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OBJ_MTL_PBR.md

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OBJ Material Entries (MTL file)

Keep in mind that this repository is using the following customized OBJ related three.js files: OBJ Loader, MTL Loader and OBJ Exporter.

These files have been synchronized to work with each other and process all custom PBR entries found in the MTL file, as listed further below, aiming at providing the GLTF alike functionality in the OBJ Viewer by taking advantage of the THREE.MeshPhysicalMaterial and a modified GLTF Loader.

All the entries have been set as such so they are compatible with three.js as well as the assimp library being used in this repository:

  • Note that the assimp library was heavily customized with features currently not available in the official repository but available in the updates branch

Most, if not all, of the available viewers do include the OBJ Exporter, thus can try to export their models to OBJ + MTL + textures format with whatever limitations there might be (like animations for example).

As for the textures, any of the following image formats should work with OBJ models when specified in the MTL file:

  • BMP, DDS, GIF, JPEG, KTX2, PNG, TGA, WEBP:
    • DDS, KTX2 and TGA textures will be converted to PNG format
  • Test model can be found here and each of the zip files can be loaded as such:
    • clicking each of these zip files will take you to the page where you should look for 3 dots button in the top right corner and its option to copy permalink of the file to the clipboard which can then be pasted into the URL textbox of my OBJ Viewer
    • the other option would be to just download the file to a hard drive and load it with the Browse option of the OBJ Viewer

This document would represent a summary of PBR entries that can be found in the exported MTL file, with mentioning of some possibly utilized standard entries, whose description can be seen online here, as well as just a few FYI Blender adopted entries as per these proposals.

Since the MTL file is a text file, then it could be edited manually and any entries changed if required.

Standard Entries:

  • Ka, Kd, Ke, Ks, Ns, Ni, Tr, Tf, d, illum, -bm, -s, -o
  • map_Ka, map_Kd, map_Ke, map_Ks, bump, map_bump, map_d, norm, disp

PBR Entries with associated textures (maps):

  • a - alphaTest (alphaCutoff)

  • s - material side

  • Pe - emissive intensity

  • Prf - reflectivity

  • Pac / Pad - attenuationColor / attenuationDistance

  • Pm - metalness

    • map_Pm - metalnessMap

  • Pr - roughness

    • map_Pr - roughnessMap

  • map_Px - replaces both map_Pm and map_Pr when they are identical

  • Pa/ Par - anisotropy / anisotropy rotation (BLENDER: aniso / anisor)

    • map_Pa - anisotropyMap

  • Pi / Pii - iridescence / iridescenceIOR

    • map_Pi - iridescenceMap

  • Pitx / Pity - iridescenceThicknessMinimum / iridescenceThicknessMaximum (aka iridescenceThicknessRange[x, y] in three.js)

    • map_Pit - iridescenceThicknessMap

  • Pcc / Pcr - clearcoat / clearcoat roughness (BLENDER: Pc / Pcr)

    • map_Pcc - clearcoatMap
    • map_Pcn - clearcoatNormalMap
    • map_Pcr - clearcoatRoughnessMap

  • Ps / Psr - sheenColor [RGB] / sheenRoughness (BLENDER: using Ps for sheen layer intensity instead)

    • map_Psc - sheenColorMap (BLENDER: map_Ps)
    • map_Psr - sheenRoughnessMap`

  • Pth - thickness

    • map_Pth - thicknessMap

  • Ptr - transmission

    • map_Ptr - transmissionMap

  • Psp - specularColor [RGB]

    • map_Psp - specularColorMap

  • Psi - specularIntensity

    • map_Psi - specularIntensityMap

Entries currently not compatible with the assimp library but working fine in the OBJ + MTL Viewer:

  • -c - texture center of rotation
  • -r - texture rotation value
  • -w - texture wrapping parameters (wrapS and wrapT)
  • Pns - normalScale
  • Pcn - clearcoatNormalScale
  • Pdt - depthTest
  • Pli - lightMapIntensity
  • disp_b - displacementBias
  • disp_s - displacementScale
  • Pbr_ps - sheen (layer intensity)
  • Pbr_pl_map - lightMap

Keep in mind that other OBJ + MTL viewers will probably NOT be able to interpret most of these PBR entries.

Testing

This can be tested by using the Khronos Group glTF v2.0 examples and following these instructions:

  • Use my GLTF Viewer to load one of those example GLTF files via the viewer's URL option, here is an example url:
    • https://github.com/KhronosGroup/glTF-Sample-Models/blob/master/2.0/IridescenceMetallicSpheres/glTF/IridescenceMetallicSpheres.gltf
  • Use the viewer's OBJ export option to get OBJ + MTL + textures in a zip file
  • Extract all the files to a folder on your device (this needs to be done since my viewers currently don't support loading of zip files)
  • Examine the MTL file to see what the entries look like and then do the following:
    • Use my OBJ Viewer or ASSIMP Viewer and select its Browse option to load these extracted files all at once - your device will have to allow loading local files
    • OBJ Viewer additionally supports loading of ZIP files
  • After the model is loaded then make sure to check the Eq box in the viewer to get the environment texture / lights

Most of the examples will work properly but some might NOT.

Credits

Trademarks

  • Any and all trademarks, either directly or indirectly mentioned, belong to their respective owners.