20200129.mandelbrot-external-arguments.c

#include <stdio.h>
#include <math.h>
#include <complex.h>
#include <assert.h>

#ifndef WIDTH
#define WIDTH 800
#endif

#ifndef HEIGHT
#define HEIGHT 800
#endif

#ifndef MAXITER
#define MAXITER 1024
#endif

#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif

int
main (void)
{
  printf ("P3\n%d %d\n255\n", WIDTH, HEIGHT);
  for ( int i=0 ; i<HEIGHT ; i++ )
    {
      for ( int j=0 ; j<WIDTH ; j++ )
	{
#if 1
	  double complex c
	    = (((double)j)/(WIDTH-1) * 2 - 1) * 1.3 - 0.75
	    - I * (((double)i)/(HEIGHT-1) * 2 - 1) * 1.3;
#else
	  double complex c
	    = (((double)j)/(WIDTH-1) * 2 - 1) * 0.1 + 0.36
	    - I * (((double)i)/(HEIGHT-1) * 2 - 1) * 0.1 + I * 0.60;
#endif
	  double complex z = c;
	  double complex argtab[MAXITER];
	  int iter;
	  for ( iter=0 ; iter<MAXITER ; iter++ )
	    {
	      argtab[iter] = carg(z) / (M_PI*2);
	      if ( cabs(z) >= 1.e6 )
		break;
	      z = z*z + c;
	    }
	  if ( iter < MAXITER )
	    {
	      double arg = carg(z) / (M_PI*2);
	      for ( int k=0 ; k<iter ; k++ )
		{
		  arg /= 2;
		  double altarg = arg + (signbit(arg) ? 0.5 : -0.5);
		  // We have two determinations of the argument: arg
		  // and altarg: pick the one closest to the argument
		  // of the corresponding past iterate.
		  double checkarg = argtab[iter-k-1];
		  if ( ( altarg > checkarg-0.25 && altarg < checkarg+0.25 )
		       || ( altarg > checkarg-1.25 && altarg < checkarg-0.75 )
		       || ( altarg > checkarg+0.75 && altarg < checkarg+1.25 ) )
		    arg = altarg;
		}
	      // The rest is just color gradient...
	      unsigned char colr, colg, colb;
	      double p = arg*6 + 3;
	      while ( p >= 6 )
		p -= 6;
	      while ( p < 0 )
		p += 6;
	      if ( p < 1 )
		{
		  colr = 0;
		  colg = 255*(1-p);
		  colb = 255;
		}
	      else if ( p < 2 )
		{
		  colr = 255*(p-1);
		  colg = 0;
		  colb = 255;
		}
	      else if ( p < 3 )
		{
		  colr = 255;
		  colg = 0;
		  colb = 255*(3-p);
		}
	      else if ( p < 4 )
		{
		  colr = 255;
		  colg = 255*(p-3);
		  colb = 0;
		}
	      else if ( p < 5 )
		{
		  colr = 255*(5-p);
		  colg = 255;
		  colb = 0;
		}
	      else
		{
		  colr = 0;
		  colg = 255;
		  colb = 255*(p-5);
		}
	      printf ("\t%d %d %d", colr, colg, colb);
	    }
	  else
	    printf ("\t0 0 0");
	}
      printf ("\n");
    }
}

https://gitlab.com/adammajewski/parameter_external_angle