What does running a DX9 game on a DX8 card mean for performance?

[QAGuy]

Please feel free to correct any of my assumptions if they are incorrect.

I assume that a DX9 game running on a DX8 video card will have to utilize the CPU more and will perform worse. Assuming that this is true, what is this performance penalty likely to be?
I suppose that features of DX9 would not be implemented, as well. If so, what features would be lost?

Thanks!

 

[Pete]

No, a game that has DX9 effects will not be able to run those effects on a DX8 card. The game will either switch to lower-quality DX8 versions of those effects, or not show them at all. Neither option should result in higher CPU usage, just as running DX8 effects on a DX9 card won't save you CPU usage. You cannot trade CPU power for higher DX compatability. Well, technically, you might be able to with vertex shaders--but pixel shaders create the pretty effects, and I'm pretty sure those are the ones you're referring to.

As for what effects might be lost, it depends on the game. Some may offer lower-quality versions of DX9 shaders using DX8, but I'd imagine the big reason to use DX9 is to do something DX8 can't achieve, in which case you'll just miss that fancy effect.

 

[McArra]

Exactly

 

[BrotherAaron]

woe is me with my directx7 card... anyone know where I could get a radeon 9600 mobile?

 

[nRollo]

 

[nRollo]

The game will either switch to lower-quality DX8 versions of those effects, or not show them at all.

How big of a deal this is remains to be seen. The famous "shinier water" screenshot in HL2 didn't exactly blow me away, nor did the movie I downloaded. It's a nice looking game, but I would expect it to be given what's invested in it.

 

[Pete]

Yeah, technically it'll be lower-precision. Whether that will manifest itself as lower-quality remains to be seen. But not all DX9 effects have a DX8 counterpart, so you'd still be missing out on some things.