# cv::cvtColorTwoPlane error with steps

**URL:** <https://forum.opencv.org/t/cv-cvtcolortwoplane-error-with-steps/2808>\
**Category:** C++\
**Tags:** cvtcolor\
**Created:** [April 16, 2021, 10:58pm UTC](https://forum.opencv.org/t/cv-cvtcolortwoplane-error-with-steps/2808 "2021-04-16T22:58:19Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![warpstar22](https://avatars.discourse-cdn.com/v4/letter/w/a5b964/32.png) [@warpstar22](https://forum.opencv.org/u/warpstar22)\
**Post date:** [April 16, 2021, 10:58pm UTC](https://forum.opencv.org/t/cv-cvtcolortwoplane-error-with-steps/2808/1 "2021-04-16T22:58:19Z")

</div>

I made a class using GStreamer to get frames from some cameras. I have both MIPI CSI and UVC cameras. This is on the Jetson Nano so they have some other GstElements. The simplified pipelines:

CSI: `nvarguscamerasrc ! video/x-raw(memory:NVMM), format=NV12 ! nvvidconv ! video/x-raw ! appsink`

UVC: `v4l2src ! image/jpeg, format=MJPG ! nvv4l2decoder mjpeg=true ! nvvidconv ! video/x-raw ! appsink`

These pipelines will give me the data in YUV NV12 so I use `cv::cvtColorTwoPlane` with `cv::COLOR_YUV2BGR_NV12`. The class has three functions: one to play the pipeline; another to decode the last frame received in the pipeline; and a function to return the decoded image. I had a test program where these are working fine. I tried to include it in my main program and I get the following error for all my cameras:  
`OpenCV(4.4.0) /tmp/build_opencv/opencv/modules/imgproc/src/color_yuv.dispatch.cpp:409: error: (-2:Unspecified error) in function 'void cv::cvtColorTwoPlaneYUV2BGRpair(cv::InputArray, cv::InputArray, cv::OutputArray, int, bool, int)'`

```auto
> (expected: 'ysrc.step == uvsrc.step'), where
> 'ysrc.step is 1920
> must be equal to
> 'uvsrc.step' is 960

```

The error occurs at `CV_CheckEQ(ysrc.step, uvsrc.step, "");` in `color_yuv.dispatch.cpp`.

It’s pretty self-explanatory where the Y Mat’s step needs to be equal to the UV Mat’s step. The only problem is that it is different in my test program which doesn’t give me an error. I printed the steps with `nv12_y.step` and `nv12_uv.step` and they print 1920 and 960 respectively, but somehow pass the `CV_CheckEQ` statement in my test program but not in my main application. The actual decoding code should be fine because the frames appear properly in my test program. Here is the decode code:

```auto
GstBuffer *gbuffer = gst_sample_get_buffer(m_sample);
GstMapInfo *gmap;
gst_buffer_map(gbuffer, &gmap, GST_MAP_READ);

cv::Mat nv12_y, nv12_uv;
nv12_y = cv::Mat(height, width, CV_8UC1, (uchar *) gmap.data);
nv12_uv = cv::Mat(height/2, width/2, CV_8UC1, ( (uchar *) gmap.data ) + (height * width));

cv::cvtColorTwoPlane(nv12_y, nv12_uv, frame, cv::COLOR_YUV2BGR_NV12);

```

My test code looks like the following in pseudcode:

```auto
while(true) {
    play all camera pipelines for a few milliseconds; pause pipelines

    decode latest frames from each pipeline

    cv::imshow each frame
}

```

My main application is doing the same thing, but with a lot of other stuff going on around it. The only difference would be the time between each step. The pipelines and cameras are the exact same.

---

<div class="post-metadata">

**Author:** ![warpstar22](https://avatars.discourse-cdn.com/v4/letter/w/a5b964/32.png) [@warpstar22](https://forum.opencv.org/u/warpstar22)\
**Post date:** [April 19, 2021, 9:32pm UTC](https://forum.opencv.org/t/cv-cvtcolortwoplane-error-with-steps/2808/2 "2021-04-19T21:32:49Z")

</div>

Well, I fixed it by creating nv12\_uv with CV\_8UC2 rather than CV\_8UC1. The steps are equal now. Still not sure why it worked in my test program though.