material specification

gistfile1.txt

Storage for materials:
Version A. compact/hardcoded for certain shading types

type: 1 byte (phong=0, pbrMetallicRoughnes=1, pbrSpecularGlossiness=2, ...)
  if ( type == 0 )
     4 floats diffuseColor
     string   diffuseTexture
     1 byte   diffuseTexture texcoord channel
     4 bytes  diffuseTexture sampler parameters
  .....
  if ( type == 1)
     4 floats baseColorFactor
    ....
    
version B:
predefined values for parameter names and parameters are stored as array.
#define DIFFUSE_COLOR 1
#define DIFFUSE_TEXTURE 2

Parameter have types like float, float2, float3, float4, texture, int, bool, string, etc.
These types define what information is stored in data section.

type: 1 byte (phong=0, pbrMetallicRoughnes=1, pbrSpecularGlossiness=2, ...)
number_of_parameters: 2 bytes
   name_of_parameter 1: 4 bytes
   data_of_parameter 1: x bytes
   ...   
   name_of_parameter n: 4 bytes
   data_of_parameter n: x bytes

diffuseTexture parameter example

4 bytes: (1) DIFFUSE_TEXTURE
string: filename
1 byte: texcoord channel
4 bytes: sampler parameters

version C:
similar to B, but there are no predefined values for shading type and params, they are strings.

types are predefined:
#define PARAMETER_TYPE_COLOR 1
#define PARAMETER_TYPE_TEXTURE 2

string 'pbrMetallicRoughness'
4byte  number_of_parameters
   1 byte: parameter type of param0
   string parameter_name 
   x bytes: data based on type

....

diffuseTexture parameter example

1byte PARAMETER_TYPE_TEXTURE
string 'diffuseTexture'
string: filename
1 byte: texcoord channel
4 bytes: sampler parameters

version D:
similar to C, but store materials (in a separate file) in text format.


-----------------------------------------------


                         material parameters in different obj, gltf2, opengex and fbx
                         ------------------------------------------------------------

wavefront obj
----------------------
ka, map_ka: diffuse color/texture 
ks, map_ks: specular color/texture 
ke, map_ke: emissive color/texture 
ns, map_ns: specular exponent value/texture
d/tr, map_d: opacity/transparency value/texture
map_bump/bump: bumpmap

gltf2 - pbrMetallicRoughness 
----------------------------
baseColorFactor - rgba color in linear space
baseColorTexture - rgb(a) texture in srgb space
metallicFactor
roughnessFactor
metallicRoughnessTexture. channel b - metallic, g - roughness 

Factor and texture are multiplied in linear space. 
vertex color attrib COLOR_0 (assumed to be in linear space) also multiplies basecolor.

normalTexture - tangent space normal map. rgb in [0,1]. extra scale property
occlusionTexture - in .r channel linear space. extra strength property (0.0 - means no occlusion )
emissiveFactor - rgb color
emissiveTexture - rgb in srgb space

each texture has a texcoord channel and a sampler.
sampler properties: minFilter, magFilter, wrapS, wrapT

alphaMode - OPAQUE, MASK (discard with alphaCutoff), BLEND
alphaCutoff - [0,1]
doubleSided

gltf2 - pbrSpecularGlossiness
-----------------------------
same as: pbrMetallicRoughness but it has no baseColor, metallic and roughness.

diffuseFactor - color in linear space
diffuseTexture - rgb(a) texture in srgb space
specularFactor - color in linear space
glossinessFactor - scalar value
specularGlossinessTexture - .rgb specular in srgb space, .a - glossiness

gltf2 - unlit
-------------
baseColorFactor, baseColorTexture and COLOR_0 is used to compute color with no lighting calculation.

opengex
-------
phong shading model

two_sided
diffuse color/texture
specular color/texture
specular_power value/texture
emission color/texture
opacity color/texture - rgb
transparency value/texture
normal - tangent space normal map

color and texture values are multiplied together.
texture have texture coordinate index and and transform.

fbx (with openfbx)
---
diffuse color/texture
normal texture

it currently only exposes these parameters but there are much more here:
http://docs.autodesk.com/FBX/2014/ENU/FBX-SDK-Documentation/index.html?url=files/GUID-3E0DCCD0-5F9A-44D0-8D5E-423237D49DB6.htm,topicNumber=d30e10256
My assumption is that the realtime friendly parameters are similar to what opengex defines.