Thanks for the response Jason.
Jason Daly wrote:
Roger
James wrote:
So Collada is giving us emission, ambient, diffuse, and specular
elements which can be either a fixed colour or a variable colour
acquired from a texture map. In the OpenGL pipeline the lighting
calculations are done prior to the fragment shading. The only exception
is the specular highlighting which we can optionally postponed until
after the fragment shading phase. So I would suggest that the starting
position is that any textures in the emission, ambient, and specular
elements are ignored. If a texture exists in the diffuse element then a
default value (probably white) can be applied as the fixed diffuse
colour and the texture used to modulate (GL_MODULATE) the incoming
fragment colour in the fixed function fragment shader. The common
profile does not allow multiple techniques to used in an effect, i.e.
no multitexturing.
I've seen light maps and ambient occlusion maps placed as a texture in
the ambient channel along with a regular diffuse texture in the diffuse
channel. I've implemented this in my own code with simple
fixed-function multitexturing before. Is there a reason not to do
this?
I was assuming that because the lighting equation (blinn) has already
been applied in the pipeline before textures are applied, that if we
wanted to use a texture map to supply the ambient terms to that
equation, then the lighting equation would have to be somehow
re-implemented in a fragment shader. I did not think the the fixed
functionality could be configured to handle it, especially the
contributions from light sources. I am happy to be wrong in this.
If the diffuse channel contains a texture and
the shininess element is non zero then specular highlighting should be
postponed.
I cannot think of a way of handling the index_of_refraction. Has anyone
got any thoughts on this?
That would have to be done with a shader implementing Snell's Law (I'd
copy it here, but I don't know how to type a "theta" character. Just
look it up on wikipedia if you're interested :-) )
I think I will just put a comment in about that and leave it for
someone more experienced to implement!
Reflective and reflectivity need further
study. Anyone got any ideas?
I think these are intended for environment maps. "Reflective" holds
the map itself (or just an overall color), and "reflectivity" specifies
the blending factor between the surface and the environment.
Same as above :-)
That leaves the thorny issues surrounding the
transparent and transparency elements. This is what the Collada spec
has to say about then in general terms.
It sounds like you've done more research than me here, so I don't think
I can contribute much more :-)
The transparency bit is the bit that needs fixing. We are just not
handling transparent textures at the moment. I would really appreciate
your input in this area, as I feel that I am just wandering about in
the wilderness on it at the moment.
Roger
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