Applying different materials to submeshes. (obj)

My obj file contains two objects that are body and floor.

body.jpg and floor.png files should be mapped to body and floor respectively.

Body part renders OK.

But, floor portion always renders in black.

In particular, floor.png file contains shadow and the rest area is transparent and floor mesh is a simple plain with just 4 vertices.

Any idea why its floor renders always in black?

Here is code snipet and info from obj and mtl files.

let scatterFuncFloor = MDLScatteringFunction()
let floorProperty = MDLMaterialProperty(name: floorTextureFile, semantic: MDLMaterialSemantic.baseColor, url: floorTextureURL)
let floorMaterial = MDLMaterial(name: NOLOC("FLOOR"), scatteringFunction: scatterFuncFloor)
floorMaterial.setProperty(floorProperty)
.............

        for submesh in modelObj.submeshes! {
            if let submesh = submesh as? MDLSubmesh {
                if (index == 0) { // Floor
                    submesh.material = floorMaterial
                    print(" submesh[0] name:", submesh.name)
                    index += 1
                }
                else { // body 
                    submesh.material = material
                    print(" submesh[1] name:", submesh.name)
                }
            }
        }

Here is material info.

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

newmtl FLOOR

Kd 0.800000 0.800000 0.800000

illum 1

map_Kd floor.png

map_d floor.png

newmtl BODY

Ka 1.000000 1.000000 1.000000

Kd 0.800000 0.800000 0.800000

illum 1

map_Ka body.jpg

map_Kd body.jpg

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

From obj file

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

o Plane.1

v -10 0 10

v 10 0 10

v -10 0 -10

v 10 0 -10

# 4 vertices

vt 0.00000 0.00000

vt 0.00000 1.00000

vt 1.00000 1.00000

vt 1.00000 0.00000

# 4 texture coordinates

vn 0 1 -0

# 1 vertex normals

usemtl FLOOR

f 2/4/1 4/3/1 3/2/1 1/1/1

# 1 facets

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

PS:

By the way,

If I put the obj into SCN file and load the scene from SCN file, then it renders the floor OK, though.