read roughness from pincipled bsdf

[zichenwang01]

Hi,
I want to read specifically the roughness value from Mitsuba's principled bsdf within my custom renderer. I see there is a build-in function for reflectance

albedo = bsdf.eval_diffuse_reflectance(itx)

I wonder how can I do it for roughness? Thanks!

[njroussel]

Hi @zichenwang01

You should be able to do bsdf.eval_attribute_1('roughness', si active).
The eval_attribute_N methods can evaluate any attribute/parameter that you would typically get when using mi.traverse(). The N serves as an indicator for the number of dimensions of the desired output value.

[zichenwang01]

Thanks for this very helpful information! I have a bit of trouble calling this function inside Python--

AttributeError: 'mitsuba.cuda_ad_rgb.BSDFPtr' object has no attribute 'eval_attribute_1'

where my code looks like

itx : SurfaceInteraction3f = scene.ray_intersect(ray)
bsdf : mi.BSDF = itx.bsdf()
roughness = bsdf.eval_attribute_1('roughness', itx)

[njroussel]

Hi @zichenwang01

Sorry for the delay. Have you figured this out?
I just gave it another go, and it seems to run without any issues:

import mitsuba as mi
import drjit as dr

mi.set_variant('cuda_ad_rgb')

scene = mi.load_dict(mi.cornell_box())
ray = mi.Ray3f(o=[0, 0, 3], d=[0, 0, -1])
si = scene.ray_intersect(ray)
bsdf = si.bsdf()
roughness = bsdf.eval_attribute_1('roughness', si)
print(f"{roughness=}") # return 0 but at least there aren't any errors

[Start1er]

Hi, I was also confusing in this, so I may need for some help. For diffuse reflectence, put it onto the image was simple in the following code:

            block = sensor.film().create_block()
            block.set_coalesce(block.coalesce() and spp >= 4)
            diffuse_alpha = dr.select(active, mi.Float(1), mi.Float(0))
            block.put(pos, ray.wavelengths, diffuse, diffuse_alpha)

This was like you do in Integrator render function. But for single channel like roughness or depth, could there be some way to put it on to a imageblock and output? Any response would be helpful. I try simple put single channel var into RGB image block, but I was scared about the value could be changed automaticlly when I extend one channel attribute into RGB image block. The same discusstion I have created #1394
Any response would be helpful!