direct_reparam

[zichenwang01]

Hi,

I used the built-in direct_reparam to run some experiments but could not get the correct result.

My scene consists of a sphere and a plane at the back. The camera is at the front, and the point light is also at the front.

<!-- INTEGRATOR -->
<integrator type="direct_reparam">
    <integer name="max_depth" value="2"/>
</integrator>

<!-- CAMERA -->
<sensor type="perspective">
    <float name="fov" value="45"/>

    <!-- init camera position -->
    <transform name="to_world">
        <lookat origin="0.5,0.5,1" target="0.5,0.5,0.5" up="0,1,0"/>
    </transform>

    <!-- init sampler -->
    <sampler type="independent">
        <!-- samples per pixel -->
        <integer name="sample_count" value="32"/>
    </sampler>

    <!-- init film -->
    <film type="hdrfilm">
        <integer name="width" value="512"/>
        <integer name="height" value="512"/>
        <rfilter type="gaussian">
            <float name="stddev" value="0.5"/>
        </rfilter>
        <boolean name="sample_border" value="true"/>
    </film>
</sensor>

<!-- OBJECTS -->
<shape type="sphere">
    <float name="radius" value="0.1"/>
    <point name="center" x="0.5" y="0.5" z="0.5"/>
</shape>

<shape type="rectangle" id="Background">
    <transform name="to_world">
        <scale value="1.0"/>
        <lookat origin="0.5,0.5,0" target="0.5,0.5,0.5" up="0,1,0"/>
    </transform>
</shape>


<!-- LIGHT -->
<!-- point light -->
<emitter type="point">
    <spectrum name="intensity" value="1"/>
    <point name="position" x="0.5" y="0.5" z="1"/>
</emitter>

Now I have a single parameter that moves the plane to its right and want to forward it to the rendered image. In theory, I should expect the image gradient to be zero everywhere, but instead I see a circle of gradients around the sphere.

"""move right plane"""
params = mi.traverse(scene)
dr.enable_grad(params)

p = mi.Float(1e-3)
transform : mi.Transform4f = params['Background.to_world']
transform.matrix[0,3] += p
params['Background.to_world'] = transform
params.update()

image = mi.render(scene, params, spp=64)
dr.enable_grad(image)
dr.set_grad(p, 1)
dr.forward_to(image)
grad = dr.grad(image)

Am I misusing direct_reparam? Many thanks in advance.

[njroussel]

Hi @zichenwang01

No, this is the correct utilization. I believe this is simply a limitation of the method (project page).