platelocal.c Example C Language Program

/*
This example program uses a blue local light to
illuminate a white flat plate.  By removing the
definition of FIXED_LIGHT in the first line of
the program, the light source will maintain its
position relative to the plate.  This
demonstrates an important concept:  the position
of a light source (or direction if using an
infinite light source) is transformed by the
current transformation matrix at the time it is
bound.

Try changing the local light to an infinite
light and run the program again.  Notice how the
color across the plate is now constant at any
given instant.  Since the plate surface material
has no specular reflectance, we did not use a
local viewer (remember diffuse reflection is
independent of viewer position).
*/

/* Example C Language Program platelocal.c */
/*
This program draws a flat plate with a simple
local light.  If the line at the top of the file
is left in, the light is fixed, and the plate
moves.  Thus the bright spot on the plate will
appear to move around (on the plate).
Sometimes, the plate gets in front of the light,
and almost disappears, since only the back is
lit.  It does not quite disappear, since there
is a small ambient component for the default
material.

If the statement "#define FIXED_LIGHT" is
deleted, the light is effectively attached to
the moving plate, so the lighted portion of the
plate moves with the plate.
*/

#define FIXED_LIGHT 
#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  white_material[] =   {
  DIFFUSE,  1.0, 1.0, 1.0,
  SPECULAR, 0.0, 0.0, 0.0,
  LMNULL};

float  local_blue_light[] = {
  LCOLOR, 0.0, 0.0, 1.0,
  POSITION, 0.5, 0.5, 0.1, 1.0,
  LMNULL};

/*
** draw_plate draws a flat plate covering
** the range -1.0 <= x, y <= 1.0 and z = 0.0
** 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,
** the lighting calculation includes a LOCAL light.
*/
def_light_calc()
{
  lmdef(DEFLMODEL, 1, 0, NULL);
  lmdef(DEFMATERIAL, 1, 9, white_material);
  lmdef(DEFLIGHT, 1, 10, local_blue_light);
}

/*
** Tell the Graphics Library to USE the lighting
** calculation that we defined earlier.
*/
use_light_calc()
{
  lmbind(LMODEL, 1);
  lmbind(LIGHT0, 1);
  lmbind(MATERIAL, 1);
}

main()
{
  int i;    
  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();    
  for (i = 0; i < 1800; i++) {
    cpack(0);
    clear();
    pushmatrix();
    rot(i*0.5, 'Z');
    rot(i*0.5, 'Y');

    #ifndef FIXED_LIGHT
    lmbind(LIGHT0, 1);

    #endif FIXED_LIGHT
    draw_plate(10);
    popmatrix();
    swapbuffers();
  }

}

Related Information

Advanced Lighting Capabilities in GL3.2 Version 4 for AIX: Programming Concepts describes how to manipulate material emission, ambient light, run-time lighting capabilities (Imcolor Subroutine), local viewer, local lights, and light attenuation.