nvidia option question

[nala-naj]

i have an NVidia GeForce4  440 Go card and if I go into my settings and check the option "Enable conformant OpenGL texture clamp behavior", i go from 300+ fps (unchecked) in my game, to 5 fps.

anyone know what this option does or if i could have my game assure that the option is off via some opengl setting?

thanks

[cfmdobbie]

Googled from some nVidia docs:

Enable conformant OpenGL texture clamp behavior.  Enabling this option activates conformant OpenGL texture clamping. "Texture clamping" refers to how texture coordinates are handled when they fall outside the body of the texture. Texture coordinates can be clamped to the edge or within the image.

As for why it slows your application down, no idea!  Most sites Google returned mention it as a speed-up.  Just clamping texture coordinates in a certain way shouldn't cause that much of a performance hit, anyway.  You might want to bounce a question off nVidia and see what they say, or at least ask on some nVidia forums.

I would assume there is no way of checking whether or not it's enabled from code.

[cfmdobbie]

Right, a bit more research - from someone on Usenet:

that "comformant opengl texture clamp behavior" has something to do with the way the graphics are drawn onto your screen, some textures are drawn outside of the viewable screen, "over the edge" and clamp behavior clamps those textures right to the border of your monitor letting none escape.

It's probably interpolating between vertices to create vertices on the screen boundary, then interpolating the texture coords to make the new vertex texture correctly.  Nothing that shouldn't be done lightning fast in hardware, I'd have thought?

Well, whatever.  It's the kind of thing that you shouldn't worry about at all.  You as the developer have no way of knowing what the effect of that setting is on the performance of the application!  Just because your system with an MX440 on those particular drivers has a bad time of it, doesn't mean any other system will suffer.  Hopefully anyone who messes with the driver's OpenGL settings and notices a slowdown like that will have the good sense to click "Restore Factory Settings" and learn to leave things alone unless they know what they're doing.

[nala-naj]

it is really that extreme... very weird.  i gave a demo to mojomonkey and he was getting 9fps, and he has the same card as i have... and that is how we figured out the settings.... anyway, strange is all i can say.