Smeenk, R.J.M. (Roland) wrote:
Hello Roger,
I did not make a study of the Collada transparency intricacies like
you did. The assumption I made was that a texture for diffuse
specifies the diffuse components to use in the texture. The fact that
texture coincidently was packed with an alpha channel (which of course
happens a lot) should not influence that.
When you consider the chain <texture><sampler><surface><image> the
responsibilities are clearly dividided. It could be possible that
somebody decides to pack an ambient occlusion or specular value in the
alpha channel of an image. The semantics of the data source should
be specified by the texture elements in ambient, diffuse, specular,
etc. That's why I added code to only enable blending if a texture was
specified in the transparant element.
What you say makes sense, but I've never seen it done this way in any of
the files I've used or created via COLLADA plugins. I think we need to
support the "RGBA texture in diffuse channel only" model, just because
so many files and tools work that way. This isn't ideal, but it's
probably necessary.
I've been very suprised to see suggested everywhere that the common
profile maps to the fixed function pipeline, but this easily falls
short if you need to support both blinn and phong. Well OpenGL FFP
does not even support phong shading. Gouraud is the closest we get
with per vertex lighting.
It clearly doesn't map to FFP, but it's as close as COLLADA comes. The
biggest problem with handling profile_COMMON with shaders is that it
makes it hard for an OSG developer to use his/her own shaders higher up
in the scene graph. I think this is what drives the desire to handle
profile_COMMON with FFP.
I don't know how to map textures specified for both ambient, diffuse,
specular etc. to the Ogl FFP. Texture combiners might bring us a
little closer, but I hardly consider this the FFP.
Well, I believe ARB_texture_env_combine was made a core feature in
OpenGL 1.3, so many would consider it FFP. But I digress...
Besides what happens if the surface contains a channels specification
other that rgb or rgba? It gets even worse if it is allowed to
compose 3 channel input for a diffuse texture combining components of
three different source images. I am not sure if this is allowed in the
profile_COMMON. Though very flexible, who is going to support this?
I don't think that is allowed (the schema says only one color or texture
per channel). That would get out of hand quickly. I don't think anyone
here expects that to work anyway. We're just talking about a single
RGBA texture in the diffuse channel.
--"J"
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