[Mesa3d-dev] Fog on tdfx (was: GL_EXT_fog_coord, etc.)

[Brian P. <br...@va...>]

Michael Vance wrote:
> 
> On Fri, Oct 27, 2000 at 12:46:32PM -0600, Keith Whitwell wrote:
> 
> > Not really.  The FX driver basically turns this code off as the hardware
> > normally fogs from the z or 1/w values (I can't remember which).  I'm sure
> > it's just a matter of setting up the fog table correctly for orthographic
> > projections & making sure that some usable value gets into the vertex format.
> 
> The w-value is always 1 (or 0, can't remember) for vertices in an
> orthographic projection, and the 3Dfx card uses the w-value for
> indexing into the fog table--thus everything is fogged with the fog
> color from entry 1 (or 0).
> 
> In the past, Brian and Daryll mentioned using an alternatve fogging
> method available in Glide3 (a z fogging method), but I don't believe
> it was pursued.


In glide.h we see this:

typedef FxI32 GrFogMode_t;
#define GR_FOG_DISABLE                     0x0
#define GR_FOG_WITH_TABLE_ON_FOGCOORD_EXT  0x1
#define GR_FOG_WITH_TABLE_ON_Q             0x2
#define GR_FOG_WITH_TABLE_ON_W             GR_FOG_WITH_TABLE_ON_Q
#define GR_FOG_WITH_ITERATED_Z             0x3
#define GR_FOG_WITH_ITERATED_ALPHA_EXT     0x4
#define GR_FOG_MULT2                       0x100
#define GR_FOG_ADD2                        0x200

So using GR_FOG_WITH_TABLE_ON_Q or GR_FOG_WITH_TABLE_ON_W is the
same and neither works with orthographic projection since the W
value is always 1.0.

I'm thinking that if we're using orthographic projection that we
could use GR_FOG_WITH_TABLE_ON_FOGCOORD_EXT (available on Voodoo3
and later, don't know about Banshee or Voodoo1/2) if we added a
fog coord to the Glide vertices and computed it from the vertex Z
somewhere in the vertex setup code.

I think this would be much more easily done in the new tdfx-3-0-0
driver than the current driver.

Gareth, could we put GR_FOG_COORD into the vertex struct where
we currently have a GLuint padding?  I haven't examined the vertex
layouts very much yet.

I can't find any documentation for GR_FOG_WITH_ITERATED_Z but I'd
guess that f (fog factor) is just a multiple of the fragment's Z and
can only be used for GL_LINEAR fog if near=0 and far=1.

-Brian