Is the if-statement particularly slow ?

[Korval]

The GeForce FX series isn't able to do dynamic branching in pixel shaders.

For the sake of clarification, the GeForce FX 6800 is still an FX, and it can do dynamic branching in the fragment program. What he really means is that 5xxx cards can't do fragment program branching.

[wizzo]

Hi again,

Now that i had some time to look at it, i'd like to get some precision. I'm quiet a beginner so dont hesitate to explain to me like you would to a kid =)

So here are the steps :

1 - We render our first pass with depthAmbiant shader ON, rendering for shadowed aeras

2 - XXXX

3 - Finally we do our second pass with lighting shader ON only where stencil value says, so that lit fragments are actually lit.

That's the second step i'm missing :

the if shader return as a color param the distance from the light to the fragment.
And i know thats when the stencil buffer decide whether a fragment is lit or not, but i dont get how.

thx to anyone
wizzo

[Jesse Hall]

Korval: It's the GeForce 6800; no "FX". FX <=> 5xxx. </nitpick>

[Korval]

Korval: It's the GeForce 6800; no "FX". FX <=> 5xxx. </nitpick>

Hmmm. I could have sworn I checked that before posting it. Oh well.

[Humus]

Originally posted by dingo_aus:
BTW the dynamic branching doesn't seem to work with my NV5900 (forceware 62.71).

Is there something I should do to help it run properly?

It seems this is actually the 5x00 hardware not being capable of doing early stencil rejection.

[Humus]

Originally posted by Corrail:
The GeForce FX series isn't able to do dynamic branching in pixel shaders. You won't get it work on your card. But you can either buy a new graphic card or use a software rasterizer (NVemmulate).

Well, the point of that demo is to show that you can implement dynamic branching for some common situations without special hardware support, and still get as good or actually better performance than real dynamic branching.

[Humus]

Originally posted by wizzo:
Hi again,

Now that i had some time to look at it, i'd like to get some precision. I'm quiet a beginner so dont hesitate to explain to me like you would to a kid =)

So here are the steps :

1 - We render our first pass with depthAmbiant shader ON, rendering for shadowed aeras

2 - XXXX

3 - Finally we do our second pass with lighting shader ON only where stencil value says, so that lit fragments are actually lit.

That's the second step i'm missing :

the if shader return as a color param the distance from the light to the fragment.
And i know thats when the stencil buffer decide whether a fragment is lit or not, but i dont get how.

thx to anyone
wizzo

The result from the if-shader is in the alpha. By using the alpha test you kill fragments that are unlit. So the stencil is only updated for lit fragments.

[dingo_aus]

It seems this is actually the 5x00 hardware not being capable of doing early stencil rejection.

Does these mean the ATI cards can do early stencil rejection?

[Humus]

Yes, and the 6800 too (though it seems a bit more sensitive).

[dingo_aus]

Ah good to know - pity a 6800 at the moment would cost more than the rest of my PC combined

(and depreciate a lot faster too)