localatten.c Example C Language Program

/*
This program demonstrates the effect of light attenuation
by continuously moving a local light toward a flat plate.

It draws a flat green plate at z = 0; -1.0 > x, y > 1.0.
The eye is 6 units above, looking down.  A
light bounces up and down in the range 0.1 > z > 1.5, and
x = y = 0.  The lighting model attenuates intensity with
distance, so the center of the plate gets brighter as
the light moves closer.  The character string printed at
the lower left of the plate shows the height of the
light.
Note that the color is set after the cmov()
command -- the cmov() actually sends a vertex through the
transformation, and it will set the current color.  
If you move the cpack() command just above the cmov()
command, the character string will be lighted and will
appear in varying shades of green.
*/

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

Matrix idmat = {
   1.0,0.0,0.0,0.0, 
   0.0,1.0,0.0,0.0, 
   0.0,0.0,1.0,0.0, 
   0.0,0.0,0.0,1.0};

float  green_material[] = {
   DIFFUSE,  0.0, 1.0, 0.0,
   LMNULL};

float  local_white_light[] = {
   LCOLOR, 1.0, 1.0, 1.0,
   POSITION, 0.0, 0.0, 1.0, 1.0,
   LMNULL};

float  light_model[] = {
   AMBIENT, 0.0, 0.0, 0.0,
   LOCALVIEWER, 0.0,
   ATTENUATION, 1.0, 1.0,
   LMNULL};

/*
** draw_plate draws a flat plate covering the
** range -1.0 <= x, y <= 1.0 and z = 0. using
** n^2 rectangles.  All the normal vectors are
** perpendicular to the plate.
*/
draw_plate(n)
long n;
{
   long i, j;
   float p0[3], p1[3], p2[3], p3[3];
   float n0[3];    

   n0[0] = n0[1] = 0.0;
   n0[2] = 1.0;
   p0[2] = p1[2] = p2[2] = p3[2] = 0.0;    
   for (i = 0; i < n; i++) {
      p0[0] = p1[0] = -1.0 + 2.0*i/n;
      p2[0] = p3[0] = -1.0 + 2.0*(i+1)/n;
      for (j = 0; j < n; j++) {
         p0[1] = p3[1] = -1.0 + 2.0*j/n;
         p1[1] = p2[1] = -1.0 + 2.0*(j+1)/n;
         bgnpolygon();
         n3f(n0); 
         v3f(p0);
         n3f(n0); 
         v3f(p1);
         n3f(n0); 
         v3f(p2);
         n3f(n0); 
         v3f(p3);
         endpolygon();
      }
   }

}

/*
** Tell the Graphics Library to DEFINE a
** lighting calculation that accounts for
** diffuse and ambient reflection.  In
** addition, this lighting calculation
** includes a local light whose emitted
** light is attenuated as a function of
** distance from the object.
*/
def_light_calc()
{
   lmdef(DEFLMODEL, 1, 10, light_model);
   lmdef(DEFMATERIAL, 1, 5, green_material);
   lmdef(DEFLIGHT, 1, 10, local_white_light);
}

/*
** Tell the Graphics Library to USE the lighting
** calculation that we defined earlier.
*/
use_light_calc()
{
   lmbind(LMODEL, 1);
   lmbind(LIGHT1, 1);
   lmbind(MATERIAL, 1);
}

main()
{
   float dist;
   long flag = 1;
   char str[32];    

   keepaspect(1, 1);
   prefposition(XMAXSCREEN/4,XMAXSCREEN*3/4,YMAXSCREEN/4,
       YMAXSCREEN*3/4);
   winopen("local");
   RGBmode();
   doublebuffer();
   gconfig();

   /*
    ** Use mmode() to set up projection and
    ** viewing matrices for lighting.
     */
   mmode(MVIEWING);
   perspective(400, 1.0, 0.5, 10.0);
   loadmatrix(idmat);
   lookat(0.0,0.0,6.0,0.0,0.0,0.0,0);    
   def_light_calc();
   use_light_calc();    
   dist = 1.0;    
   while (TRUE) {
      if (flag) {
         dist += .01;
         if (dist > 1.5) flag = 1 - flag;
      }
      else {
         dist -= .01;
         if (dist < 0.1) flag = 1 - flag;
      }

      cpack(0);
      clear();
      sprintf(str, "Light Distance: %1.2f", dist);
      cmov2(-1.5, -1.5);
      cpack(0xffffff);
      charstr(str);
      pushmatrix();

             /*
            ** Change the position of the local light
            ** by REDEFINING and REBINDING the light.
            ** Repositioning the light changes the
            ** illumination of the plate for two reasons:
            ** 1) the affect of attenuation, and 
            ** 2) the light direction vector from a
            ** vertex on the plate to the repositioned
            ** light source has changed.
            */
      local_white_light[7] = dist;
      lmdef(DEFLIGHT, 1, 10, local_white_light);
      lmbind(LIGHT1, 1);
      draw_plate(20);
      popmatrix();
      swapbuffers();
   }

}

Related Information

The cpack subroutine , loadmatrix subroutine , RGBmode subroutine .