PBRMaterial <-> SimpleMaterial conversion seems to be asymmetric

[ankuPRK]

Hello,

I see the material conversion from SimpleMaterial to PBRMaterial here, where we convert glossiness into roughness:

trimesh/trimesh/visual/material.py

Lines 198 to 215 in 634c608

	def to_pbr(self):
	"""
	Convert the current simple material to a
	PBR material.
	Returns
	------------
	pbr : PBRMaterial
	Contains material information in PBR format.
	"""
	# convert specular exponent to roughness
	roughness = (2 / (self.glossiness + 2)) ** (1.0 / 4.0)
	return PBRMaterial(
	roughnessFactor=roughness,
	baseColorTexture=self.image,
	baseColorFactor=self.diffuse,
	)

But the PBRMaterial to SimpleMaterial conversion here doesn't map roughness -> specular:

trimesh/trimesh/visual/material.py

Lines 675 to 686 in 634c608

	def to_simple(self):
	"""
	Get a copy of the current PBR material as
	a simple material.
	Returns
	------------
	simple : SimpleMaterial
	Contains material information in a simple manner
	"""
	return SimpleMaterial(image=self.baseColorTexture, diffuse=self.baseColorFactor)

The conversion formula seems to be reversible. Any insights about why we have this asymmetry would be great.

Thanks!

[mikedh]

Hey yeah agreed that seems like it would be pretty easy and would make sense to be bidirectional, it's just not implemented. PR's with a fix and test would be super welcome!!