View Full Version : How to check the visibility of a box

03-12-2002, 11:03 AM

I'm new on 3D graphics, so I'm yet understanding all that stuff.

As far as I learned, I prefer octrees (for an easy view frustum culling). I feel comfortable with that structure (BSP is a bit confusing for me).

Now I'm thinking on how to minimize overdraws, so I was thinking this way:

1 Check what octrees comes inside frustum.
2 Push those boxes onto a stack sorted by distance.
3 Start drawing from front to back. I'd start with the box where the camera lies.
4 Pop the nearest box from the stack.
Before drawing polys from a box, check his visibility(not draw it). If the box is not visible at all(i.e. completely occluded), I don't draw the polys inside it.
5 Remove that box from the stack.
6 Check if the entire screen is drawn. If yes, we stop the render.
7 Goto 4.

This way, I supposely don't need PVS (render stops when all the scene is draw).

My questions are:
+It's possible to do that?
+That strategy will be fast enough?
-It's better a PVS?
+Is there any OpenGL method to do points 4 & 6 ?
Coding point 4 maybe can take much CPU time. Point 6 is not critical.

Thanks in advance.

03-12-2002, 11:24 PM
yes you are right - points 4 & 6 could be very difficult...

6 is easier because you can just count "new" pixels drawn to the screen buffer (assuming whole screen has to be filled and there's no mistake) - anyway i don't find it a good approach

and point 4 is possible using ray tracing along edges of already drawn boxes and BSP or your octree but it's not easy and you could end up in 0(n*n) or even more complex algorithm...

>+Is there any OpenGL method to do points 4 & 6 ?
don't think of OpenGL like of magic box being able to do everything - then it wouldn't be as good as it is

try to get familiar with PVS, portals, sectors etc, which is used by all professional games where speed is very important


03-13-2002, 01:36 AM
I found on Delphi3D a demo with something like I thought.
It's with the GL_HP_occlusion_test. But it's only for NVidia cards http://www.opengl.org/discussion_boards/ubb/frown.gif

If anyone have used that, it improves the rendering speed? How much?