Question

I’m trying to perform hidden line removal using polygon offset fill. The code works perfectly if I render directly to the window buffer but fails to draw the lines when passed through a FBO as shown below

The code I use to draw the objects

void drawCubes (GLboolean removeHiddenLines)
{
  glLineWidth(2.0);
  glPushMatrix();
    camera.ApplyCameraTransform();
    for(int i = 0; i < 50; i ++){
    glPushMatrix();
      cube[i].updatePerspective();
      glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
      glColor3f(1.0,1.0,1.0);
      cube[i].draw();
      glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);

      if(removeHiddenLines){
          glEnable(GL_POLYGON_OFFSET_FILL);
          glPolygonOffset(1.0, 1.0);
          glColor3f(1.0, 0.0, 0.0);  //fill polygons for hidden line removal
          cube[i].draw();
          glDisable(GL_POLYGON_OFFSET_FILL);
      }
    glPopMatrix();
  }
  glPopMatrix();
}

For this example, the first pass involves rendering to both the window buffer and a FBO.

void firstPass()
{
  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  glViewport(0, 0, fboWidth, fboHeight);

  glEnable(GL_DEPTH_TEST);
  glDisable(GL_TEXTURE_2D);
  drawParticleView(GL_TRUE);
  glDisable(GL_DEPTH_TEST);

  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebufferID[1]);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  glViewport(0, 0, fboWidth, fboHeight);

  glEnable(GL_DEPTH_TEST);
  glDisable(GL_TEXTURE_2D);
  drawParticleView(GL_TRUE);
  glDisable(GL_DEPTH_TEST);
}

Second pass renders FBO back to window buffer.

void secondPass()
{
  glEnable(GL_TEXTURE_2D);

  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
  glBindTexture(GL_TEXTURE_2D, renderTextureID[0]);

  glViewport(fboWidth, 0, fboWidth, fboHeight);
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();

  glBegin(GL_QUADS);
    glTexCoord2i(0, 0);
    glVertex2f(-1.0f, -1.0f);
    glTexCoord2i(1, 0);
    glVertex2f(1.0f, -1.0f);
    glTexCoord2i(1, 1);
    glVertex2f(1.0f, 1.0f);
    glTexCoord2i(0, 1);
    glVertex2f(-1.0f, 1.0f);
  glEnd();

  glDisable(GL_TEXTURE_2D);
}

Setup FBO’s

void setupRC()
{
  setupTextures();
  glGenFramebuffersEXT(2, framebufferID);

  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebufferID[0]);
  glGenRenderbuffersEXT(1, &renderbufferID);
  glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, renderbufferID);
  glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT32, fboWidth, fboHeight);
  glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, renderbufferID);
  glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, renderTextureID[0], 0);
  GLenum fboStatus = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
  if(fboStatus != GL_FRAMEBUFFER_COMPLETE_EXT){
    fprintf(stderr, "FBO #1 Error!");
  }
  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebufferID[1]);
  glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, renderTextureID[1], 0);
  fboStatus = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
  if(fboStatus != GL_FRAMEBUFFER_COMPLETE_EXT){
     fprintf(stderr, "FBO #2 Error!");
  }
  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
}

Setup textures

void setupTextures(void)
{
  glGenTextures(2, renderTextureID);

  for (GLint i = 0; i < 2; i++){
    glBindTexture(GL_TEXTURE_2D, renderTextureID[i]);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    // this may change with window size changes
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, fboWidth, fboHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
  }
  glBindTexture(GL_TEXTURE_2D, 0);
}

I don’t understand why the two views wouldn’t be the same? Am I missing something (obviously I am)?
Thanks

Answer 1

The problem is that I was rendering to a FBO without a depth attachment. Setting up the second FBO the same as the first gave the correct results.