🐛 Bounding box of volume grid is showing in renders

[vucenovic]

Hello, I am trying to run a volume optimization with a slightly modified version of mitsuba. When rendering my reference picutres or gradients, the edges of my meshes are always shown in the render as if they would absorb energy.

Here is an example:

Here is the relevant scene definition:

   	<float name="scale" value="$scalematobj"/>
   	<volume name="sigma_t" type="gridvolume">
   		<transform name="to_world">
               <translate x="-0.5" y="-0.5" z="-0.5"/>
   			<scale x="0.01" y="0.01" z="0.01"/>
   			<translate x="0" y="$od" z="0"/>
   			<rotate y="1" angle="-90"></rotate>
           </transform>
   		<boolean name="use_grid_bbox" value="false" />
   		<boolean name="raw" value="true"/>
   		<string name="filename" value="sigmat_gt_pyramide_randomized_0001.vol" />
   	</volume>
   	<!-- <volume name="albedo" type="gridvolume">
   		<transform name="to_world">
   			<translate x="0" y="$od" z="0"/>
               <translate x="-0.5" y="-0.5" z="-0.5"/>
   			<scale x="0.01" y="0.01" z="0.01"/>
           </transform>
   		<boolean name="use_grid_bbox" value="false" />
   		<string name="filename" value="albedo.vol" />
   	</volume> -->
   	<phase type="isotropic" />
   </medium>


   <shape type="obj">
   	<string name="filename" value="unit_cube_centered.obj"/>
       <transform name="to_world">
           <scale x="0.01" y="0.01" z="0.01"/>
   		<!-- <rotate x="1" angle="45.0"/> -->
   		<translate x="0" y="$od" z="0"/>
       </transform>
   	<bsdf type="null"></bsdf><!--
   		<medium type="homogeneous" name="interior">
   			<float name="scale" value="$scalematobj"/>
   			<spectrum name="sigma_t" filename="$matobj"/>
   			<spectrum name="albedo" value="0.0"/>
   			<phase type="isotropic"/>
   		</medium>
               <ref id="volume" name="interior"/>
     </shape>

  Am I doing something wrong here, or how could this problem otherwise occure?