Extending Wavefront OBJ MTL Material Definition for Physically-Based Rendering

Originally published on the now defunct Exocortex blog. Republished here for posterity.

Updates

• 2015: TinyObjLoader adopts this proposal.
• 2019: This proposal was included in the US Library of Congress official description of the MTL material format.
• 2022: These parameters were incorporated into Blender's MTL importer/exporter, thanks to Aras Pranchevicius.

Original Proposal

Over the last couple of years, the game industry has moved from the traditional Blinn-Phong shading model towards the Physically-based or Physically-plausible Rendering method that uses new material definitions that make it easier for artists to create realistic (e.g. physical) results.

But there is a significant issue in trying to develop assets for PBR-based renderers. There is no well defined format for transmitting the underlying Physically-based Material parameters between various creation tools (e.g. Clara.io, Unreal Engine, CryEngine, Unity, Maya, 3DS Max, Modo.)

In this blog post, we propose a simple solution. We suggest simply extending the Wavefront MTL material format with a number of new parameters that capture the model proposed by Disney for Physically-Based Rendering.

To represent a PBR material, we suggest using these existing well defined Wavefront MTL material properties:

• Kd/map_Kd (base/diffuse) // reuse
• Ks/map_Ks (specular) // reuse
• d or Tr (opacity) // reuse
• map_d/map_Tr (opacitymap) // reuse
• Tf (translucency) // reuse
• bump/bm (bump map) // reuse
• disp (displacement map) // reuse

And we propose adding these additional properties, which refer to PBR-specific parameters as defined by the Disney PBR paper:

• Pr/map_Pr (roughness) // new
• Pm/map_Pm (metallic) // new
• Ps/map_Ps (sheen) // new
• Pc (clearcoat thickness) // new
• Pcr (clearcoat roughness) // new
• Ke/map_Ke (emissive) // new
• aniso (anisotropy) // new
• anisor (anisotropy rotation) // new
• norm (normal map) // new

The various parameters are to be parsed in the same manner as the original MTL parameters.