linux + opencv + cuvid中使用cv::cuda::GpuMat类的一些坑

1.我最终成功实现了opencv中利用cuvid实现GPU视频解码：
核心代码是：

cv::cuda::GpuMat d_frame;
    cv::Ptr<cv::cudacodec::VideoReader> d_reader = cv::cudacodec::createVideoReader(mp4_file_name);
for (;;)
{
    if (!d_reader->nextFrame(d_frame))  //BRGA格式
        break;
    gpu_frame_count++;
    cv::Mat frame2;
    d_frame.download(frame2);
    cv::imwrite("xxx.png", frame2);
}

2.GupMat类的参考地址是：
  https://docs.opencv.org/master/d0/d60/classcv_1_1cuda_1_1GpuMat.html

  源码在： opencv-master/modules/core/include/opencv2/core/cuda.hpp

  GPUMat类的成员变量都是public的，就算没有提供访问的方法也没关系。

  一些重要的成员变量和成员函数是：

class CV_EXPORTS_W GpuMat
{
public:

    /** @brief Performs data download from GpuMat (Blocking call)

    This function copies data from device memory to host memory. As being a blocking call, it is
    guaranteed that the copy operation is finished when this function returns.
    */
    CV_WRAP void download(OutputArray dst) const;

    /** @brief Performs data download from GpuMat (Non-Blocking call)

    This function copies data from device memory to host memory. As being a non-blocking call, this
    function may return even if the copy operation is not finished.

    The copy operation may be overlapped with operations in other non-default streams if p stream is
    not the default stream and p dst is HostMem allocated with HostMem::PAGE_LOCKED option.
    */
    CV_WRAP void download(OutputArray dst, Stream& stream) const;

    //! the number of rows and columns
    int rows, cols;

    //! a distance between successive rows in bytes; includes the gap if any
    CV_PROP size_t step;

    //! pointer to the data
    uchar* data;

    //! helper fields used in locateROI and adjustROI
    uchar* datastart;
    const uchar* dataend;

};

data是GPU内存中，存储图像数据的指针

datastart的地址与data相同

dataend指向图像存储空间的结束位置。（很可惜，这里是错误的）

rows 是图片的高度

cols是图片的宽度

channels() 返回4， 说明每个像素是四个字节，格式是BGRA

step是图片每行的字节数。注意：这个值是按2的幂对齐的。我测试中使用的图片，宽度是480，每像素四字节的话，一行应该是1920; 而此处的step值是2048， 每行多出来32像素，这些像素的alpha通道值为0。

因此，虽然看起来dataend-datastart是GPU内存所占空间大小，但实际的所占空间是：step*rows

3. GpuMat类使用dowmload()方法后，Mat类会去掉多余的对齐的像素
   具体怎么做到的呢？搜索了很久终于找到源码原来在：opencv-master/modules/core/src/cuda/gpu_mat.cu

   download方法的源码是：

void cv::cuda::GpuMat::download(OutputArray _dst) const
{
    CV_DbgAssert( !empty() );

    _dst.create(size(), type());
    Mat dst = _dst.getMat();

    CV_CUDEV_SAFE_CALL( cudaMemcpy2D(dst.data, dst.step, data, step, cols * elemSize(), rows, cudaMemcpyDeviceToHost) );
}

直接这样拷贝也是可以的：
cudaMemcpy(host_data, d_frame.data,  d_frame.rows * d_frame.step , cudaMemcpyDeviceToHost);
但要注意：
#include <cuda_runtime.h>
cudaGetDeviceCount(&num_devices);
cudaSetDevice(cuda_device);
//调用各种函数来初始化cuda运行环境，否则一执行就崩溃