Program to 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;
}

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