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JDX_John

OpenGL
Standard emissive+ambient+diffuse+specular lighting model... how do texture and material settings combine?

5 posts in this topic

The standard lighting used in OpenGL fixed-function (and I guess in D3D) basically boils down to:

color = material.emissive + material.ambient * light.ambient
      + material.diffuse * diffuseValue + material.specular * specularValue

Where diffuseValue and specularValue are calculated values.

 

But when you add a texture-map, how is this traditionally integrated i.e. how does it affect the emissive, ambient, diffuse and specular terms?

 

I assume emissive should not be modified. Should we replace material.ambient with material.ambient * textureValue and material.diffuse with material.diffuse * textureValue? Should specular BE affected by the texture map or not?

 

I'm aware there's probably a million valid ways it CAN be done. I'm mainly asking how the standard fixed-function pipeline does it.

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It just turns the specific value in question into something that can vary over the surface of a model instead of being uniform. Granted, ambient texture maps don't generally make sense, but it's just a question of scale and artistic intent.

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Granted, ambient texture maps don't generally make sense
Ambient lighting itself doesn't make sense wink.png

 

Ambient light is just diffuse light that comes from a magical "background"/bounce light, so both the "diffuse" and "ambient" results from this lighting model should be multiplied by the diffuse texture.

 

If present, the emissive term would be multiplied by an emissive map, and the specular term by a specular map.

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You could check this link on the d3d lighting model.

I'm not 100% sure if this is how the fixed pipeline does it though.

http://msdn.microsoft.com/en-us/library/windows/desktop/bb147178(v=vs.85).aspx

 

That's a great link, but they nowhere mention how textures fit in. Only material and vertex colours are discussed :( Maybe material is synomous with texture, but I thought they were both supported simultaneously.

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Hi,
Looking in an old OpenGL 2.0 spec, it seems that active texture replace fragment color, chapter 3.8.15:
 
"This texture value is used along with the incoming fragment in computing the texture function
indicated by the currently bound texture environment. The result of this function
replaces the incoming fragment’s primary R, G, B, and A values"
 
If you look, in chapter 2.14.1, you have the complete lighting equations. 
 
Hope this helps !
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