Converting cv::Mat image from BGR to YUV using ffmpeg - opencv

Question

I am trying to convert BGR image into YUV420P, but when I try to view each of the YUV planes separately, this is what I see.
Shouldn't cv::Mat::data and AVFrame::data[0] be packed in the same way? I should be able to do a direct memcpy. Am I missing something?
Any ideas?
Mat frame;
VideoCapture cap;
if(!cap.open(0)){
return 0;
}
// capture a frame
cap >> frame;
if( frame.empty() ) return 0;
cv::Size s = frame.size();
int height = s.height;
int width = s.width;
// Creating two frames for conversion
AVFrame *pFrameYUV =av_frame_alloc();
AVFrame *pFrameBGR =av_frame_alloc();
// Determine required buffer size and allocate buffer for YUV frame
int numBytesYUV=av_image_get_buffer_size(AV_PIX_FMT_YUV420P, width,
height,1);
// Assign image buffers
avpicture_fill((AVPicture *)pFrameBGR, frame.data, AV_PIX_FMT_BGR24,
width, height);
uint8_t* bufferYUV=(uint8_t *)av_malloc(numBytesYUV*sizeof(uint8_t));
avpicture_fill((AVPicture *)pFrameYUV, bufferYUV, AV_PIX_FMT_YUV420P,
width, height);
// Initialise Software scaling context
struct SwsContext *sws_ctx = sws_getContext(width,
height,
AV_PIX_FMT_BGR24,
width,
height,
AV_PIX_FMT_YUV420P,
SWS_BILINEAR,
NULL,
NULL,
NULL
);
// Convert the image from its BGR to YUV
sws_scale(sws_ctx, (uint8_t const * const *)pFrameBGR->data,
pFrameYUV->linesize, 0, height,
pFrameYUV->data, pFrameYUV->linesize);
// Trying to see the different planes of YUV
Mat MY = Mat(height, width, CV_8UC1);
memcpy(MY.data,pFrameYUV->data[0], height*width);
imshow("Test1", MY); // fail
Mat MU = Mat(height/2, width/2, CV_8UC1);
memcpy(MU.data,pFrameYUV->data[1], height*width/4);
imshow("Test2", MU); // fail
Mat MV = Mat(height/2, width/2, CV_8UC1);
memcpy(MV.data,pFrameYUV->data[2], height*width/4);
imshow("Test3", MV); // fail
waitKey(0); // Wait for a keystroke in the window

Answer 1

For sws_scale() third parameter should not be pFrameYUV->linesize but rather pFrameBGR->linesize, i.e. pointer to width*3.