Question about BRDFS

[saeedhd96]

I have two questions regarding the BRDFs:

1. Is it possible to represent roughplastic alpha also using a texture or it is only float?
2. For the principled BSDF, what parameters should create perefectly diffuse? I can never get perfectly diffuse surface using principled.

For example:

import mitsuba as mi
scene = mi.load_dict(mi.cornell_box())
brdf_1 = mi.load_dict({'type': 'principled',
'base_color': {
    'type': 'rgb',
    'value': [0.8, 0.8, 0.8],
},
'flatness': 0.0,
'specular': 0.0,
'roughness': 1.0
})
brdf_2 = mi.load_dict({'type': 'diffuse',
'reflectance': {
    'type': 'rgb',
    'value': [0.8, 0.8, 0.8]
}})
rays = mi.Ray3f(o = 0.0, d = mi.warp.square_to_uniform_sphere(0.5))
si = scene.ray_intersect(rays)
s1 =  mi.Float(0.5)
s2 =  mi.Point2f(0.5)
_, principled_W = brdf_1.sample(mi.BSDFContext(), si, s1, s2)
_, diffuse_W = brdf_2.sample(mi.BSDFContext(), si, s1, s2)
print(principled_W)
print(diffuse_W)

The output of this is
[[1.3195570707321167, 1.3195570707321167, 1.3195570707321167]]
[[0.800000011920929, 0.800000011920929, 0.800000011920929]]

which means the principled is not producing perfectly diffuse weights ...

[saeedhd96]

OK, I guess by definition it has two lobes and thus never will be perfectly diffuse.

[Speierers]

Is it possible to represent roughplastic alpha also using a texture or it is only float?

This will depend on the BSDF. Take a look at the plugin reference. If the Type of the parameter says texture, it means that this parameter can be textured.

For the principled BSDF, what parameters should create perefectly diffuse?

I believe you can make it perfectly diffuse by setting the specular parameter to 0.0.

[saeedhd96]

But in my example above I have set the specular to 0.0 and it still produces non diffuse results. Could that be a bug then?

[Speierers]

According to this diagram, you will always get the retro-reflection with the diffuse. That's probably the reason why this doesn't match the simple diffuse BSDF.