May I ask what are these PNG pictures for?

After installing the software I downloaded from the official website, the material inside is like this folder. May I ask what are these PNG pictures for?

These are preview icons for the materials that can be found in the .mdl files. Some applications are able to display those preview icons for the materials in the material browaser, so you already know what the material preset looks like. If you remove those, the material catalog will still work, however the individual preview icons will be gone and replaced by a generic symbol inside the application. In the attachment you can see, how this works in Iray for Rhino.

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But in a folder there is usually only one or several.mdl files, but there are many PNG files.How can an MDL produce so many looks???

[quote=“Ruediger”]

These are preview icons for the materials that can be found in the .mdl files. Some applications are able to display those preview icons for the materials in the material browaser, so you already know what the material preset looks like. If you remove those, the material catalog will still work, however the individual preview icons will be gone and replaced by a generic symbol inside the application. In the attachment you can see, how this works in Iray for Rhino.

但是在一个文件夹里一般只有一个或几个.mdl文件,但是却有很多PNG文件。只有一个mdl怎么可以产生这么多的外观呢???

One .mdl file can contain more than one material. You can see by the name of the .png file from which .mdl file file it comes from. The naming scheme goes as follows:

<MDL_SOURCE_FILE>.<MATERIAL_NAME>.png

That way you can identify the material name and the source file name by reading the name of the filename of the .png

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How is Omni PBR different from regular PBR format? Do I need to convert my PBR materials to MDL format? If yes then how?

Hello @user126085,
“Omni” defines it just as an omniverse material. It is a regular PBR format.
All materials in Omniverse are supported using MDL. There is no need to convert formats. You can use textures like albedo, reflectivity, ao, emissive, opacity, and normal for the physically-based material.

I like to show you this documentation that could help to understand everything that you need to know.