c++ - Trying image blurring with cuda

Question

I'm trying to blur an image with cuda using stbi_image as library for loading and saving the image. I get no error when I compile my code, but when I try to see the result it's just a blank image. That is the code.

#include "lodepng.h"
#define STB_IMAGE_IMPLEMENTATION
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image.h"
#include "stb_image_write.h"
#include <iostream>
#define BLUR_SIZE 7
#define R 0
#define G 1
#define B 2

__global__ void blurKernel(unsigned char* in, unsigned char* out, int width, int height, int num_channel, int channel) {

  int col = blockIdx.x * blockDim.x + threadIdx.x;
  int row = blockIdx.y * blockDim.y + threadIdx.y;

  if(col < width && row < height) {
    int pixVal = 0;
    int pixels = 0;

    for(int blurRow = -BLUR_SIZE; blurRow < BLUR_SIZE + 1; ++blurRow) {
      for(int blurCol = -BLUR_SIZE; blurCol < BLUR_SIZE + 1; ++blurCol) {
        int curRow = row + blurRow;
        int curCol = col + blurCol;
        if(curRow > -1 && curRow < height && curCol > -1 && curCol < width) {
          pixVal += in[curRow * width * num_channel + curCol * num_channel + channel];
          pixels++;
        }
      }
    }
    out[row * width * num_channel + col * num_channel + channel] = (unsigned char)(pixVal/pixels);
  }
}

int main() {

  int width, height,n;
  unsigned char *image = stbi_load("image4.png",&width,&height,&n,0);
  unsigned char *output = (unsigned char*)malloc(width * height * n *sizeof(unsigned char));
  
  unsigned char* Dev_Input_Image = NULL;
  unsigned char* Dev_Output_Image = NULL;
  cudaMalloc((void**)&Dev_Input_Image, sizeof(unsigned char)* height * width * n);
  cudaMalloc((void**)&Dev_Output_Image, sizeof(unsigned char)* height * width * n);

  cudaMemcpy(Dev_Input_Image, image, sizeof(unsigned char) * height * width * n, cudaMemcpyHostToDevice);

  //kernel call
  dim3 blockSize(16, 16, 1);
  dim3 gridSize(width/blockSize.x, height/blockSize.y,1);
  blurKernel <<<gridSize, blockSize>>>(Dev_Input_Image, Dev_Output_Image, width, height,n,R);
  blurKernel <<<gridSize, blockSize>>>(Dev_Input_Image, Dev_Output_Image, width, height,n,G);
  blurKernel <<<gridSize, blockSize>>>(Dev_Input_Image, Dev_Output_Image, width, height,n,B);
  
  cudaDeviceSynchronize();

    cudaMemcpy(image, Dev_Output_Image, sizeof(unsigned char) * height * width * n, cudaMemcpyDeviceToHost);

  stbi_write_png("output_stbimage.png", width, height, n, image, width * n);

  cudaFree(Dev_Input_Image);
  cudaFree(Dev_Output_Image);

  return 0;
}

I tried every possible ways, but I can't get where I'm stuck at. I tried doing it in a serial-way and it perfectly works with the same logic (I mean processing the blur on every channel). Hope somebody can help me

question from:https://stackoverflow.com/questions/65929954/trying-image-blurring-with-cuda

Answer 1

A problem with your code is that you are not copying (or setting) the alpha channel from input to output image in your kernel code (or anywhere else). The alpha channel is effectively uninitialized. If it happens to end up at zero, you won't see anything interesting in the output picture, regardless of the other channels.

When I fix your code like this:

#include "lodepng.h"
#define STB_IMAGE_IMPLEMENTATION
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image.h"
#include "stb_image_write.h"
#include <iostream>
#define BLUR_SIZE 7
#define R 0
#define G 1
#define B 2
#define A 3

__global__ void blurKernel(unsigned char* in, unsigned char* out, int width, int height, int num_channel, int channel, int copy_A) {

  int col = blockIdx.x * blockDim.x + threadIdx.x;
  int row = blockIdx.y * blockDim.y + threadIdx.y;

  if(col < width && row < height) {
    int pixVal = 0;
    int pixels = 0;
    if (copy_A)
      out[row*width*num_channel+col*num_channel+A] = in[row*width*num_channel+col*num_channel+A];
    for(int blurRow = -BLUR_SIZE; blurRow < BLUR_SIZE + 1; ++blurRow) {
      for(int blurCol = -BLUR_SIZE; blurCol < BLUR_SIZE + 1; ++blurCol) {
        int curRow = row + blurRow;
        int curCol = col + blurCol;
        if(curRow > -1 && curRow < height && curCol > -1 && curCol < width) {
          pixVal += in[curRow * width * num_channel + curCol * num_channel + channel];
          pixels++;
        }
      }
    }
    out[row * width * num_channel + col * num_channel + channel] = (unsigned char)(pixVal/pixels);
  }
}

int main() {

  int width, height,n;
  unsigned char *image = stbi_load("image4.png",&width,&height,&n,0);
  unsigned char *output = (unsigned char*)malloc(width * height * n *sizeof(unsigned char));
  unsigned char* Dev_Input_Image = NULL;
  unsigned char* Dev_Output_Image = NULL;
  cudaMalloc((void**)&Dev_Input_Image, sizeof(unsigned char)* height * width * n);
  cudaMalloc((void**)&Dev_Output_Image, sizeof(unsigned char)* height * width * n);

  cudaMemcpy(Dev_Input_Image, image, sizeof(unsigned char) * height * width * n, cudaMemcpyHostToDevice);

  //kernel call
  dim3 blockSize(16, 16, 1);
  dim3 gridSize(width/blockSize.x, height/blockSize.y,1);
  blurKernel <<<gridSize, blockSize>>>(Dev_Input_Image, Dev_Output_Image, width, height,n,R,0);
  blurKernel <<<gridSize, blockSize>>>(Dev_Input_Image, Dev_Output_Image, width, height,n,G,0);
  blurKernel <<<gridSize, blockSize>>>(Dev_Input_Image, Dev_Output_Image, width, height,n,B,1);
  
  cudaDeviceSynchronize();

    cudaMemcpy(image, Dev_Output_Image, sizeof(unsigned char) * height * width * n, cudaMemcpyDeviceToHost);
  cudaFree(Dev_Input_Image);
  cudaFree(Dev_Output_Image);
  stbi_write_png("output_stbimage.png", width, height, n, image, width * n);


  return 0;
}

And compile and run it using this picture:

I get an output picture that looks like this: