Depth Buffer Reading

OpenGL OpenGL: Advanced Coding
1

Hello all

I’m implementing the dynamic LOD algorithm Occluder Shadows, which consists of reading back from the depth buffer (on my Geforce II). I’m currently using glReadPixels(), but it’s very slow. Is there a faster way of reading back from the depth buffer, or would I be better to use Software Rasteraztion instead.

thnx

Lloyd

2

As far as I know, glReadPixels is the only way to access the depth buffer…

Sorry !

I do not know what the algorithm you are using involves but what do you exactly do when reading the buffer ?

  1. Do you read the whole buffer at once and then work on it ?
  2. Do you read parts of the buffer several time ?

If you are doing 2), perhaps you should try to read the whole buffer only once as described in 1)… It should be faster than multiple “mini”-glReadPixels (on this one, nVidia guys will answer better than me !).

If you need to read it at different stages of your rendering process, then I guess you have to stick to multiple “mini”-reads…

Now, if you are already reading the whole buffer at once and it is too slow, I have no idea…

I don’t think switching to software mode would help: you do not have direct access to the depth buffer either if you use Microsoft GDI Generic implementation (although I am sure there is a dirty hack to find in this case !).

Best regards.

Eric

3

How slow is very slow? That should actually run at a reasonable speed (tens of megapixels per second). Make sure you’re reading back into a fixed-point format, though. Floats will introduce extra conversions.

  • Matt
4

In the game I am working on we used glReadPixels to check depth buffer values for determining visibility of certain things in screen-space coordinates. Certain hardware handles this nicely (read: very little speed hit), nvidia hardware for example. Other hardware takes a massive speed hit for reading from the back buffer (3Dfx comes to mind). This is definiately a feature of your app that is definately going to vary greatly across hardware.

5

sorry for the incoherency in my post, I just read it and it shows how sleep deprived I am feeling right now, heh.

6

You could also use the accumulation buffer to grab the depth buffer which might be faster but I don’t know whether this would help with what you are doing or not