// write png image with stb_image_write.h

#include <stdio.h>

#define STB_IMAGE_STATIC
#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"

#define STB_IMAGE_WRITE_STATIC
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

int main(int argc, char *argv[])
{
  if (argc != 2)
  {
    printf("Usage\n\t02_writeimage.exe IN.png");
  }

  auto filename = argv[1];

  unsigned char *pixels;
  int width;
  int height;
  int bpp;

  // load image
  pixels = stbi_load(filename, &width, &height, &bpp, 0);

  printf("Image size %d x %d\n", width, height);
  printf("Image bpp %d\n", bpp);

  for (int y = 0; y < height; y++)
    for (int x = 0; x < width; x++)
    {
      int idx = (y * width + x) * bpp;

      unsigned int r = (unsigned int)pixels[idx + 0];
      unsigned int g = (unsigned int)pixels[idx + 1];
      unsigned int b = (unsigned int)pixels[idx + 2];

      // convert greyscale
      if (1)
      {
        double grey = 0.299 * (double)r + 0.587 * (double)g + 0.114 * (double)b;
        if (grey < 0)
          grey = 0;
        if (grey > 255)
          grey = 255;
        unsigned int ui_grey = (unsigned int)grey;

        pixels[idx + 0] = ui_grey;
        pixels[idx + 1] = ui_grey;
        pixels[idx + 2] = ui_grey;
      }
    }

  // write image
  if (stbi_write_png("out.png", width, height, 3, pixels, width * 3))
  {
    printf("Save out.png\n");
  }
  else
  {
    printf("Failure save.\n");
  }

  stbi_image_free(pixels);
}