[🐛 bug report] Apparent medium scattering difference between gpu_spectral and scalar_spectral renders.

[einburger]

Summary

The gpu_spectral variant's output seems to lack the medium scattering effects that the same scene rendered with the scalar_spectral variant has.

System configuration

• Platform: CentOS 7
• Compiler: GCC 9.3.1
• Mitsuba version: 2.2.1
• Python version: 3.6.8
• Compiled variants:
  • scalar_spectral
  • gpu_spectral

Description

I created a basic scene with three shapes. Two of the shapes are boxes with a diffuse bsdf and and the last shape is a box acting as an area emitter that uses a bitmap for spatially varying radiance values. The sensor is a perspective lens that has a homogeneous medium attached to it for a smokey environment effect. When using the scalar_spectral variant the scene looks as one would expect (lots of scattering due to the medium) but when using the gpu_spectral variant the scene lacks this scattering.

Things tried (but didn't change the difference):

• Specifying different sample counts, samples per pass, and sampler types
• Swapped the volpath integrator for the volpathmis integrator
• Removing the emissive texture and using a single radiance value for the emitter

Curiosities:

• When I use an area emitter with a bitmap mitsuba generates warnings about not finding a sensor or integrator but when I remove the bitmap and just rely on a value for the radiance, the warnings disappear
• The profiler output shows the gpu variant spends a lot less time on medium sampling than the scalar variant (0.35% vs. 10.95%)

With the scalar_spectral variant:

With the gpu_spectral variant:

With the scalar_spectral variant with no emissive "texture" (just a radiance value):

With the gpu_spectral variant with no emissive "texture" (just a radiance value):

Here's my xml scene file:

<scene version="2.1.0">
	<default name="resx" value="512"/>
	<default name="resy" value="512"/>
	<default name="spp" value="64"/>

	<integrator type="volpath" id="integrator">
		<integer name="samples_per_pass" value="4"/>
		<integer name="max_depth" value="8"/>
	</integrator>
	<sensor type="perspective" id="sensor">
		<float name="near_clip" value="0.100000"/>
		<float name="far_clip" value="10000.000000"/>
		<transform name="to_world">
			<rotate x="1" angle="118.66886777196044"/>
			<rotate y="1" angle="4.4630245556769734e-07"/>
			<rotate z="1" angle="165.76382806152043"/>
			<translate value="-1.509772 -6.421305 0.993045"/>
		</transform>
		<film type="hdrfilm">
			<rfilter type="lanczos"/>
			<integer name="width" value="$resx"/>
			<integer name="height" value="$resy"/>
		</film>
		<sampler type="independent">
			<integer name="sample_count" value="$spp"/>
		</sampler>
		<string name="focal_length" value="10.0mm"/>
		<medium type="homogeneous">
			<integer name="scale" value="1"/>
			<spectrum value="0.200000" name="sigma_t"/>
			<spectrum value="0.700000" name="albedo"/>
			<phase type="isotropic"/>
		</medium>
	</sensor>

	<shape type="obj" id="shape1">
		<string name="filename" value="meshes/LightBox.obj"/>
		<transform name="to_world">
			<matrix value="1.000000 0.000000 0.000000 0.000000 0.000000 1.000000 0.000000 0.000000 0.000000 0.000000 1.000000 0.000000 0.000000 0.000000 0.000000 1.000000"/>
		</transform>
		<bsdf type="diffuse"/>
		<emitter type="area">
			<texture type="bitmap" name="radiance">
				<string name="filename" value="textures/lbox_tex.pfm"/>
			</texture>
		</emitter>
	</shape>

	<shape type="obj" id="shape2">
		<string name="filename" value="meshes/LBox.obj"/>
		<transform name="to_world">
			<matrix value="1.000000 0.000000 0.000000 0.000000 0.000000 1.000000 0.000000 0.000000 0.000000 0.000000 1.000000 0.000000 0.000000 0.000000 0.000000 1.000000"/>
		</transform>
		<bsdf type="diffuse">
			<spectrum value="0.500000" name="reflectance"/>
		</bsdf>
	</shape>
	<shape type="obj" id="shape3">
		<string name="filename" value="meshes/RBox.obj"/>
		<transform name="to_world">
			<matrix value="1.000000 0.000000 0.000000 0.000000 0.000000 1.000000 0.000000 0.000000 0.000000 0.000000 1.000000 0.000000 0.000000 0.000000 0.000000 1.000000"/>
		</transform>
		<bsdf type="diffuse">
			<spectrum value="0.300000" name="reflectance"/>
		</bsdf>
	</shape>
</scene>

Steps to reproduce

1. Set up a basic spectral scene
2. Attach a medium to the scene's sensor
3. Give one shape in the scene an area emitter with a bitmap texture for its radiance
4. Render the scene with scalar_spectral and gpu_spectral

[Speierers]

Hi @einburger ,

If I remember correctly there are currently some known issues regarding volume rendering with gpu_* variants. Unfortunately we are focussing on the release of the next version of Mitsuba, which will better support volume rendering.