Simple raytracer in C

circle.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <png.h>
#include <assert.h>

int width, height;
int main(int argc, char *argv[]){
   if (argc != 4) {printf("usage: %s Pz c r\n", argv[0]); return 1;}

   width = 1000, height = 1000;

   FILE *fp = fopen("t.png", "wb");
   assert(fp!=NULL);

   png_structp png = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
   assert(png);
   png_infop info = png_create_info_struct(png);
   assert(info);
   assert(!setjmp(png_jmpbuf(png)));

   png_init_io(png, fp);
   png_set_IHDR(
      png,
      info,
      width, height,
      8,
      PNG_COLOR_TYPE_RGBA,
      PNG_INTERLACE_NONE,
      PNG_COMPRESSION_TYPE_DEFAULT,
      PNG_FILTER_TYPE_DEFAULT);
   png_write_info(png, info);

   png_bytep *row = NULL;
   row = malloc(sizeof(png_bytep)*height);
   assert(row!=NULL);

   for (int y = 0; y < height; y++){
      row[y] = malloc(sizeof(png_bytep)*width);
      assert(row[y]!=NULL);
   }

   for (int y = 0; y < height; y++){
      png_bytep line = row[y];
      for (int x = 0; x < width; x++){
         double Px = x-500, Py = y-500, Pz = atof(argv[1]), c = atof(argv[2]), r = atof(argv[3]);
         double delta = pow(2.0*Pz*c, 2.0)-((Px*Px+Py*Py+Pz*Pz)*(c*c-r*r));
         if (delta>0.0)
            {*(line+(x*4)+0)=0xFF;*(line+(x*4)+1)=0xFF;*(line+(x*4)+2)=0xFF;*(line+(x*4)+3)=0xFF;}
         else if (delta<0.0)
            {*(line+(x*4)+0)=0x00;*(line+(x*4)+1)=0x00;*(line+(x*4)+2)=0x00;*(line+(x*4)+3)=0xFF;}
         if (delta==0.0)
            {*(line+(x*4)+0)=0xFF;*(line+(x*4)+1)=0xFF;*(line+(x*4)+2)=0xFF;*(line+(x*4)+3)=0xFF;}
   /*
   double t_star_pos =  ((2.0*Pz*c)+sqrt(delta))/(2.0*(Px*Px+Py*Py+Pz*Pz));
   double t_star_neg =  ((2.0*Pz*c)-sqrt(delta))/(2.0*(Px*Px+Py*Py+Pz*Pz));
   printf("%lf %lf\n", t_star_pos, t_star_neg);
   */
      }
   }

   png_write_image(png, row);
   png_write_end(png, NULL);

   for (int y = 0; y < height; y++) free(row[y]);
   free(row);

   fclose(fp);
   png_destroy_write_struct(&png, &info);
   return 0;

}