Debug volume-caustic scene

[wahn]

For debugging we use a lower resolution and the bdpt integrator, instead of mlt:

diff --git a/assets/scenes/caustic.pbrt b/assets/scenes/caustic.pbrt                                                                        
index 382670a..c17389a 100644                                                                                                               
--- a/assets/scenes/caustic.pbrt                                                                                                            
+++ b/assets/scenes/caustic.pbrt                                                                                                            
@@ -3,11 +3,12 @@ MakeNamedMedium "vol"                                                                                                     
         "rgb sigma_a" [ 0.0199999996 0.0199999996 0.0199999996 ]                                                                           
         "rgb sigma_s" [ 0.3499999940 0.3499999940 0.3499999940 ]                                                                           
 Film "image"                                                                                                                               
-        "integer xresolution" [ 800 ]                                                                                                      
-        "integer yresolution" [ 800 ]                                                                                                      
-        "string filename" [ "caustic.exr" ]                                                                                                
-Integrator "mlt"                                                                                                                           
-        "integer mutationsperpixel" [ 16384 ]                                                                                              
+        "integer xresolution" [ 400 ]                                                                                                      
+        "integer yresolution" [ 400 ]                                                                                                      
+        "string filename" [ "f16-22a.exr" ]                                                                                                
+Sampler "02sequence"                                                                                                                       
+        "integer pixelsamples" [ 64 ]                                                                                                      
+Integrator "bdpt"                                                                                                                          
 MediumInterface "" "vol"                                                                                                                   
 LookAt -0.75 0.800000012 -1.25                                                                                                             
         -1 0.800000012 0                                                                                                                   
@@ -30,7 +31,7 @@ AttributeEnd                                                                                                              
 AttributeBegin                                                                                                                             
     Rotate -60 0 0 1                                                                                                                       
     LightSource "infinite"                                                                                                                 
-            "string mapname" [ "textures/skylight-day.hdr" ]                                                                               
+            "string mapname" [ "textures/skylight-day.exr" ]                                                                               
             "rgb scale" [ 1.5000000000 1.5000000000 1.3999999762 ]                                                                         
 AttributeEnd                                                                                                                               
 LightSource "spot"                                                                                                                         

We can render the scene with bdpt (bi-directional path tracing) but the results of the C++ and Rust versions render very differently:

> imf_diff -d f16-22a_cpp.png f16-22a_rust.png diff.jpg
differing pixels:	100.000% (160000 of 160000)
average difference:	 51.499%
maximum difference:	 86.603%
Summary: Many pixels differ strongly.
== "f16-22a_cpp.png" and "f16-22a_rust.png" are different

[wahn]

$ git diff assets/scenes/caustic.pbrt
diff --git a/pbrt/assets/scenes/caustic.pbrt b/pbrt/assets/scenes/caustic.pbrt
index c17389a..972e6a8 100644
--- a/pbrt/assets/scenes/caustic.pbrt
+++ b/pbrt/assets/scenes/caustic.pbrt
@@ -8,7 +8,7 @@ Film "image"
         "string filename" [ "f16-22a.exr" ] 
 Sampler "02sequence" 
         "integer pixelsamples" [ 64 ] 
-Integrator "bdpt" 
+Integrator "path" 
 MediumInterface "" "vol"
 LookAt -0.75 0.800000012 -1.25
         -1 0.800000012 0

Let's first focus on the difference between the C++ version and the Rust version using path tracing, which doesn't support volume scattering:

The left side is rendered by C++, the right by the current Rust version. Here is the difference:

differing pixels:	  1.335% (2136 of 160000)
average difference:	 23.079%
maximum difference:	 86.603%
Summary: Some pixels differ strongly.
== "volume_caustic_path_cpp.png" and "volume_caustic_path_rust.png" are different

[wahn]

Since the last commit (4db2f1f) there should be no difference between the C++ and the Rust version (using path tracing):

$ grep Integrator assets/scenes/caustic.pbrt
Integrator "path"
$ imf_diff -d -f pbrt_cpp.png pbrt_rust.png diff.jpg
pbrt_cpp.png pbrt_rust.png: no differences.
== "pbrt_cpp.png" and "pbrt_rust.png" are identical

[wahn]

Now we have to focus on the two remaining problems:

The left side is rendered by C++, the right by the current Rust version. Both use bdpt (bi-directional path tracing). Here is the difference:

differing pixels:	100.000% (160000 of 160000)
average difference:	 51.525%
maximum difference:	 86.603%
Summary: Many pixels differ strongly.
== "volume_caustic_bdpt_cpp.png" and "volume_caustic_bdpt_rust.png" are different

1. While using path tracing we get the contribution of the image used with the infinite light source, during bdpt those pixels where no geometry gets hit render black.
2. The homogeneous medium is not visible in the Rust version.

But maybe both problems are related.

[wahn]

Background pixels render fine now (after b181dac) with bdpt:

> imf_diff -f -d pbrt_cpp.png pbrt_rust.png diff.jpg
differing pixels:	 83.276% (133241 of 160000)
average difference:	  6.899%
maximum difference:	 61.337%
Summary: Many pixels differ strongly.
== "pbrt_cpp.png" and "pbrt_rust.png" are different

We are still missing the caustics within the homogeneous medium though ...

[wahn]

After commit cd4f481 we still do not see any caustics:

> imf_diff -d -f pbrt_cpp.png pbrt_rust.png diff.jpg
differing pixels:	 90.904% (145447 of 160000)
average difference:	  8.160%
maximum difference:	 61.337%
Summary: Many pixels differ strongly.
== "pbrt_cpp.png" and "pbrt_rust.png" are different

[wahn]

After commit 043d25f the result got slightly better, but still no caustics:

> imf_diff -d -f pbrt_cpp.png pbrt_rust.png diff.jpg
differing pixels:	 77.629% (124207 of 160000)
average difference:	  7.115%
maximum difference:	 59.770%
Summary: Many pixels differ strongly.
== "pbrt_cpp.png" and "pbrt_rust.png" are different

[wahn]

After commit 5958635 we can finally render caustics with bdpt:

Let's neglect the differences for now:

> imf_diff -d -f pbrt_cpp.png pbrt_rust.png diff.jpg
differing pixels:	  1.002% (1603 of 160000)
average difference:	  2.784%
maximum difference:	 36.462%
Summary: Some pixels differ strongly.
== "pbrt_cpp.png" and "pbrt_rust.png" are different

Let's go back to the task to implement the MLTIntegrator now (see issue #43).