Incorrect Motion Blur with SPPM

[paulgrassler]

Hey there,
I am a bit confused, hopefully someone can help me. I am trying to write my own SPPM implementation and currently I am working on Motion Blur. To have some kind of reference, I took a look at how PBRT implements this. From what I understand, you sample a time value for each camera ray and photon ray independently and that is what I initially tried as well. What I found, however, is that the Motion Blur with SPPM seems incorrect when doing this. At least it does not match the same scene rendered with the path integrator and I am pretty sure they should not look this different, should they? Is this known or am I misunderstanding something? I will include my scene description and the 2 renders (one with sppm [bottom one] and one with path [top one]).

As you can see, the sphere is generally darker and the blur certainly looks somewhat weird for SPPM. I would appreciate any clarification on this! Also, if this is indeed a known issue, how would you go about implementing it correctly?

Thank you for your help!

Scene:

Integrator "path"
#Integrator "sppm"
#    "float radius" [ 0.005 ]
Transform [ 1 -0 -0 -0 -0 1 -0 -0 -0 -0 -1 -0 -0 -1 6.8 1  ]
Sampler "paddedsobol"
    "integer pixelsamples" [ 64 ]
Film "rgb"
    "string filename" [ "cornell-box.png" ]
    "integer yresolution" [ 1024 ]
    "integer xresolution" [ 1024 ]
Camera "perspective"
    "float fov" [ 19.5 ]
TransformTimes 0 1


WorldBegin

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    "rgb reflectance" [ 0.75 0.25 0.25 ]
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    "string type" [ "diffuse" ]
    "rgb reflectance" [ 0.725 0.71 0.68 ]
MakeNamedMaterial "Ceiling"
    "string type" [ "diffuse" ]
    "rgb reflectance" [ 0.725 0.71 0.68 ]
MakeNamedMaterial "BackWall"
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    "rgb reflectance" [ 0.725 0.71 0.68 ]
MakeNamedMaterial "ShortBox"
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    "rgb reflectance" [ 0.25 0.75 0.25 ]
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    "rgb reflectance" [ 0.25 0.75 0.25 ]
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    "rgb reflectance" [ 0 0 0 ]
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	Translate 0 1.9 0
	AreaLightSource "diffuse" "rgb L" [ 40 40 40 ] 
	NamedMaterial "Light" 
	Shape "sphere" "float radius" [ 0.1 ] 
AttributeEnd
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    "point3 P" [ -1 0 1 -1 2 1 -1 2 -1 -1 0 -1 ]
    "integer indices" [ 0 1 2 0 2 3 ]

AttributeBegin
    NamedMaterial "ShortBox"
    Translate 0 0.2 0
    ActiveTransform EndTime
    Translate 0 0.2 0
    Shape "sphere"
        "float radius" [ 0.2 ]
AttributeEnd

[paulgrassler]

Hey, I implemented a straightforward fix for this and created a pull request #441. Please take a look! Instead of sampling separate timestamps for each ray and photon, one time sample is taken at the beginning of the SPPM iteration and used for all rays and photons. Each iteration therefore estimates the radiance in the scene at that timestamp and because of the increasing number of iterations/samples, it converges to the correct results and avoids sampling artifacts. I am fairly certain that this is also how they did it in the original paper.

In the pull request, I included images of the scene above with and without the fix compared to the path tracer ground truth. I also included the contact information of my supervisor (Reinhold Preiner) and myself (Paul Graßler). I found this issue while working on a seminar paper about Photon Mapping at the Institute of Computer Graphics and Knowledge Visualisation, Graz University of Technology.

[pbrt4bounty]

I checked here and, indeed, the result seems very accurate.