Z-Buffer FAQ?

[Jimmers]

Is there a good explanation of z-buffer anywhere on the net?

If not, here's what I'm asking about. I use RadLinker to get program specific driver settings. There's a pull down menu titled "Z-Buffer Depth", and it has the following options.
- 16
- 16,24
- 16,24;8 bit stencil
- 16,24,32
- 16,24,32;8 bit stencil
What do there different options do? Are these referring to FP16, FP24, FP32, etc?

 

[K.I.L.E.R]

Depth precision = Z/W-Buffer.

Radeon's don't have a W-Buffer though.

It has nothing to do with color precision.

Stencil buffer is used for shadows/reflections.

Some games have an option for 24bZ/32bZ.
24b Z Buffer is the highest accuracy you can achieve, the extra 8-bits are for the stencil buffer.

That doesn't apply for all cards, eventually we will see a 64b Z-Buffer + 24b stencil.

I'm just assuming you have an R3xx/X8xx.

 

[j^aws]

A link to Z-buffer

Edit:

24b Z Buffer is the highest accuracy you can achieve, the extra 8-bits are for the stencil buffer.

For what tech/cards are you referring to for 24 bit the highest accuracy? :?

 

[KimB]

The short of it:
A z-buffer is a buffer that stores the depth (z) value of each pixel on the screen. This allows triangles to be sent to the GPU in any order, with those behind already-rendered triangles being thrown out, and those in front being rendered, updating the depth stored in the z-buffer at the same time.

The different numbers associated with the z-buffer are precision values. More precision means objects can get closer before the hardware has a hard time figuring out which one is in front. When they get too close for the chosen precision, you'll typically get flickering. Ever seen a decal (explosion blast or blood) in UT or some other game flicker? Particularly when viewed from a large distance (i.e. through a sniper rifle sight)? That's a z-buffer error.

The W-buffer basically acts like a z-buffer, but has a different spread of where the errors occur. Stencil buffers are typically connected to the z-buffer, and can be used for any number of effects. They have the same characteristics as a z-buffer (i.e. they can be used to accept or reject pixels on a per-pixel basis), but can be modified in many ways. One way is to render shadows, where the stencil buffer is used to determine whether a pixel is in shadow or not.

 

[K.I.L.E.R]

A link to Z-buffer

Edit:

24b Z Buffer is the highest accuracy you can achieve, the extra 8-bits are for the stencil buffer.

For what tech/cards are you referring to for 24 bit the highest accuracy? :?

"I'm just assuming you have an R3xx/X8xx."

 

[KimB]

I'm pretty sure no consumer card currently supports higher than a 24-bit z-buffer. I do know that 3DLabs supports a 32-bit z-buffer, but consumer cards elect to pack the z- and stencil-buffers into one to save performance, which limits the possible precision.

 

[j^aws]

A link to Z-buffer

Edit:

24bit z buf

 

[KimB]

It's just 24-bit z and 8-bit stencil.

 

[j^aws]

It's just 24-bit z and 8-bit stencil.

Thanks for clearing that up...the last PC card I owned was a 3Dlabs which had 32 bit z. Thought the industry had moved onto 32 bit z by now.

 

[KimB]

Nope. I think GPU manufacturers have decided the performance hit of separating the stencil and z-buffers isn't worth the increase in visual quality for games.

 

[DeanoC]

There are also options for integer or float depth buffers. A float buffer using 1-z is very close to a w-buffer without the problems.

Many modern cards support very few actual depth buffers options any more.

D24S8 24 bit integer depth, 8 bit stencil
D24X8 24 bit integer depth, 8 bit unused
F24S8 24 bit float depth, 8 bit stencil
F24X8 24 bit float depth, 8 bit stencil.

Apart from bandwidth, another reason for sticking both buffers together is related to hierachical Z, seperate them and you generally lose hierachical stencil.

 

[Mendel]

I too recently installed radlinker and I was wondering especially about which to choose from these 3 modes?

- 16,24;8 bit stencil
- 16,24,32
- 16,24,32;8 bit stencil

So could you please tell me in point by point fashion if there is any difference among them? As far as I could tell according to discussion here so far, they should all act the same, 24 bit z buffer + 8 bits for stencil but then again I'm confused.

 

[Simon F]

I'm pretty sure no consumer card currently supports higher than a 24-bit z-buffer. I do know that 3DLabs supports a 32-bit z-buffer, but consumer cards elect to pack the z- and stencil-buffers into one to save performance, which limits the possible precision.

Kyro has a 4 bit stencil. What might that suggest to you?

A link to Z-buffer

Hmmm... I'm tempted to add a few comments and references to that.

 

[KimB]

Since the Kyro stores its depth buffer on-board, I see no reason why it has to link the two buffers at all. Regardless, I said, "currently." The Kyro is sufficiently defunct to not be considered a current consumer video card anymore.

 

[Simon F]

Since the Kyro stores its depth buffer on-board, I see no reason why it has to link the two buffers at all.

Think about it.

 

[KimB]

Doesn't matter either way. It's not a current consumer card.

Still, got a doc that states the depth accuracy of the Kyro?

 

[Quitch]

I too recently installed radlinker and I was wondering especially about which to choose from these 3 modes?

- 16,24;8 bit stencil
- 16,24,32
- 16,24,32;8 bit stencil

So could you please tell me in point by point fashion if there is any difference among them? As far as I could tell according to discussion here so far, they should all act the same, 24 bit z buffer + 8 bits for stencil but then again I'm confused.

ChrisW is the author of RadLinker IIRC and he visits these boards sometimes. Might be worth sending him a PM (and let me know the result ).

Can someone explain the stencil concept a bit more. Right now I see there's no purpose to selecting a 32 option, that's either future proofing, or redundant registry values. However, what's the difference between selecting 24 bit and 24bit 8bit stencil?

I also wonder, if you select a value too high for the card, what does it fall back to? Do the values above suggest that it tries 32, then 24 then 16?

 

[KimB]

Basically, you get to use the stencil buffer. You can increase or decrease the value in the stencil buffer, as well as perform a test against the stencil buffer (for example, throw a pixel out if the value in the stencil buffer is nonzero).

This has been used for some time for shadowing, and Humus recently produced a demo that used the stencil buffer to implement a multipass dynamic branching technique.

 

[Quitch]

Does that mean that 24bit 8bit stencil indicates that 8bits of those 24 are being dedicated to the stencil buffer? Is that a good trade?

 

[Simon F]

Does that mean that 24bit 8bit stencil indicates that 8bits of those 24 are being dedicated to the stencil buffer? Is that a good trade?

No, it means 24+8 so that it fits into a nice round power of 2.