Generate a color palette

Generate a color palette

#include <stdio.h>
#include <error.h>
#include <math.h>
#include <gd.h>

#define RGB2INT(r,g,b)  (b + 256 * g + 256 * 256 * r)

int main(int argc, char *argv[]) {
 FILE *fp = {0};
 gdImagePtr img = {0};
 char *fname = NULL;
 int x, y, w, h, color;
 int red, green, blue;
 int palette[256] = {0};

 if(argc != 2)
  error(1, 0, "Usage: gdpalette image.png");
 else
  fname = argv[1];

 w = 256, h = 100;
 img = gdImageCreateTrueColor(w, h);

 for(x = 0; x < 256; x++) {
  red   = (int)(128.0 + 128 * sin(3.1415 * x / 16.0));
  green = (int)(128.0 + 128 * sin(3.1415 * x / 32.0));
  blue  = (int)(128.0 + 128 * sin(3.1415 * x / 64.0));
  palette[x] = RGB2INT(red, green, blue);
 }

 for(x = 0; x < w; x++)
  for(y = 0; y < h; y++) {
   red   = gdImageRed(img, palette[x]);
   green = gdImageGreen(img, palette[x]);
   blue  = gdImageBlue(img, palette[x]);
   color = gdImageColorAllocate(img, red, green, blue);
   gdImageSetPixel(img, x, y, color);
  }

 if((fp = fopen(fname, "w")) == NULL)
  error(1, 0, "Error - fopen(): %s", fname);
 else {
  gdImagePng(img, fp);
  fclose(fp);
 }

 gdImageDestroy(img);
 return 0;
}