB2YUV:
    // ...
    // 1 if it is BGR, 0 if it is RGB
    bidx = code == CV_BGR2YCrCb || code == CV_BGR2YUV ? 0 : 2; 
    //... converting to YUV with the only difference that brings 
    //    order of Blue and Red channels (variable bidx)

But there is one more thing to say.
There is currently a bug in conversion CV_BGR2YUV and CV_RGB2YUV in OpenCV branch 2.4.* .

At present, this formula is used in implementation:

Y = 0.299B + 0.587G + 0.114R
U = 0.492(R-Y)
V = 0.877(B-Y)

What it should be (according to wikipedia):

Y = 0.299R + 0.587G + 0.114B
U = 0.492(B-Y)
V = 0.877(R-Y)

The channels Red and Blue are misplaced in the implemented formula.

Possible workaround to convert BGR->YUV while the bug is not fixed :

  cv::Mat source = cv::imread(filename, CV_LOAD_IMAGE_COLOR);
  cv::Mat yuvSource;    
  cvtColor(source, yuvSource, cv::COLOR_BGR2RGB); // rearranges B and R in the appropriate order
  cvtColor(yuvSource, yuvSource, cv::COLOR_BGR2YUV);
  // yuvSource will contain here correct image in YUV color space