Keith Whitwell wrote:
Ian Romanick wrote:
Over the past year an issue of OpenGL versioning has come up a few times.
Basically, we have conflicting goals of wanting to advertise OpenGL
1.3 or
1.4 but not wanting to advertise extensions that aren't hardware
accelerated
(cube textures and shadow maps come to mind).
I believe that a solution exists, but it will require some trivial
changes
to Mesa. Basically, we advertise whatever OpenGL version we want
(assuming
that the driver knows how to fall-back for unsported functionality), but
only advertise extension strings for functionality that is hardware
accelerated. For example, advertise version 1.2, but only advertise
GL_EXT_texture3D in the extension string if it's hardware supported.
This may sound silly, but I think it's a good compromise. It turns
out that
this is what Nvidia does. That's not to say that Nvidia is always right,
but I think this is better than being stuck back at 1.2. :)
Does GL_EXT_texture3D still work, even though it's not listed?
If not, that takes all the hard work out of advancing mesa -- no need to
code stuff up, just bump the version number...
Darn! Why didn't I think of that. All these years of wasted effort!!!
It's a nice idea Ian, but it's not foolproof. Using 3D textures as an
example, the R200 has 3D textures, but not mip-mapped 3D textures. So
GL_[LINEAR/NEAREST]_MIPMAP_[LINEAR/NEAREST] would be a software fallback.
I'm a bit hesitant to stop advertising extension strings for corresponding
OpenGL 1.x features because there are older apps out there depend upon
them. As recently as this past spring, SGI Performer on Linux didn't use
texture objects if GL_EXT_texture_object wasn't listed (it would fall back
to display lists)!
As much as I hate inventing new environment variables, perhaps an env
var could control whether we advertise OpenGL 1.3/1.4 with a bunch of s/w
fallbacks vs OpenGL 1.2 where almost everything is in hardware.
I'm open to ideas.
-Brian
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