Npp step error when using cuda Npp color conversion function

Question

I use nppiBGRToYCbCr420_8u_C3P3R to convert RGB image to YUV420. The function parameters like this:

nppiBGRToYCbCr420_8u_C3P3R(const Npp8u pSrc, int nSrcStep, Npp8u pDst[3], int rDstStep[3], NppiSize oSizeROI)

I want to copy the d_array[0] to host_array to imshow Y channel image and check it,but I found that nppiBGRToYCbCr420_8u_C3P3R return error NPP_STEP_ERROR"(pitch is 921600 BGR.step is 4096(In opencv,image step is Nth power of 2)).So I hope some one can help me.

Answer 1

You have two major problems here:

1. nppiBGRToYCbCr420_8u_C3P3R converts a BGR image with interleaved BGR pixel values to one Y image, one Cb image and one Cr image. I.e. the image is output in three separated planes, thus the P in “C3P3”.
2. Due to the 420 coding, the color information is subsampled, meaning the image plane for Cb and Cr has only half the size of the original image.

And using nppiMalloc_8u_C1 to allocate the device output image would give something like (omitting error checking for simplicity and written here in browser without checking):

    Mat temp = imread("1.jpg",1);
    Npp8u *d_arrayY, *d_arrayCB, *d_arrayCR;
    GpuMat BGR(temp);

    unsigned char *host_array = (unsigned char*)malloc(temp.cols * temp.rows * sizeof(unsigned char ));
    memset(host_array,0,temp.cols * temp.rows * sizeof(unsigned char));

    size_t pitchY, pitchCB, pitchCR ;
    d_arrayY = nppiMalloc_8u_C1(temp.cols, temp.rows, &pitchY); 
    d_arrayCB = nppiMalloc_8u_C1(temp.cols/2, temp.rows/2, &pitchCB); 
    d_arrayCR = nppiMalloc_8u_C1(temp.cols/2, temp.rows/2, &pitchCR); 

    int Dstep[3] = {pitchY,pitchCB,pitchCR};
    Npp8u* d_ptrs[3] = {d_arrayY, d_arrayCB, d_arrayCR};

    NppiSize ds;
    ds.height = temp.rows;
    ds.width  = temp.cols;
    nppiBGRToYCbCr420_8u_C3P3R(BGR.ptr<Npp8u>(), BGR.step, d_ptrs, Dstep, ds);

    cudaMemcpy2D(host_array, temp.cols, d_arrayY, pitchY, temp.cols * sizeof(Npp8u), temp.rows, cudaMemcpyDeviceToHost);