search

mmp / pbrt-v4

Source code to pbrt, the ray tracer described in the forthcoming 4th edition of the "Physically Based Rendering: From Theory to Implementation" book.
https://pbrt.org
Apache License 2.0
2.82k stars 436 forks source link

Meandering Motion Blur with Instancing #206

Open shadeops opened 2 years ago

shadeops commented 2 years ago

Fun one!

When motion blurring instances the objects seem to be going on a bit of a trip. In the image below, the red object is an instanced sphere, the blue has the same ActiveTransforms applied but is not an instance.

motion_blur 0219

Both spheres should be following the same path but the instanced sphere is being adventurous.

Example Scene

Film "rgb"
    "integer yresolution" [ 512 ]
    "integer xresolution" [ 512 ]
    "string filename" [ "motion_blur.0219.png" ]
PixelFilter "gaussian"
    "float yradius" [ 2 ]
    "float xradius" [ 2 ]
Sampler "zsobol"
    "integer pixelsamples" [ 128 ]
Integrator "volpath"
    "integer maxdepth" [ 5 ]

Accelerator "bvh"
Transform [ -1 0 0 0 0 0 -1 0 0 1 0 0 74 -6 6 1  ]
Camera "perspective"
    "float screenwindow" [ -1 1 -1 1 ]
    "float fov" [ 30 ]

WorldBegin

AttributeBegin
    Rotate -45 0 1 0
    Rotate 40 1 0 0
    LightSource "distant"
        "float scale" [ 10 ]
AttributeEnd

MakeNamedMaterial "red"
    "rgb reflectance" [ 1 0 0 ]
    "string type" [ "diffuse" ]

MakeNamedMaterial "blue"
    "rgb reflectance" [ 0 0 1 ]
    "string type" [ "diffuse" ]

ObjectBegin "test"
AttributeBegin
    NamedMaterial "red"
    Shape "sphere"
        "float radius" [ 0.5 ]
AttributeEnd
ObjectEnd

AttributeBegin
    ActiveTransform StartTime
        Translate 73 0.4 6.0
        Rotate -50 0 0 1
        Rotate -200 0 1 0
        Rotate -144 1 0 0
        Scale 0.123457 0.123457 0.123457
    ActiveTransform EndTime
        Translate 73.4 0.4 6.0
        Rotate -30 0 0 1
        Rotate -144 0 1 0
        Rotate -65 1 0 0
        Scale 0.123457 0.123457 0.123457
    ActiveTransform All 

    ObjectInstance "test"

    AttributeBegin
        NamedMaterial "blue"
        Shape "sphere"
            "float radius" [ 0.5 ]
    AttributeEnd
AttributeEnd
shadeops commented 2 years ago

My current theory is this is due to the way the inverse transforms are applied after the interpolation. ie) https://github.com/mmp/pbrt-v4/blob/47201aa309a6b6390f7ee148c892bb50adf29cec/src/pbrt/util/transform.h#L458-L463

As an example of how the curved path can occur -

Right side locators are the inverse of the left side. Moving locator on the right side is calculated by interpolating the transforms of the two left locators, then inverting. example_2

In this example, as before the right side locators are the inverse of the left side, however the moving locator on the right is calculated by first taking the inverse of each transform, then interpolating that result. example_1

shadeops commented 2 years ago

I think I narrowed this down further and at least fixed the "artifacts" that I was perceiving. The above theory was in the ballpark but not exactly right.

The issue relates to the space in which the interpolation happens,

https://github.com/mmp/pbrt-v4/blob/47201aa309a6b6390f7ee148c892bb50adf29cec/src/pbrt/scene.cpp#L370-L376

The interpolation has the worldFromRender applied. The quick hack that I did to address this was in the AnimatedTransform::Interpolate() to remove the worldFromRender transform, decompose both startTransform and endTransform, interpolate, then reapply the worldFromRender transform. Which caused this this to become a very expensive function, but at least validated what was causing the issue.

Master (47201aa30)

orig

AnimatedTransform::Interpolate() fix

fix

Example Scene

Film "rgb"
    "integer yresolution" [ 256 ]
    "integer xresolution" [ 256 ]
    "string filename" [ "motion_blur_simple.png" ]
PixelFilter "gaussian"
    "float yradius" [ 2 ]
    "float xradius" [ 2 ]
Sampler "zsobol"
    "integer pixelsamples" [ 128 ]
Integrator "volpath"
    "integer maxdepth" [ 1 ]

Accelerator "bvh"

Translate 0 0 25

Camera "perspective"
    "float screenwindow" [ -1 1 -1 1 ]
    "float fov" [ 30 ]

WorldBegin

AttributeBegin
    Rotate -45 0 1 0
    Rotate 40 1 0 0
    LightSource "distant"
        "float scale" [ 10 ]
AttributeEnd

MakeNamedMaterial "red"
    "rgb reflectance" [ 1 0 0 ]
    "string type" [ "diffuse" ]

MakeNamedMaterial "blue"
    "rgb reflectance" [ 0 0 1 ]
    "string type" [ "diffuse" ]

