Texture object fetching in CUDA - c++

Question

I can find many examples online that use CUDA texture references, but not so many that rely on texture objects. I am trying to understand why my code below always fetches 0 rather than my input texture. Am I missing something, or using a wrong setting? I simplified it as much as I could:
#include <stdio.h>
__global__ void fetch(cudaTextureObject_t tex, std::size_t width, std::size_t height)
{
for (int j = 0; j < height; j++) {
for (int i = 0; i < width; i++) {
float u = (i + 0.5f) / width;
float v = (j + 0.5f) / height;
auto p = tex2D<uchar4>(tex, u, v);
printf("i=%d, j=%d -> u=%3.2f, v=%3.2f, r=%d, g=%d, b=%d, a=%d\n", i, j, u, v, p.x, p.y, p.z, p.w);
// -> always returns p = {0, 0, 0, 0}
}
}
}
int main() {
constexpr std::size_t width = 2;
constexpr std::size_t height = 2;
// creating a dummy texture
uchar4 image[width*height];
for(std::size_t j = 0; j < height; ++j) {
for(std::size_t i = 0; i < width; ++i)
image[j*width+i] = make_uchar4(255*j/height, 255*i/width, 55, 255);
}
cudaArray_t cuArray;
auto channelDesc = cudaCreateChannelDesc<uchar4>();
cudaMallocArray(&cuArray, &channelDesc, width, height);
cudaMemcpy2DToArray(cuArray, 0, 0, image, width*sizeof(uchar4), width*sizeof(uchar4), height, cudaMemcpyHostToDevice);
struct cudaResourceDesc resDesc;
memset(&resDesc, 0, sizeof(resDesc));
resDesc.resType = cudaResourceTypeArray;
resDesc.res.array.array = cuArray;
struct cudaTextureDesc texDesc;
memset(&texDesc, 0, sizeof(texDesc));
texDesc.addressMode[0] = cudaAddressModeBorder;
texDesc.addressMode[1] = cudaAddressModeBorder;
texDesc.filterMode = cudaFilterModeLinear;
texDesc.readMode = cudaReadModeElementType;
texDesc.normalizedCoords = 1;
cudaTextureObject_t texObj = 0;
cudaCreateTextureObject(&texObj, &resDesc, &texDesc, NULL);
fetch<<<1, 1>>>(texObj, width, height);
cudaDeviceSynchronize();
cudaDestroyTextureObject(texObj);
cudaFreeArray(cuArray);
return 0;
}

Answer 1

In your code you specify the texture description as
texDesc.addressMode[0] = cudaAddressModeBorder;
texDesc.addressMode[1] = cudaAddressModeBorder;
texDesc.filterMode = cudaFilterModeLinear;
texDesc.readMode = cudaReadModeElementType;
texDesc.normalizedCoords = 1;
and the array holding the texture data is defined as
auto channelDesc = cudaCreateChannelDesc<uchar4>();
Quoting the documentation
Linear Filtering
In this filtering mode, which is only available for floating-point textures ......
You have a uchar4 texture. You can't use linear filtering on an integer texture. Either change to a floating point texture type or use another read mode.