alias_back.c Example C Language Program

#include <gl/gl.h>
#include <gl/device.h>
#include <stdlib.h>
#include <math.h>

/* 
* This program illustrates the drawing of anti-aliased lines of
* multiple foreground colors on a monochrome background.
* 
* Note that this style of drawing is limited to precisely one
* background color. The number of foreground colors is also
* limited;
* the maximum number of foreground colors is
* one-sixteenth of the total number of colormap entries.
*
* The basic idea behind the algorithm is to divide the available
* bitplanes into two groups: those containing the foreground line
* colors, and those containing the antialiasing coverage
* information.  A color-ramp is loaded in the color map ranges
* corresponding to the coverage information.  The foreground line
* drawing colors can be arbitrary.
*
* Note that proper gamma correction is absolutely vital to
* getting antialiased lines that look truly smooth to the user.
* The gamma exponent depends on the monitor (specifically, on the
* type of phosphors) and therefore needs to be tuned to the model
* of the monitor.  */

draw_fan()
{
  int i; /* draw fan */

  for (i=0; i<= 90; i+=10) {
    pushmatrix();
    rot (-(float) i, 'z');
    move (0.0, 0.0, 0.0);
    draw (0.0, 200.0, 0.0);
    popmatrix();
  }

}

#define AABLACK  0
#define AAGREEN  16
#define AARED    32
#define AAPURPLE 48
#define AAWHITE  64

/* experimentally determined gamma factor */
double gammy = 2.4;

/* a utility macro used to build the color ramp */
#define ADD_TO_RAMP(COL)                                                \ 
{                                                                       \ 
   for (j=0; j<16; j++) {                                               \ 
      col_idx = COL + j;                                                \ 
      rramp[col_idx] = (j*foreground_r + (16-j)*background_r) / 16;     \ 
      gramp[col_idx] = (j*foreground_g + (16-j)*background_g) / 16;     \ 
      bramp[col_idx] = (j*foreground_b + (16-j)*background_b) / 16;     \ 
      rramp[col_idx] = (int)                                            \ 
        (255.0 * pow (((double) rramp[col_idx])/255.0 , 1.0 / gammy));  \ 
      gramp[col_idx] = (int)                              \ 
        (255.0 * pow (((double) gramp[col_idx])/255.0 , 1.0 / gammy));  \ 
      bramp[col_idx] = (int)                                            \ 
        (255.0 * pow (((double) bramp[col_idx])/255.0 , 1.0 / gammy));  \ 
   }                                                                    \ 
}

/* This routine creates an "random" color ramp and loads it */
void load_background_cmap (short background_r,
short background_g,
short background_b)
{
  int i, j, col_idx;
  int foreground_r, foreground_g, foreground_b;
  short rramp[256], gramp[256], bramp[256];
    /* update gamma correction factor */
  if (getbutton (WKEY)) gammy +=0.02;
  if (getbutton (SKEY)) gammy -=0.02;

   /* create color ramp */

  /* AABLACK */
  foreground_r = 0;
  foreground_g = 0;
  foreground_b = 0;
  ADD_TO_RAMP (AABLACK); /* AAGREEN */
  foreground_r = 0;
  foreground_g = 255;
  foreground_b = 0;
  ADD_TO_RAMP (AAGREEN); /* AARED */
  foreground_r = 255;
  foreground_g = 127;
  foreground_b = 0;
  ADD_TO_RAMP (AARED); /* AAPURPLE */
  foreground_r = 150;
  foreground_g = 25;
  foreground_b = 120;
  ADD_TO_RAMP (AAPURPLE); /* AAWHITE */
  foreground_r = 225;
  foreground_g = 235;
  foreground_b = 255;
  ADD_TO_RAMP (AAWHITE);
  mapcolors (0, 127, rramp, gramp, bramp);
}

#define RRRANDO ( (float) (400*rand()/32767))

/* This routine draws a background of random polygons.
 * One of five colors are selected randomly.  */

void draw_foreground()
{
  int i, j;

  color (AABLACK);
  clear();
  linesmooth(TRUE);
  translate (-100.0, -100.0, 0.0);
  for (j=0; j<9; j++) {
    color (16*(5*rand()/32767));
    pushmatrix();
    translate ( RRRANDO, RRRANDO, 0.0);
    draw_fan();
    popmatrix();
  }

}

main ()
{
  int i, j, k, iinc=1, jinc=3, kinc=7;

  prefsize (400.0, 400.0);
  winopen ("antialiased lines");
  cmode();
  singlebuffer();
  gconfig();
  save_cmap();
  /* create the color ramp--the background color will be black */
  load_background_cmap (0, 0, 0);
  /* draw multicolored line drawing */
  draw_foreground();
  /* loop until the right mouse button is depressed */
  while (!getbutton(RIGHTMOUSE) && !getbutton(ESCKEY)) {
    /* cycle through a random set of background colors */
    i+=iinc;
    j+=jinc;
    k+=kinc;
    if ((i+iinc>5*255) || (i+iinc<0)) {
      iinc = -iinc;
      i+=iinc;
    }

    if ((j+jinc>5*255) || (j+jinc<0)) {
      jinc = -jinc;
      j+=jinc;
    }

    if ((k+kinc>5*255) || (k+kinc<0)) {
      kinc = -kinc;
      k+=kinc;
    }

    if ( i%5 == 0) {
      load_background_cmap ( (i/5)%255, (j/5)%255,(k/5)%255);
    }
  }
  restore_cmap();

}

/*This saves the colormap*/
#define lo_end 0
#define hi_end 255
short *CarrayR, *CarrayG, *CarrayB;

save_cmap()
{
  CarrayR = calloc (lo_end+hi_end,sizeof(short));
  CarrayG = calloc (lo_end+hi_end,sizeof(short));
  CarrayB = calloc (lo_end+hi_end,sizeof(short));

  getmcolors ((Int16 const)lo_end,(Int16 const)hi_end,
     CarrayR, CarrayG, CarrayB);
}

/*This restores the colormap*/
restore_cmap()
{
  mapcolors ((Int16 const)lo_end,(Int16 const)hi_end,
     CarrayR, CarrayG, CarrayB);
}

/*
  Changes:
    -  Added the restoring of the colormap
    -  Added escape key and right mouse exiting cleanly
*/

Related Information

The linesmooth subroutine.

Pixel Coverage in GL3.2 Version 4 for AIX: Programming Concepts.