ObjectBegin "test"
AttributeBegin
    Identity
    NamedMaterial "red"
    Shape "sphere"
        "float radius" [ 0.5 ]
AttributeEnd
ObjectEnd

AttributeBegin
    ActiveTransform StartTime
    Translate 0 5 0
    Rotate -90 0 1 0
    ActiveTransform EndTime
    Translate 0 -5 0
    Rotate 90 0 1 0
    ActiveTransform All

    ObjectInstance "test"

#    AttributeBegin
#        NamedMaterial "blue"
#        Shape "sphere"
#            "float radius" [ 0.5 ]
#    AttributeEnd
AttributeEnd

I didn't submit a PR for this because I kinda made a mess of the API by stashing the worldFromRender in AnimatedTransform :grimacing:

shadeops commented 2 years ago

This can also be addressed on the scene creation side as well. For example using the current master - 315b28bd1 we get -

inst_space

But if we apply the inverse of the camera's transform in the ObjectBegin description, then reapply it prior to calling ObjectInstance we get - camera_inst_space

Film "rgb"
    "integer yresolution" [ 256 ]
    "integer xresolution" [ 256 ]
    "string filename" [ "motion_blur_simple.png" ]
PixelFilter "gaussian"
    "float yradius" [ 2 ]
    "float xradius" [ 2 ]
Sampler "zsobol"
    "integer pixelsamples" [ 512 ]
Integrator "volpath"
    "integer maxdepth" [ 1 ]

Accelerator "bvh"

# XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
# Camera's xform
Rotate -20 1 0 0
Rotate 15 0 1 0
Translate -1.5 -3 6
# XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Camera "perspective"
    "float screenwindow" [ -1 1 -1 1 ]
    "float fov" [ 45 ]

WorldBegin

AttributeBegin
    LightSource "infinite"
AttributeEnd

MakeNamedMaterial "red"
    "rgb reflectance" [ 1 0 0 ]
    "string type" [ "diffuse" ]

MakeNamedMaterial "blue"
    "rgb reflectance" [ 0 0 1 ]
    "string type" [ "diffuse" ]

ObjectBegin "test"
    AttributeBegin

        # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        # Apply inverse of the camera's xform
        Translate 1.5 3 -6
        Rotate -15 0 1 0
        Rotate 20 1 0 0
        # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

        NamedMaterial "red"
        Shape "trianglemesh"
            "integer indices" [ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 
                                21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 ]
            "point3 P" [ 0.5 -0.5 0.5 -0.5 -0.5 0.5 -0.5 0.5 0.5 -0.5 -0.5 -0.5 0.5 -0.5 
                         -0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 0.5 0.5 -0.5 0.5 0.5 0.5 0.5 
                         -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 
                         0.5 0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 -0.5 -0.5 -0.5 0.5 -0.5 -0.5 
                         0.5 -0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 0.5 0.5 0.5 0.5 0.5 -0.5 
                         0.5 -0.5 -0.5 -0.5 -0.5 0.5 0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 0.5 
                         0.5 -0.5 0.5 0.5 -0.5 0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 -0.5 
                         -0.5 -0.5 -0.5 0.5 0.5 0.5 0.5 0.5 0.5 -0.5 0.5 ]
    AttributeEnd
ObjectEnd

AttributeBegin
    Rotate 90 1 0 0
    Shape "disk"
        "float radius" [ 100 ]
AttributeEnd

AttributeBegin
    ActiveTransform StartTime
        Translate 0 1.5 0
    ActiveTransform EndTime
        Translate 1.5 0.5 0
        Rotate -90 0 0 1
    ActiveTransform All

    # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
    # Reapply camera's xform
    Rotate -20 1 0 0
    Rotate 15 0 1 0
    Translate -1.5 -3 6
    # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    ObjectInstance "test"
AttributeEnd

AttributeBegin

    ActiveTransform StartTime
        Translate 0 1.5 0
    ActiveTransform EndTime
        Translate 1.5 0.5 0
        Rotate -90 0 0 1
    ActiveTransform All

    NamedMaterial "blue"
    Shape "trianglemesh"
        "integer indices" [ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 
                            21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 ]
        "point3 P" [ 0.5 -0.5 0.5 -0.5 -0.5 0.5 -0.5 0.5 0.5 -0.5 -0.5 -0.5 0.5 -0.5 
                     -0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 0.5 0.5 -0.5 0.5 0.5 0.5 0.5 
                     -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 
                     0.5 0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 -0.5 -0.5 -0.5 0.5 -0.5 -0.5 
                     0.5 -0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 0.5 0.5 0.5 0.5 0.5 -0.5 
                     0.5 -0.5 -0.5 -0.5 -0.5 0.5 0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 0.5 
                     0.5 -0.5 0.5 0.5 -0.5 0.5 -0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 -0.5 
                     -0.5 -0.5 -0.5 0.5 0.5 0.5 0.5 0.5 0.5 -0.5 0.5 ]
AttributeEnd