NPP Library Image Processing (erosion + dilation) seems non-deterministic

Accelerated Computing CUDA CUDA Programming and Performance
1

Hello.

I am using the NVIDIA Performance Primitives to perform some image processing on a Jetson TX1 with CUDA 8.

I just want to use a simple function like ‘nppiDilate_8u_C1R’. The issue is that the output image is different each time I apply the function while the input image is the same.

I wrote a short program to prove it (compiled with ‘nvcc prog.cu -lnppi -arch=sm_53’):

#include <iostream>
#include <npp.h>

int main(void) {    
    const int width = 2592;
    const int height = 1944;
    const int nbPixels = width * height;

    unsigned char img[nbPixels];

    for (int i = 0; i < nbPixels; i++) img[i] = i % 10 == 0 ? 255 : 0;

    unsigned char *d_img;
    unsigned char *d_mask;

    NppiSize maskSize = {3, 3};
    Npp8u mask[9] = {
        1, 1, 1,
        1, 1, 1,
        1, 1, 1
    };
    NppiPoint anchor = {1, 1};
    NppiSize sizeROI = {width, height};

    cudaMalloc(reinterpret_cast<void **>(&d_img), nbPixels * sizeof(unsigned char));
    cudaMalloc(reinterpret_cast<void **>(&d_mask), sizeof(unsigned char) * maskSize.height * maskSize.width);
    cudaMemcpy(d_mask, mask, maskSize.height * maskSize.width, cudaMemcpyHostToDevice);
    cudaMemcpy(d_img, img, nbPixels * sizeof(unsigned char), cudaMemcpyHostToDevice);

    nppiDilate_8u_C1R(d_img, width, d_img, width, sizeROI, d_mask, maskSize, anchor);

    cudaMemcpy(img, d_img, width * height * sizeof(unsigned char), cudaMemcpyDeviceToHost);

    int count = 0;
    for (int i = 0; i < nbPixels; i++) count += img[i] / 255;
    std::cout << count << std::endl;

    return 0;
}

The number of white pixels can be 3 830 585 or 3 859 498 for example.

I find this strange that it is non-deterministic and that there is a random component acting for a simple dilatation operation.

Is it the expected behavior or am I doing something wrong?

2

I have never used NPP. But from reading along for many years, a common programmer error with the use of NPP functions seems an incorrectly defined region of interest, which in turn leads to out-of-bounds memory accesses, which could explain “random” output (that are caused by data outside the image). So I would suggest double-checking ROI.

3

i have no idea that is dilatation, but if this compuattion involves floatimng-point data, the reason may be combination of

  1. FP computation results may depend on their order, they don’t comply to math laws
  2. order of grid blocks execution in the kernel is undefined
4

Thank you both for the answers.

I was doing something very very bad and stupid. This is obvious now that I carefully read my code: I tried to apply the image processing operation “in-place”. Source and destination images were the same which obviously leads to weird results.