Trouble setting up a rectangular mirror

[mohanamisra]

Hello 👋!

I am trying to replicate a somewhat simple scene with a light (Lambertian emission pattern) at the top ("ceiling"), a body at the bottom ("floor") and a mirror on the right wall.

I have been trying to check if everything is set up properly using the irradiance sensor. The irradiance on the body should of course increase with the presence of the mirror which will reflect even more light onto it, and decrease with its absence.

This is functioning properly for spherical mirrors. But I want a rectangular mirror and can't get it to work, for some reason. I am assuming the problem lies somewhere in the coordinates/orientation of the mirror, but cannot pinpoint it properly.

Would appreciate any help !

scene.xml:

<scene version="3.0.0">
    <integrator type="path">
        <integer name="max_depth" value="20"/>
    </integrator>

<!--    CAMERA-->
    <sensor type="perspective">
        <float name="fov" value="60"/>
        <transform name="to_world">
            <lookat origin="0, 0, 10" target="0, 0, 0" up="0, 1, 0"/>
        </transform>
    </sensor>

<!--    LIGHT-->
    <shape type="sphere">
        <point name="center" x="0" y="4" z="0"/>
        <float name="radius" value="0.2"/>
        <emitter type="area">
            <rgb name="radiance" value="1000"/>
        </emitter>
    </shape>

<!--    ROBOT-->
    <shape type="sphere">
        <point name="center" x="0" y="0" z="0"/>
        <float name="radius" value="1"/>
        <bsdf type="diffuse">
            <rgb name="reflectance" value="0.0, 0.2, 0.75"/>
        </bsdf>
        <sensor type="irradiancemeter"/>
    </shape>

<!--    MIRROR-->
        <shape type="rectangle">
        <transform name="to_world">
            <translate x="3" y="0" z="0"/>
            <rotate x="0" y="1" z="0" angle="90"/>
            <scale x="4" y="4" z="1"/>
        </transform>
        <bsdf type="conductor">
            <string name="material" value="none"/>
        </bsdf>
        <sensor type="irradiancemeter"/>
    </shape>
</scene>

[merlinND]

Hello @mohanamisra,

When trying to set up a scene according to a specific layout like this, I think it's easiest to create it in Blender, and then export it with the mitsuba-blender add-on: https://github.com/mitsuba-renderer/mitsuba-blender

[mohanamisra]

Thanks a lot, that works! As a note, I did have to use Blender 4.3.2 to get the add-on to work.

However, the rendering speed as well as the speed with which the sensor values are read, is pretty slow and take around a minute or half. I am currently using the llvm_ad_rgb variant. I would need to check the value of the irradiance sensor multiple times (around 300-400) for different scene configurations.

Is there a way to optimise the speed when attempting to fetch the value of each sensor?

[merlinND]

The rendering speed typically depends on:

• The rendering resolution
• The sample count (spp)
• Your hardware (CPU when using llvm_*, GPU when using cuda_*, GPU is typically faster)