How to draw rectangle on a YUV420p frame - opencv

Question

I would like to draw a red rectangle using the following function on a YUV420P frame. Following code alters the frame and I can see two black line(top and bottom) remaining black dots scattered. Any suggestions?
void draw_rectangle(uint8_t *frame, int x, int y,
int width, int height,
int img_width, int img_height)
{
cv::Mat frame_yuv;
int size[2];
Point pt1, pt2;
cv::Scalar color = Scalar(255, 0, 0);
size[0] = img_width + img_width/2;
size[1] = img_height;
frame_yuv = cv::Mat(2, size, CV_8UC1, frame);
pt1.x = x;
pt1.y = y;
pt2.x = x + width;
pt2.y = y + height;
rectangle(frame_yuv, pt1, pt2, Scalar(0, 0, 255));
}

Answer 1

Finally, I got my code working. Steps are given below for reference.
frame_yuv = cv::Mat(2, size, CV_8UC3, frame);
cv::Mat C(2,2, CV_8UC3, color);
cv::Mat C_yuv;
cvtColor(C, C_yuv, cv::COLOR_BGR2YUV_I420);
// Set the R, G, B values to C_yuv
// Extract the Y, U, V components to separate Mat's
// Apply rectange first on Y component
// Devide each points pt1, pt2 by 2
// Apply the rectange on U, V
No extra copy of the frame is done.

Answer 2

As you haven't provided any sample data, please use the file kindly provided by #zindarod with dimensions 144x176.
Here is how the YUV data look in memory:
Notice in the stream along the bottom... all the Y pixels come first. Then all the U pixels but downsampled by a factor of 4. Then all the V pixels, also downsampled by a factor of 4.
I haven't got time to write the code in OpenCV, but I can show you how to make a regular Mat out of it.
Step 1 - Extract Y channel
Take the first 144x176 bytes and put them into an 144x176 8UC1 Mat called Y.
Step 2 - Extract U channel
Skip the first 144x176 bytes and then take the next 72x88 bytes and put them into another 72x88 8UC1 Mat called U. Resize this Mat to double the width and double the height, i.e. 144x176.
Step 3 - Extract the V channel
Skip the first (144x176) + (88x72) bytes and then take the next 72x88 bytes and put them into another 72x88 8UC1 Mat called V. Resize this Mat to double the width and double the height, i.e. 144x176.
Step 4 - Merge
Take the Y, U, and V Mats and merge them into an 8UC3 Mat:
// Now merge the 3 individual channels into 3-band bad boy
auto channels = std::vector<cv::Mat>{Y, U, V};
cv::Mat ThreeBandBoy;
cv::merge(channels, ThreeBandBoy);
There is some code here that does more or less exactly what is needed for Steps 1-3.

Answer 3

I am reading this YUV image from file, which is YUV_I420.
fstream file;
file.open("yuv_i420.yuv", ios::in | ios::binary);
// size of image in RGB
size_t rows = 144, cols = 176;
if (!file.is_open())
stderr<<"Error opening file"<<endl;
else {
// get total size of file
auto size = file.tellg();
file.seekg(0,ios::end);
size = file.tellg() - size;
file.seekg(0,ios::beg);
char *buffer = new char[size];
// read image from file
if (file.read(buffer, size)) {
// create YUV Mat
Mat yuv_I420(rows + rows / 2, cols, CV_8UC1, buffer);
// draw a rectangle on YUV image, keep in mind that the YUV image is a
// single channel grayscale image, size is different than the BGR image
rectangle(yuv_I420, Point(10, 10), Point(50, 50), Scalar(255));
// convert to BGR to check validity
Mat bgr;
cvtColor(yuv_I420, bgr, cv::COLOR_YUV2BGR_I420);
cv::imshow("image", bgr);
cv::waitKey(0);
}
file.close();
delete[] buffer;
}