Infinite shadow projection

shadow.c

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

#include <GLUT/glut.h>

int window_width = 500;
int window_height = 500;

int mousex = 0;
int mousey = 0;

void reshape(int width, int height) {
  glViewport (0, 0, (GLsizei) width, (GLsizei) height);

  glMatrixMode (GL_PROJECTION);
  glLoadIdentity();
  gluOrtho2D(0, width, 0, height);

  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  
  window_width = width;
  window_height = height;
}

void shadowVert(GLfloat x, GLfloat y){
	glVertex3f(x, y, 1.0f);
	glVertex3f(x, y, 0.0f); 
}

void drawLight(GLfloat x, GLfloat y, GLfloat d, GLfloat r, GLfloat g, GLfloat b){
	glDisable(GL_BLEND);
  glColor4f(0.0f, 0.0f, 0.0f, 0.0f);
	glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);
	
	// Render shadow mask
  glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
  glClear(GL_COLOR_BUFFER_BIT);
	
	{ // set up the shadow matrix
		GLfloat mat[16] = {
			1.0f, 0.0f, 0.0f, 0.0f,
			0.0f, 1.0f, 0.0f, 0.0f,
			   x,    y, 0.0f, 1.0f,
			  -x,   -y, 0.0f, 0.0f,
		};
		
		glLoadMatrixf(mat);
	}
	
	// draw some shadow segments
  glBegin(GL_QUAD_STRIP); {
  	shadowVert(100, 200);
  	shadowVert(200, 200);
  	shadowVert(200, 100);
  } glEnd();
	
  glBegin(GL_QUAD_STRIP); {
  	shadowVert( 50, 150);
  	shadowVert(150,  50);
  } glEnd();
	
  glBegin(GL_QUAD_STRIP); {
  	shadowVert(300, 400);
  	shadowVert(400, 300);
  } glEnd();
	
	// render light map
	glEnable(GL_BLEND);
	glBlendFunc(GL_DST_ALPHA, GL_ONE);
  glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
	glLoadIdentity();
	
	glTranslatef(x, y, 0.0f);
  glBegin(GL_TRIANGLE_FAN); {
	  glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
  	glVertex2f(0.0f, 0.0f);

	  glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
  	for(int i=0; i<=30; i++){
  		GLfloat angle = 2.0f*M_PI*(GLfloat)i/30.0f;
  		glVertex2f(cosf(angle)*d, sinf(angle)*d);
  	}
  } glEnd();
}

void draw() {
  glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
  glClear(GL_COLOR_BUFFER_BIT);
	glMatrixMode(GL_MODELVIEW);
	
	drawLight(mousex, mousey, 300.0f, 1.0f, 1.0f, 1.0f);
	drawLight(250.0f, 250.0f, 200.0f, 0.8f, 0.8f, 0.8f);

  glutSwapBuffers();

	int err;
	while(err = glGetError())
		printf("%s\n", gluErrorString(err));
}

void mouse(int x, int y){
	mousex = x;
	mousey = window_height - y;
	
	glutPostRedisplay();
}

void initGL(int width, int height) {
  reshape(width, height);
}

int main(int argc, char** argv) {	  
  glutInit(&argc, argv);

  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
  glutInitWindowSize(window_width, window_height);
  glutCreateWindow("GLUT");
  glutReshapeFunc(reshape);
  glutDisplayFunc(draw);
  
  glutPassiveMotionFunc(mouse);

  initGL(window_width, window_height);

  glutMainLoop();
  return 0;
}