@rich o brien said:
ARM could be a variant of MRAO where the Red channel is Ambient Occlusion, Green channel is Reflection and Blue channel is Metallic. Its a technique used in game engines to pack 3 maps into 1 bitmap
Yeah, that kind of channel packing is one of the most popular, even if we usually refer to it as "ORM map" (Occlusion Roughness Metallic respectively in R, G and B channels).
It's used in that way for the gltf/glb format and for many UnrealEngine shaders, ThreeJS/BabylonJS shaders and so on... so it's widely adopted in the game/vr/ar/web industry.
There are of course different (more specialized) standard for channel packing, e.g. Unity Standard shader uses R for metallic, G for occlusion, Alpha for glossiness, or the WinMRTK Hololens shader which is similar to the previous one, but also add emissive intensity mask on B channel... and a lot of different "custom" packing for similar custom shaders, often using the B channel to store the DetailMask (used to drive optional detail albedo/normals).
I see all the time people struggling in attempt to use this kind of maps in Vray and such.
The real answer is "You cannot have a 1:1 translation of game engine material into a vray material", mainly because of the occlusion map.
A lot of tutorials out there say that AO map should be simply multiplied on top of the basecolor map, which is totally wrong in 99,9% of cases.
First off, because the AO map is intended to be used to occlude ONLY a (fake, not raytraced, not shadowcasting) Ambient light, which simply doesn't exsist at all in Vray&co (even a vray or corona domelight is casting some "true" raytraced shadow and GI), and also because a basecolor map itself could be meant to be a "diffuse" or an "albedo" which are similar, but not the exact same thing (and the exact difference is a bit controversial, so may vary quite a bit depending on the context).
In addition to all of this, to make "game engines" maps work 99% properly in different engines, one have to know about the difference between metallic Vs specular workflow, difference between directX vs openGL normals, difference between glossiness and roughness, difference between displacement mapping/bumpmapping/parallax occlusion and so on..
Many tutorial out there are misleading (to say the least) and it's difficult to tell someone a general rule about "where I shpuld plug the maps I downloaded to be used correctly?", whitout seeing the actual maps and without knowing how and why they were autored in first place.
One of the best resource out there to clarify this complex topic, is the Allegoritmic PBR guide, I strongly reccomend that to everyone which is into textures and materials.