Convert NvBuffer to cv::Mat

Autonomous Machines Jetson & Embedded Systems Jetson TX2
1

I am using the example code for the Nvidia jpeg decoder and after the call to decodeToBuffer() I am wondering how can I convert the resulting NvBuffer to cv::Mat. I tried assigning the data pointer in the cv::Mat to the buffer->planes[0].data which is supposed to be the pointer to the begging of the memory in the NvBuffer but I keep on getting segfaults in the cv::Mat code.

3

Hi,
You would need to create RGBA NvBuffer and then call NvBufferTransform() to convert the decoded YUV to RGBA. Please refer to this patch for mapping RGBA NvBuffer to cv::Mat:
NVBuffer (FD) to opencv Mat - #6 by DaneLLL

4

Hello,

Thank you for the answer.

After seeing this I was wondering why do I need to trasnform the NvBuffer to RGBA before I create the cv::Mat. Can I not just use cv::Mat that is in the YUV color space and then use the open cv functions to make the color space transfer.

5

Hi,

Since NvBuffer is hardware DMA buffer so the YUV420 is in 3 planes and there is alignment between each plane. Please check the example of YUV420 in 1920x1080 in
Memory for NvMap - #10 by DaneLLL

YUV420 in cv::Mat is contiguous so the data in NvBuffer cannot be mapped to it directly. And we suggest convert to single plane RGBA.

6

Thank you so much for the quick answer!

One last question, does that mean that I can use the decodeToFd() function and use the resulting FD to make the NvBufferTransform() and make my own RGBA NvBuffer from that.

7

Also the buffer returned from decodeToBuffer() seems to be a software buffer from the documentation of the function. Would I still need the NvBufferMemSyncForCpu() call in that case.

8

Hi,
Please call decodeToFd() to get decoded NvBuffer, There is reference code in 12_camera_v4l2_cuda for MJPEG decoding. Please check the sample and grep decodeToFd().

9

Hello,

When I try to create a ARGB NvBuffer in order to do the BufferTransform I get a segfault but I’ve done it exactly like it is in the example from 13_multi_camera

        int * nvbuffer = 0;
        NvBufferCreateParams input_params;
        input_params.width = width;
        input_params.height = height;
        input_params.layout = NvBufferLayout_Pitch;
        input_params.colorFormat = NvBufferColorFormat_ABGR32;
        input_params.nvbuf_tag = NvBufferTag_JPEG;
        NvBufferCreateEx(nvbuffer, &input_params);

The error I get before the segfault is

nvbuf_utils: nvbuffer Payload Type not supported

10

Hi,
You may do memset to input_params to make sure there’s no garbage data. And call like

int fd=0;
NvBufferCreateEx(&fd);

If the issue is still present, suggest you connect TX2 to a USB camera and try to successfully run 12_camera_v4l2_cuda as a reference.

12

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