View Full Version : Pipeline stall question

What's the performance impact of changing

the modelview matrix? Suppose I fire a

number of vertexes, then call glRotatef()

and the fire more vertexes, will the changing

of the matrix implicitly sync ("fence") the

previous vertexes, causing transform pipeline

bubbles?

Obviously, this is hardware dependent, and

I'm mostly interested in the answer for

GeForce2.

[This message has been edited by bgl (edited 12-02-2000).]

If I understand you correct ("Will changing the matrix affect vertices that has already been passed?"), then the answer is: NO, definitely no. This is not allowed to happen under any circumstances. Think about what can happen IF it does. Models will look totally screwed (if major changes to the matrix is done), or objects might pop up in the wrong place. If it does, something is seriously wrong.

MikeC

12-03-2000, 07:30 AM

Bob, you're right, but I think bgl was asking whether matrix state changes would stall the pipeline until all vertices issued under the previous matrix state had been completely processed.

Obviously this is implementation-dependant, but for NV hardware the answer is no. I asked a while back about the performance cost of matrix changes (and whether transform sorting made sense compared to texture/blend sorting), and Cass stated that the cost of matrix ops was down in the noise. That seems to rule out a pipeline stall.

I don't know about other architectures, but I'd be VERY surprised if any driver forced a stall in this case.

Originally posted by MikeC:

Bob, you're right, but I think bgl was asking whether matrix state changes would stall the pipeline until all vertices issued under the previous matrix state had been completely processed.

Obviously this is implementation-dependant, but for NV hardware the answer is no

Thank you; this was the answer I was looking

for.

Now, if there were only a glMatrixPointer()

extension and a glMatrixWeightsPointer() way

of applying them to vertexes. Yeah, I am

(slowly) working with skeletal animation :-)

mcraighead

12-03-2000, 10:23 AM

NV_vertex_program. You can fit a lot of matrices into 96 constant registers, and you can do paletted skinning with relative addressing.

Rumor has it that there's a leaked driver out there that _seems_ to support this extension. http://www.opengl.org/discussion_boards/ubb/smile.gif

- Matt

96/12 == 8 (just in case you forgot your

calculator at home).

It seems that using a vertex_program will

substitute "traditional" lighting, so I will

have to reduce at least one matrix to have

the program do lighting.

Would one also be blown on the modelview

transform? And how do I get the N matrix

weights into the program on a per-vertex

basis? Seems like N would be 6 in this case,

which is a respectable number.

Does this mean we will see 4- or 6-matrix

support in DX (and OGL?) in future Detonator

drivers for existing hardware? :-)

mcraighead

12-03-2000, 06:41 PM

No, actually, 96/4 = 24. All the registers are defined as xyzw vectors.

You would need to implement your own lighting. Generally, multi-matrix vertex blending occurs at the stage of the modelview transform, so you would blend between N matrixes to compute eye coordinates and eye normals, then multiply the resulting eye coordinate by a projection matrix to get clip coordinates. The eye coordinates and eye normals are used in lighting.

- Matt

That still doesn't explain how the set of

weighting values for each matrix for each

vertex gets handed to the vertex program.

Or can the vertex program issue DMA requests

for floating-point data? (Doesn't look like

it, but I haven't dived deep into it).

Michael Steinberg

12-04-2000, 01:05 PM

I'm sorry to aks here, but what's the trick with vertex blending. Didn't get the theory yet. What does it do how?

I could understand if nobody would answer my idiotic questions...

Actually, I want to blend transform matrices

for skeletal animation, not vertexes. Vertex

blending (a la Quake 2) only needs two

matrices and a fixed weight.

Anyway, skeletal animation is where you have

some "bones" (invisible) in your model,

which have swivel "joints" (one end of the

bone, typically) and some degree of freedom

of movement. Then you "tag" each vertex with

which "bone" it belongs to. At seams (joints)

a vertex belongs a little to at least two

bones; sometimes more.

The bones, in turn, are hierarchical, so that

bending your upper-arm will move the joint to

the lower-arm; then you bend the lower arm in

relation to the upper arm.

While you can pre-calculate the various

matrices for the bones, the problem is that

to "fire" an entire mesh in one go, you need

blending support for one matrix for each

bone, which at a minimum (for a biped) is

back, upper&lower left&right legs, ditto

arms, and neck, for a total of 10 matrices.

Also, even if any individual vertex only

belongs to one or two bones (i e has non-0

weights for those matrices) the problem is

that to draw a triangle, you have to involve

three vertexes, and thus you could (worst

case) need 6 matrices active at the same time

to draw that one triangle.

The current consensus is to do all of this

in model space on the host, and then write

the transformed vertexes to the card for

placement in the world, lighting, etc.

For more details, look up "skinning" on your

favourite web search tool.

Originally posted by mcraighead:

Rumor has it that there's a leaked driver out there that _seems_ to support this extension. http://www.opengl.org/discussion_boards/ubb/smile.gif

Another rumor tells me there only are Win2K and Win9x versions of this driver around... I am starting to wonder if WinNT is still supported... (just kidding here ! I asked nVidia and they said an NT version will be present for the *OFFICIAL* release !).

Regards.

Eric

Michael Steinberg

12-04-2000, 11:18 PM

Oh yeah, thanks bgl, I get what you mean. So you don't only apply only the transformation matrix for one bone, you apply the other as well. But how to weight between two matrices, from the mathematical way, I mean. Is it to "middle" the outcoming vertices, regarding the weights of the matrices?

The way the spec says it's implemented, it

calculates the transform for each of the

matrices, and then averages the result based

on the weight (i e w*V + (1.0-w)*V'). This

is for the nVidia 2-matrix extension, which

for various reasons is sub-optimal for skin

animation (but is optimal for full mesh

blending).

Oh, and in my sample, you really need another

four bones (feet, hands) to get good

animations, plus possible articulation for

fingers etc. So say 16 matrices for a good

enough game-level animation system.

I now have an answer on how the weighting

data per-vertex gets into the program,

however: glVertexAttrib4fNV(). Dunno how I

could possibly have missed that before even

though I only glanced at the docs.

Looks like, with swizzling, you can store the

weights in only 4 attribute registers for the

16 matrices. Man, I wish I was getting paid

to do this, because it looks like fun, but it

will take quite a while to get it right :-(

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