gamma.c Example C Language Program for GL

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

/* Example:  gammaramp.c */

/*
 * This example program shows the use of gamma ramps.  Gamma ramps
 * are useful for correcting the appeareance of photographs
 * (images) and antialiased lines (to correct for monitor
 * phosphors), and for implementing color-map tricks.
 *
 * Pressing PF7 or PF8 changes the gamma correction factor and 
 * loads the new gamma ramp.  A window shows the current gamma 
 * factor and a black-to-white scale.
 */

/* ----------------------------------------------------------- */

/*
 * This routine prints out the current value of gamma in a window.
 * At the bottom of the window is a color bar.  
 */

void show_val (float gam)
{

   char gamstr[32];   
   cpack (0x0);         /* black */
   clear();
   cpack (0xff);        /* red */
   cmov2 (10, 80);
   charstr ("Currently, the gamma correction factor is ");
   sprintf (gamstr, "%3.2f", gam);
   charstr (gamstr);   
   cmov2 (10, 60);
   charstr ("To lower gamma, press PF7");
   cmov2 (10, 40);
   charstr ("To raise gamma, press PF8");   /* draw a sample color bar */
   cpack (0x0);         /* black */
   pmv2 (10, 10);
   pdr2 (10, 25);
   cpack (0xffffff);    /* white */
   pdr2 (390, 25);
   pdr2 (390, 10);
   pclos ();

}

/* ----------------------------------------------------------- */

/* 
 * The following routine updates the adapter gamma ramps 
 * based on the value of gamma passed to it.  Note that the 
 * entire screen is affected (because there is only one set 
 * of ramps for the entire screen).
 */
void update_ramps (float gam) {
   short rramp[256], gramp[256], bramp[256];
   int i;
   double corr, og;   /* compute the new gamma ramp */
   og = 1.0 / (gam + 0.00001);
   corr = 255.0 * pow ((double) (1.0 / 255.0), og);
   for (i=0; i<256; i++) {
      rramp[i] = gramp[i] = bramp[i] = (short) (corr * pow((float) (i), og));
   }
   /* load the new gamma ramp */
   gammaramp (rramp, gramp, bramp);
}

/* ----------------------------------------------------------- */

main() {
   float gamma = 1.0;
   float delta = 0.05;
   short dev, val;   
   prefsize (400, 100);
   minsize (400, 100);
   maxsize (400, 100);         /* disable resizing of window */
   winopen ("gamma-ramp demo");
   RGBmode ();                 /* gamma ramps work in color map mode too! */
   gconfig ();   
   update_ramps (gamma);       /* initialize the gamma ramps */
   show_val (gamma);           /* print current value of gamma */
   qdevice (RIGHTMOUSE);       /* queue up desired devices */
   qdevice (ESCKEY);
   qdevice (F7KEY);
   qdevice (F8KEY);   
   while (TRUE) {
      dev = qread (&val);
      switch (dev) {         
      case ESCKEY:
      case RIGHTMOUSE:
         exit(0);         
      case F7KEY:
         if (val == FALSE) break;  /* do nothing for key release */
         gamma += delta;
         update_ramps (gamma);
         show_val (gamma);
         break;         
      case F8KEY:
         if (val == FALSE) break;  /* do nothing for key release */
         gamma -= delta;
         update_ramps (gamma);
         show_val (gamma);
         break;         
      case REDRAW:
         show_val (gamma);         
      default:
         break;
      }
   }

}

Related Information

Pixel Coverage in GL3.2 Version 4 for AIX: Programming Concepts describes aspects of antialiasing (smoothing lines for screen display) related to color variation in the pixels.