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Converting PBR for use with Blinn-Phong


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I have some 3D models that are textured using a PBR metallic/roughness workflow that I want to use in SL. Reading these forums it seems there will be a lengthy period of transition before the majority start using PBR enabled viewers, so I would also like to texture the models using the legacy Blinn-Phong approach, of which I know little.

Is there a way to manually edit my existing metallic/roughness PBR texture maps to work with Blinn-Phong?

I’m unable to test myself right now but I am wondering if this would work:

Blend the AO map with the Albedo to create a diffuse.

Use the normal map as is.

Invert the roughness map to create the specular. This is the bit I'm most dubious about.

Would this work? Is there a better process to use? Thanks a lot!

Edited by Orbit Darkes
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Well, I've built with the standard diff, normal, and spec maps for years.  I just started using the new PBR GLTF materials and I love the improvement, even though Firestorm does not have a PBR viewer yet.  I'm not going to worry about it.  As soon as Firestorm releases their new viewer our problem is gone.  If Firestorm takes too long to release one, people will stop using it as more and more PBR GLTF materials appear on the MP.  Right now, I'm using Alchemy and I'm really liking it, and it only needs a few more update releases to make it FS's equal.  I'm even converting some of my old builds to the new materials to get me there quicker.

If you want to use the old map style textures, don't try to convert your PBR GLTF materials.  Just use the old map textures.  It's easier.

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The RGB of the diffuse texture is the diffuse color.
The A of the diffuse texture can be used as blended opacity, hard opacity (with arbitrary cutoff), or emission. (Or ignored.)

The RGB of the normal map is normal vector data (almost, but not quite, identical to the PBR spec...the exact difference is technical, subtle and still beyond me)
The A of the normal map is glossiness (i.e. smoothness, or inverted roughness)

The RGB of the specular map is the specular tint.
The A of the specular map is the environment map strength (i.e. how much the fake skybox reflects on it, for a chrome/mirrored look)

So, yeah, roughness should be inverted and added to the normal texture as an alpha channel. Ideally, you may also want to apply a forward or reverse curve correction if one channel is perceptual and the other is linear.

Metallic doesn't translate directly. For highly metallic parts you'll probably want a combination of some environment reflection plus a specular tint based on the diffuse color, while nonmetallic is no environment and white/gray specular. Expect to do a lot of hand-tuning here.

If your PBR has both translucency and emission, you'll have to give up one.

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If you have substance designer there is a node for PBR to Specular conversion, simply plug in your metal and rough and use the Glossiness output to get your specular map from.

But in all honesty I'm finding going back to the old workflow icky, Diffuse/normal/specular is just dank now.

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