How to convert downloaded PBR materials for OpenSim

Here is how you can convert downloaded PBR materials for use in OpenSim.

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Step 1: Download and unpack materials.

One good source for PBR materials is ambientCG (https://ambientcg.com/) which offers lots of materials licensed under Creative Commons CC0 (https://docs.ambientcg.com/license/).

However, these materials cannot be used in OpenSim as they are. They will have to be converted first.

Step 2: Download and unpack a converter.

There are various converters made by the Second Life community that can convert downloaded PBR materials into PBR materials compatible with Second Life and OpenSim. For example:

* GLTF Packer (Windows only; https://aiaicapta.in/gltf-packer/; see here: https://community.secondlife.com/forums/topic/513330-useful-free-programs-for-pbr-material-creation)

* MakeGLTF (Windows/Linux; https://github.com/Graven-Hearts/MakeGLTF; see here: https://community.secondlife.com/forums/topic/507687-makegltf-for-linux-based-on-gltf-packer/)

Step 3: Start the converter and put the image files from the downloaded materials into the appropriate boxes.

This is mostly self-explanatory. For example, in MakeGLTF, the "Color" map goes into the "Base Colour" box, the "Roughness" map goes into the "Roughness" field etc.

You don't have to use all files, and you don't have to use all boxes. But you should use all files that have corresponding boxes.

Notice: ambientCG gives you two normal maps: "NormalGL" for OpenGL and "NormalDX" for DirectX. Since OpenSim uses OpenGL, it's the NormalGL map that you need.

Step 4: Run the converter. It should generate a subfolder with a glTF PBR material which you can upload to OpenSim plus the necessary number of image files.

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You are not done yet, though. Not everyone can see PBR materials, especially those whose machines aren't powerful enough for Firestorm 7. If you only add a PBR material to an object, then Firestorm older than 7 will only show a plain grey surface, as will similarly old viewers. In order for non-PBR viewers to show textures, you will have to add non-PBR maps yourself. And you'll first have to make these maps.

If you want to get close to PBR without PBR, you may want to make use of all three so-called Blinn-Phong maps: the texture (or diffuse map), the normal map and the specular map.

Step 5: Rename two of these three textures.

The "Color" map can be used as the texture, and the "NormalGL" map can be used as the normal map; you may want to make copies and rename them so you have a full set of three Blinn-Phong textures.

The specular map is a bit trickier. According to here (https://community.secondlife.com/forums/topic/508876-converting-pbr-for-use-with-blinn-phong), you can approximate it by inverting the Roughness map.

Step 6: Prepare the specular map.

This requires a more advanced image editor, e.g. GIMP (https://gimp.org) or Photoshop or the like.

Open the Roughness map in the image editor. Invert the colours. Save the result as a copy with a new name, ideally one that hints at it being a specular map.

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Congrats, now you have a PBR material for OpenSim and a set of Blinn-Phong maps to go along with it.