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#include "common.h"

///////////////////////////////////////////////////////////////////////////////
/// \brief downscale image
///
/// CUDA kernel, relies heavily on texture unit
/// \param[in]  width   image width
/// \param[in]  height  image height
/// \param[in]  stride  image stride
/// \param[out] out     result
///////////////////////////////////////////////////////////////////////////////
__global__ void DownscaleKernel(int width, int height, int stride, float *out, cudaTextureObject_t texFine)
{
    const int ix = threadIdx.x + blockIdx.x * blockDim.x;
    const int iy = threadIdx.y + blockIdx.y * blockDim.y;

    if (ix >= width || iy >= height) {
        return;
    }

    float dx = 1.0f / (float)width;
    float dy = 1.0f / (float)height;

    float x = ((float)ix + 0.5f) * dx;
    float y = ((float)iy + 0.5f) * dy;

    out[ix + iy * stride] = 0.25f
                          * (tex2D<float>(texFine, x - dx * 0.25f, y) + tex2D<float>(texFine, x + dx * 0.25f, y)
                             + tex2D<float>(texFine, x, y - dy * 0.25f) + tex2D<float>(texFine, x, y + dy * 0.25f));
}

///////////////////////////////////////////////////////////////////////////////
/// \brief downscale image
///
/// \param[in]  src     image to downscale
/// \param[in]  width   image width
/// \param[in]  height  image height
/// \param[in]  stride  image stride
/// \param[out] out     result
///////////////////////////////////////////////////////////////////////////////
static void
Downscale(const float *src, int width, int height, int stride, int newWidth, int newHeight, int newStride, float *out)
{
    dim3 threads(32, 8);
    dim3 blocks(iDivUp(newWidth, threads.x), iDivUp(newHeight, threads.y));

    cudaTextureObject_t texFine;
    cudaResourceDesc    texRes;
    memset(&texRes, 0, sizeof(cudaResourceDesc));

    texRes.resType                  = cudaResourceTypePitch2D;
    texRes.res.pitch2D.devPtr       = (void *)src;
    texRes.res.pitch2D.desc         = cudaCreateChannelDesc<float>();
    texRes.res.pitch2D.width        = width;
    texRes.res.pitch2D.height       = height;
    texRes.res.pitch2D.pitchInBytes = stride * sizeof(float);

    cudaTextureDesc texDescr;
    memset(&texDescr, 0, sizeof(cudaTextureDesc));

    texDescr.normalizedCoords = true;
    texDescr.filterMode       = cudaFilterModeLinear;
    texDescr.addressMode[0]   = cudaAddressModeMirror;
    texDescr.addressMode[1]   = cudaAddressModeMirror;
    texDescr.readMode         = cudaReadModeElementType;

    checkCudaErrors(cudaCreateTextureObject(&texFine, &texRes, &texDescr, NULL));

    DownscaleKernel<<<blocks, threads>>>(newWidth, newHeight, newStride, out, texFine);
}
