CODEWITHSUNDEEP
c++imageopencvcamerav4l2
titch
titch

Reputation: 101

OpenCV capture YUYV from camera without RGB conversion

I trying to use openCV/c++ to capture the left and right image from a LI-USB30_V024 stereo camera without automatically converting it to RGB. The camera outputs images in YUYV format.

I have tried using videoCapture.set(CV_CAP_PROP_CONVERT_RGB, false) but I get the message "HIGHGUI ERROR: V4L: Property (16) not supported by device".

The reason I want to avoid the conversion to RGB is because the camera packages the left and right video together in to a single YUYV image. Both cameras are monochrome, and as far as I can tell the left image information is encoded in the Y channels while the right image is encoded in the U and V channels. For example if I run guvcview I get a single image that contains both the left and right images superpositioned. It looks like a black and white image (the left image is encoded in the Y channels) with a green and pink image lying on top (the right camera is encoded in the UV channels). I know this sounds pretty unusual so if you have any other ideas/questions about it then don't hesitate.

My goal is to capture the image as YUYV so that I can use split() to separate the left image (Y channels) from the right image (U and V channels) and display them both as monochrome images. However, once the image has been converted to RGB the luminance and chominance channels get blended together and it becomes impossible to separate the two images.

To sum it up, I need to capture the video without converting it to RGB so that the YUYV format is preserved. This will allow me to separate the left and right images.

OR I need a way of capturing the left and right images separately, but I think this is unlikely.

I think this would be possible in v4l, but I would prefer to only use openCV if possible.

Thanks!

Upvotes: 4

Views: 19525

Answers (4)

YU Chen Shih
YU Chen Shih

Reputation: 85

Yes you can do it but need build opencv c++ by urself

v4l2-ctl --list-formats #make sure your cam support yuyv

apt install libv4l-dev

source opencv 3.4.16

DWITH_GSTREAMER=OFF must off

cmake .. -G"Unix Makefiles" -DCMAKE_INSTALL_PREFIX=/opt/opencv -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_FLAGS_RELEASE="-O3 -g" -DCMAKE_CXX_FLAGS_RELEASE="-O3 -g" -DBUILD_PERF_TESTS=OFF -DBUILD_TESTS=OFF -DBUILD_JPEG=ON -DBUILD_PNG=ON -DBUILD_ZLIB=ON -DWITH_LIBV4L=ON -DBUILD_opencv_python3=ON -DBUILD_EXAMPLES=ON -DWITH_GSTREAMER=OFF -DWITH_CUDA=OFF
cmake --build . --config Release --target install -- -j$CPU_NUM VERBOSE=1

//try trace into cap_libv4l.cpp
cv::VideoCapture cap(0);
cap.set(CV_CAP_PROP_FRAME_WIDTH,    800);
cap.set(CV_CAP_PROP_FRAME_HEIGHT,   600);
cap.set(CV_CAP_PROP_MODE,           CV_CAP_MODE_YUYV);

if ( cap.grab() )
{
            cv::Mat yuyv;
            cv::Mat cvimg;
            cap.retrieve(yuyv);
            cvtColor(yuyv,cvimg,CV_YUV2RGB_YUY2);
...
}

this way is ok I tried!!! and trace to 
static int _capture_V4L2 (CvCaptureCAM_V4L *capture, const char *deviceName)
~~~run to
  case CV_CAP_MODE_YUYV:
    requestedPixelFormat = V4L2_PIX_FMT_YUYV;
    break;

//no error
if (-1 == xioctl (capture->deviceHandle, VIDIOC_S_FMT, &capture->form)) {
      fprintf(stderr, "VIDEOIO ERROR: libv4l unable to ioctl S_FMT\n");
      return -1;
  }

/*If you wan to know how libv4l color convert look this
libv4l using c to convert color is slow
v4l2_ioctl
v4l2_dequeue_and_convert
v4lconvert_convert
v4lconvert_convert_pixfmt
https://github.com/philips/libv4l/blob/master/libv4lconvert/libv4lconvert.c
opencv can do cvtYUV422toRGB parallel faster than libv4l
https://forum.opencv.org/t/capture-rgb-frame-directly-using-open-cv-apis/4197/11
*/

Upvotes: 0

InquisitiveMonkey
InquisitiveMonkey

Reputation: 1

In header file videoio.hpp:

// Generic camera output modes.
// Currently, these are supported through the libv4l interface only.
enum { CAP_MODE_BGR  = 0, // BGR24 (default)
       CAP_MODE_RGB  = 1, // RGB24
       CAP_MODE_GRAY = 2, // Y8
       CAP_MODE_YUYV = 3  // YUYV
     };

Usage:

cv::VideoCapture camera = cv::VideoCapture();
camera.open(0);
camera.set(CV_CAP_PROP_MODE, CV_CAP_MODE_YUYV);

Upvotes: -2

titch
titch

Reputation: 101

I dont think there is a way to do this in openCV. In the end it wasn't too much trouble to capture frames with V4L2 and store them in openCV Mats.

Upvotes: 3

TerryLu
TerryLu

Reputation: 136

I used to wrok on a camera which produce YUV420 data,and got some problem in configuring it. But this function works for me.

videoCapture.set(CV_CAP_PROP_CONVERT_RGB, 0)

Can you check if there is any other configuration that cause the problem.

Upvotes: 0

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