OpenGL screencast 19: blending

019.c

#include "screencasts.h"

/*
 *  main()
 *  ----
 *  Start up GLUT and tell it what to do
 */
int main(int argc,char* argv[])
{
  initializeGlobals();
  /* screencast specific variables */
  windowName = "OpenGL screenscasts 19: Blending";
  screencastID = 19;
  toggleAxes = 0;
  toggleAnimation = 0;

  glutInit(&argc,argv);
  glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
  glutInitWindowSize(windowWidth,windowHeight);
  glutInitWindowPosition(450,350);
  glutCreateWindow(windowName);

  glutDisplayFunc(display);
  glutReshapeFunc(displayReshape);
  glutKeyboardFunc(windowKey);
  glutSpecialFunc(windowSpecial);

  initializeTextures();
  initializeObjs();
  timer(toggleAnimation);

  redisplayAll();
  glutMainLoop();
  return 0;
}

draw.c

/*
 * drawScene()
 * ------
 * Draws the scene with everything we need in it
 */
void drawScene(void)
{
  int i,j,k;
  drawAxes();
  drawParameters();
  
  currentTexture = textures[TEX_DEFAULT];
  drawLight();

  opaque = 1;
  for (i = -1; i <= 1; i += 2)
    for (j = -1; j <= 1; j += 2)
      for (k = -1; k <= 1; k += 2)
	if (((i+j+k)&2)==0) {
	  cube(i,j,k,1,1,1,0);
	}
  
  currentTexture = textures[TEX_STAINGLASS];
  opaque = 0;
  for (i = -1; i <= 1; i += 2)
    for (j = -1; j <= 1; j += 2)
      for (k = -1; k <= 1; k += 2)
	if (((i+j+k)&2)!=0) {
	  cube(i,j,k,1,1,1,0);
	}

  currentTexture = textures[TEX_DEFAULT];
  opaque = 1;
}

shapes.c

/*
 *  cube
 *  ------
 *  Draw a cube
 *     at (x,y,z)
 *     dimensions (dx,dy,dz)
 *     rotated th about the y axis
 *  TODO: Refactor this!!!
 */
void cube(double x,double y,double z,
	  double dx,double dy,double dz,
	  double th)
{
  /*  Cube vertices */
  GLfloat vertA[3] = { 0.5, 0.5, 0.5};
  GLfloat vertB[3] = {-0.5, 0.5, 0.5};
  GLfloat vertC[3] = {-0.5,-0.5, 0.5};
  GLfloat vertD[3] = { 0.5,-0.5, 0.5};
  GLfloat vertE[3] = { 0.5, 0.5,-0.5};
  GLfloat vertF[3] = {-0.5, 0.5,-0.5};
  GLfloat vertG[3] = {-0.5,-0.5,-0.5};
  GLfloat vertH[3] = { 0.5,-0.5,-0.5};

  glPushMatrix();
  /*  Transform */
  glTranslated(x,y,z);
  glRotated(th,0,1,0);
  glScaled(dx,dy,dz);

  if (!opaque) {
    glEnable(GL_BLEND);  // remember to enable/disable blending
    glColor4f(1,1,1,alpha);
    if (aone)
      // glBlendFunc(SOURCE, DESTINATION)
      // Controls how color values in the fragment being processed (source) are
      // combined with those already in the framebuffer (destination)
      // See Table 6-1 in the Red book
      //   Older book version here, but same table: http://glprogramming.com/red/chapter06.html

      // Constants available
      // GL_ZERO, GL_ONE, GL_DST_COLOR, GL_SRC_COLOR, GL_ONE_MINUS_DST_COLOR, GL_ONE_MINUS_SRC_COLOR
      // GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, GL_SRC_ALPHA_SATURATE
      glBlendFunc(GL_SRC_ALPHA,GL_ONE);
    else
      glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
    glDepthMask(GL_FALSE); // make the depth buffer read-only

    glEnable(GL_TEXTURE_2D);
    /* using the current texture */
    glBindTexture(GL_TEXTURE_2D,currentTexture);
  }

  /* Cube */
  /* front => ABCD */
  glBegin(GL_QUADS);
  if (opaque) glColor3f(1,0,0);
  glNormal3f(0,0,1);
  glTexCoord2f(0,0); glVertex3fv(vertC);
  glTexCoord2f(1,0); glVertex3fv(vertD);
  glTexCoord2f(1,1); glVertex3fv(vertA);
  glTexCoord2f(0,1); glVertex3fv(vertB);
  glEnd();

  /* back => FEHG */
  glBegin(GL_QUADS);
  if (opaque) glColor3f(1,1,0);
  glNormal3f(0,0,-1);
  glTexCoord2f(0,0); glVertex3fv(vertH);
  glTexCoord2f(1,0); glVertex3fv(vertG);
  glTexCoord2f(1,1); glVertex3fv(vertF);
  glTexCoord2f(0,1); glVertex3fv(vertE);
  glEnd();
  
  /* right => EADH */
  glBegin(GL_QUADS);
  if (opaque) glColor3f(1,0,1);
  glNormal3f(1,0,0);
  glTexCoord2f(0,0); glVertex3fv(vertD);
  glTexCoord2f(1,0); glVertex3fv(vertH);
  glTexCoord2f(1,1); glVertex3fv(vertE);
  glTexCoord2f(0,1); glVertex3fv(vertA);
  glEnd();

  /* left => BFGC */
  glBegin(GL_QUADS);
  if (opaque) glColor3f(0,1,0);
  glNormal3f(-1,0,0);
  glTexCoord2f(0,0); glVertex3fv(vertG);
  glTexCoord2f(1,0); glVertex3fv(vertC);
  glTexCoord2f(1,1); glVertex3fv(vertB);
  glTexCoord2f(0,1); glVertex3fv(vertF);
  glEnd();

  /* top => EFBA */
  glBegin(GL_QUADS);
  if (opaque) glColor3f(0,0,1);
  glNormal3f(0,1,0);
  glTexCoord2f(0,0); glVertex3fv(vertB);
  glTexCoord2f(1,0); glVertex3fv(vertA);
  glTexCoord2f(1,1); glVertex3fv(vertE);
  glTexCoord2f(0,1); glVertex3fv(vertF);
  glEnd();

  /* bottom => DCGH */
  glBegin(GL_QUADS);
  if (opaque) glColor3f(0,1,1);
  glNormal3f(0,-1,0);
  glTexCoord2f(0,0); glVertex3fv(vertD);
  glTexCoord2f(1,0); glVertex3fv(vertC);
  glTexCoord2f(1,1); glVertex3fv(vertG);
  glTexCoord2f(0,1); glVertex3fv(vertH);
  glEnd();

  glPopMatrix();
  if (!opaque) {
    glDisable(GL_BLEND);
    glDepthMask(GL_TRUE);
    glDisable(GL_TEXTURE_2D);
  }
}