How to prevent rgb conversion for gray scale camera?

C++
1

Hi
I have a camera with 1600x1300 resolution at 8 or 16 bits grayscale.
I grab the frames using this program:

	VideoCapture cap1;
	Mat CameraFrame1;

	cap1.open(0);
	
	cap1.set(cv::CAP_PROP_FRAME_WIDTH, 640);
	cap1.set(cv::CAP_PROP_FRAME_HEIGHT, 480);
	//cap1.set(cv::CAP_PROP_CONVERT_RGB, 0);
	//cap1.set(cv::CAP_PROP_FORMAT, CV_8UC1);

	int grabFormat = cap1.get(cv::CAP_PROP_FORMAT);
	cout << "Format = " << type2str(grabFormat) << endl;
   
	int frame = 0;
	for (;;)
	{
		cap1 >> CameraFrame1;

		int numRows = CameraFrame1.rows;
		int numCols = CameraFrame1.cols;
		int type = CameraFrame1.type();
  
		// some frame processing
		....
		
		frame++;
		if (frame == 100) {
			cout << "type = " << type2str(type) << " c,r = " << numCols << "," << numRows << endl;
			frame = 0;
		}

		// Wait for Escape keyevent to exit from loop
		char keypressed = (char)waitKey(10);
		if (keypressed == 27)
			break;
   }

I get this output:

Format = 8UC1
type = 8UC3 c,r = 640,480
type = 8UC3 c,r = 640,480
type = 8UC3 c,r = 640,480
type = 8UC3 c,r = 640,480
type = 8UC3 c,r = 640,480
type = 8UC3 c,r = 640,480
....

So the VideoCapture indicates format 8UC1, but the captured frames are of type 8UC3!
Enabling the line

cap1.set(cv::CAP_PROP_CONVERT_RGB, 0);

or line

cap1.set(cv::CAP_PROP_FORMAT, CV_8UC1);

does not make any difference.

Also strange is that if I do not set the resolution to 640x480, the captured frames are
1600x1300 of 8UC1 type! So then it is ok!

Note that at the start of the program I get these two warnings:

[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (933) open OpenCV | GStreamer warning: Cannot query video position: status=0, value=-1, duration=-1
[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (1034) getProperty OpenCV | GStreamer warning: unhandled property: 8

Removing the lines

int grabFormat = cap1.get(cv::CAP_PROP_FORMAT);
cout << "Format = " << type2str(grabFormat) << endl;

will remove the second warning. But the frames are still of type 8UC3.

How can I prevent the rgb conversion?