Order independent transparency (Front to back depth peeling)

I had been looking through a lot of docs on order independent transparency. I started off with http://developer.nvidia.com/object/order_independent_transparency.html
also called Front to Back Depth Peeling.
This is how it works.

You need three fbos, two fbos for depth sorting and one for final rendering. You need five fbo attachements in all, two for depth and two for front/back fragments and one texture for the final output. These are attached to the fbo as given in the setup code:

 
void setup_FBO() {
glGenFramebuffersEXT(2, fbo); 
glGenTextures (2, texID);
glGenTextures (2, depthTexID);
for(int i=0;i<2;i++) {
  glBindTexture(GL_TEXTURE_RECTANGLE_ARB, depthTexID[i]);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_MAG_FILTER, GL_NEAREST);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_MIN_FILTER, GL_NEAREST);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_WRAP_S, GL_CLAMP);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_WRAP_T, GL_CLAMP); 
  glTexImage2D(GL_TEXTURE_RECTANGLE_ARB , 0,GL_DEPTH_COMPONENT32F_NV, width, height, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);

  glBindTexture(GL_TEXTURE_RECTANGLE_ARB,texID[i]);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_MAG_FILTER, GL_NEAREST);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_MIN_FILTER, GL_NEAREST);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_WRAP_S, GL_CLAMP);
  glTexParameteri(GL_TEXTURE_RECTANGLE_ARB , GL_TEXTURE_WRAP_T, GL_CLAMP);
  glTexImage2D(GL_TEXTURE_RECTANGLE_ARB , 0,GL_RGBA, width, height, 0, GL_RGBA, GL_FLOAT, NULL);

  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo[i]);
  glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT,     GL_TEXTURE_RECTANGLE_ARB, depthTexID[i], 0);
  glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_ARB, texID[i], 0);
}
 
glGenTextures(1, &colorBlenderID);
glBindTexture(GL_TEXTURE_RECTANGLE_ARB, colorBlenderID);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_RECTANGLE_ARB, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_FLOAT, 0);

glGenFramebuffersEXT(1, &colorBlenderFBOID);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, colorBlenderFBOID);
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_TEXTURE_RECTANGLE_ARB, depthTexID[0], 0);
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_RECTANGLE_ARB, colorBlenderID, 0);

GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if(status == GL_FRAMEBUFFER_COMPLETE_EXT )
 printf("OK for render to texture\n");
else
 printf("Problem");

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
}

Next, the rendering code uses this information as follows

//matrix setup and other stuff
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, colorBlenderFBOID);
glDrawBuffer(g_drawBuffers[0]);
glClearColor(0, 0, 0, 1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
//In the first pass, we render normally with depth test enabled to get the nearest surface
glEnable(GL_DEPTH_TEST);
g_shaderFrontInit.bind();
g_shaderFrontInit.setUniform("Alpha", (float*)&g_opacity, 1);
  RenderScene(angle); //do your rendering here
g_shaderFrontInit.unbind();
// ---------------------------------------------------------------------
// 2. Depth Peeling + Blending
// ---------------------------------------------------------------------
int numLayers = (NUM_PASSES - 1) * 2;
for (int layer = 1; g_useOQ || layer < numLayers; layer++) {
  int currId = layer % 2;
  int prevId = 1 - currId;

  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo[currId]);
  glDrawBuffer(g_drawBuffers[0]);
  glClearColor(0, 0, 0, 0);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glDisable(GL_BLEND);
  glEnable(GL_DEPTH_TEST);
  if (g_useOQ) {
     glBeginQuery(GL_SAMPLES_PASSED_ARB, g_queryId);
  }
  g_shaderFrontPeel.bind();
  g_shaderFrontPeel.bindTextureRECT("DepthTex", depthTexID[prevId], 0);
  g_shaderFrontPeel.setUniform("Alpha", (float*)&g_opacity, 1);
     RenderScene(angle);
  g_shaderFrontPeel.unbind();

  if (g_useOQ) {
    glEndQuery(GL_SAMPLES_PASSED_ARB);
  }
  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, colorBlenderFBOID);
  glDrawBuffer(g_drawBuffers[0]);
  glDisable(GL_DEPTH_TEST);
  glEnable(GL_BLEND);
  glBlendEquation(GL_FUNC_ADD);
  glBlendFuncSeparate(GL_DST_ALPHA, GL_ONE,GL_ZERO, GL_ONE_MINUS_SRC_ALPHA);
  g_shaderFrontBlend.bind();
  g_shaderFrontBlend.bindTextureRECT("TempTex", texID[currId], 0);
    DrawFullScreenQuad();
  g_shaderFrontBlend.unbind();
  glDisable(GL_BLEND);
  if (g_useOQ) {
   GLuint sample_count;
   glGetQueryObjectuiv(g_queryId, GL_QUERY_RESULT_ARB, &sample_count);
   if (sample_count == 0) {
 break;
   }
  }
 }

// ---------------------------------------------------------------------
// 3. Final Pass
// ---------------------------------------------------------------------
//remove the render to texture and set the back buffer as render target 
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
glDrawBuffer(GL_BACK);
glDisable(GL_DEPTH_TEST); 
glDisable(GL_BLEND);

//attach the blending shader
g_shaderFrontFinal.bind();
g_shaderFrontFinal.setUniform("BackgroundColor", g_backgroundColor, 3);
g_shaderFrontFinal.bindTextureRECT("ColorTex", colorBlenderID, 0);
   DrawFullScreenQuad();
g_shaderFrontFinal.unbind();

Thats it, this gives the following output.


Next, I will post infor on how to do dual depth peeling in opengl