Copy cv::Mat into CMSampleBufferRef - ios

Question

How can I copy cv::Mat data back into the sampleBuffer?
My scenario as follow :
I create a cv::Mat from pixelBuffer for landmark detection and add the landmarks to cv::Mat image data. I'd like to copy this cv::Mat into the sample buffer to be shown with the landmark.
Is this possible ?
I achieved this with dlib but need to know how to do it with cv::mat:
char *baseBuffer = (char *)CVPixelBufferGetBaseAddress(imageBuffer);
img.reset();
long position = 0;
while (img.move_next()) {
dlib::bgr_pixel& pixel = img.element();
long bufferLocation = position * 4; //(row * width + column) * 4;
char b = baseBuffer[bufferLocation];
char g = baseBuffer[bufferLocation + 1];
char r = baseBuffer[bufferLocation + 2];
dlib::bgr_pixel newpixel(b, g, r);
pixel = newpixel;
position++;
}

Answer 1

I am answering my own question.
First thing, you need to access the pixel data of cv::mat Image, I followed this great solution
Then you need to copy pixel into the buffer starting from the basebuffer. Following code should help you achieve this :
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
char *baseBuffer = (char *)CVPixelBufferGetBaseAddress(imageBuffer);
long position = 0;
uint8_t* pixelPtr = (uint8_t*)targetImage.data;
int cn = targetImage.channels();
cv::Scalar_<uint8_t> rgbPixel;
for(int i = 0; i < targetImage.rows; i++)
{
for(int j = 0; j < targetImage.cols; j++)
{
long bufferLocation = position * 4;
rgbPixel.val[0] = pixelPtr[i*targetImage.cols*cn + j*cn + 0]; // B
rgbPixel.val[1] = pixelPtr[i*targetImage.cols*cn + j*cn + 1]; // G
rgbPixel.val[2] = pixelPtr[i*targetImage.cols*cn + j*cn + 2]; // R
baseBuffer[bufferLocation] = rgbPixel.val[2];
baseBuffer[bufferLocation + 1] = rgbPixel.val[1];
baseBuffer[bufferLocation + 2] = rgbPixel.val[0];
position++;
}
}
Some things to take note of
make sure you CVPixelBufferLockBaseAddress and
CVPixelBufferUnlockBaseAddress before and after the operation. I
am doing this on CV_8UC3, you might want to check your cv::mat
type.
I haven't done the performance analysis but I am getting smooth output with this.