Rendering an irradiance map

[Gzhji]

Dear Mitsuba community,

I am exploring the Irradiance meter, and I am hoping to get an irradiance map for the rendered image. Taking the cbox as the example (as shown below), I was wondering how could we render an image that showing per-pixel irradiance value? I assume we can render a grayscale image with irradiance value?

Thanks in advance!

Guanzhou

<scene version="3.0.0">
    <default name="spp" value="128"/>
    <default name="res" value="256"/>
    <default name="max_depth" value="6"/>
    <default name="integrator" value="path"/>

    <integrator type='$integrator'>
        <integer name="max_depth" value="$max_depth"/>
    </integrator>

    <sensor type="perspective" id="sensor">
        <string name="fov_axis" value="smaller"/>
        <float name="near_clip" value="0.001"/>
        <float name="far_clip" value="100.0"/>
        <float name="focus_distance" value="1000"/>
        <float name="fov" value="39.3077"/>
        <transform name="to_world">
            <lookat origin="0,  0,  4"
                    target="0,  0,  0"
                    up    ="0,  1,  0"/>
        </transform>
        <sampler type="independent">
            <integer name="sample_count" value="$spp"/>
        </sampler>
        <film type="hdrfilm">
            <integer name="width"  value="$res"/>
            <integer name="height" value="$res"/>
            <rfilter type="tent"/>
            <string name="pixel_format" value="rgb"/>
            <string name="component_format" value="float32"/>
        </film>
    </sensor>

    <!-- BSDFs -->

    <bsdf type="diffuse" id="gray">
        <rgb name="reflectance" value="0.85, 0.85, 0.85"/>
    </bsdf>

    <bsdf type="diffuse" id="white">
        <rgb name="reflectance" value="0.885809, 0.698859, 0.666422"/>
    </bsdf>

    <bsdf type="diffuse" id="green">
        <rgb name="reflectance" value="0.105421, 0.37798, 0.076425"/>
    </bsdf>

    <bsdf type="diffuse" id="red">
        <rgb name="reflectance" value="0.570068, 0.0430135, 0.0443706"/>
    </bsdf>

    <bsdf type="dielectric" id="glass"/>

    <bsdf type="conductor" id="mirror"/>

    <!-- Light -->

    <shape type="obj" id="light">
        <string name="filename" value="meshes/cbox_luminaire.obj"/>
        <transform name="to_world">
            <translate x="0" y="-0.01" z="0"/>
        </transform>
        <ref id="white"/>
        <emitter type="area">
            <rgb name="radiance" value="18.387, 13.9873, 6.75357"/>
        </emitter>
    </shape>

    <!-- Shapes -->

    <shape type="obj" id="floor">
        <string name="filename" value="meshes/cbox_floor.obj"/>
        <ref id="white"/>
    </shape>

    <shape type="obj" id="ceiling">
        <string name="filename" value="meshes/cbox_ceiling.obj"/>
        <ref id="white"/>
    </shape>

    <shape type="obj" id="back">
        <string name="filename" value="meshes/cbox_back.obj"/>
        <ref id="white"/>
    </shape>

    <shape type="obj" id="greenwall">
        <string name="filename" value="meshes/cbox_greenwall.obj"/>
        <ref id="green"/>
    </shape>

    <shape type="obj" id="redwall">
        <string name="filename" value="meshes/cbox_redwall.obj"/>
        <ref id="red"/>
    </shape>

    <shape type="sphere" id="mirrorsphere">
        <transform name="to_world">
            <scale value="0.5"/>
            <translate x="-0.3" y="-0.5" z="0.2"/>
        </transform>
        <ref id="mirror"/>
    </shape>

    <shape type="sphere" id="glasssphere">
        <transform name="to_world">
            <scale value="0.25"/>
            <translate x="0.5" y="-0.75" z="-0.2"/>
        </transform>
        <ref id="glass"/>
    </shape>
</scene>

[njroussel]

Hi @Gzhji

I can't think of a straightforward way of doing something like this. The irradiancemeter attaches to an entire shape, so you'd need split shapes depending on the camera view, and the stitch back togeteher all the individual 1x1 films of the irradiancemeters into one larger film size corresponding to your perspective sensor's one.
Also, a single pixel can cover more than a single shape, would you want the combined result?

If I were to do this, I would trace a single ray in the middle of each pixel and get back the set of intersection points. Then, I would use a custom spherical sensor with a 1x1 film that I place at one of those intersection points, and repeat this for every intersection point. Essentially I'd be building tiny irradiancemeters at each point. The last step would then be to stitch together all of those 1x1 films into the original film.
Obviously, only using a single point per pixel isn't totally accurate, you might want to sample many of them. But I guess that depends on your use case.

[Gzhji]

Thanks njroussel!

Meanwhile, I am trying to figure out the scene rendered with irradiancemeter, and I only apply the sensor to the sphere (at the end of xml file).
Did I render it correctly? (I assume not) It looks like standard rendering.
I am just curious about what the result from irradancemeter looks like.

Thanks!

<scene version="3.0.0">
    <default name="spp" value="128"/>
    <default name="res" value="256"/>
    <default name="max_depth" value="6"/>
    <default name="integrator" value="path"/>

    <integrator type='$integrator'>
        <integer name="max_depth" value="$max_depth"/>
    </integrator>

    <sensor type="perspective" id="sensor">
        <string name="fov_axis" value="smaller"/>
        <float name="near_clip" value="0.001"/>
        <float name="far_clip" value="100.0"/>
        <float name="focus_distance" value="1000"/>
        <float name="fov" value="39.3077"/>
        <transform name="to_world">
            <lookat origin="0,  0,  4"
                    target="0,  0,  0"
                    up    ="0,  1,  0"/>
        </transform>
        <sampler type="independent">
            <integer name="sample_count" value="$spp"/>
        </sampler>
        <film type="hdrfilm">
            <integer name="width"  value="$res"/>
            <integer name="height" value="$res"/>
            <rfilter type="tent"/>
            <string name="pixel_format" value="rgb"/>
            <string name="component_format" value="float32"/>
        </film>
    </sensor>

    <!-- BSDFs -->

    <bsdf type="diffuse" id="gray">
        <rgb name="reflectance" value="0.85, 0.85, 0.85"/>
    </bsdf>

    <bsdf type="diffuse" id="white">
        <rgb name="reflectance" value="0.885809, 0.698859, 0.666422"/>
    </bsdf>

    <bsdf type="diffuse" id="green">
        <rgb name="reflectance" value="0.105421, 0.37798, 0.076425"/>
    </bsdf>

    <bsdf type="diffuse" id="red">
        <rgb name="reflectance" value="0.570068, 0.0430135, 0.0443706"/>
    </bsdf>

    <bsdf type="dielectric" id="glass"/>

    <bsdf type="conductor" id="mirror"/>

    <!-- Light -->

    <shape type="obj" id="light">
        <string name="filename" value="meshes/cbox_luminaire.obj"/>
        <transform name="to_world">
            <translate x="0" y="-0.01" z="0"/>
        </transform>
        <ref id="white"/>
        <emitter type="area">
            <rgb name="radiance" value="18.387, 13.9873, 6.75357"/>
        </emitter>
    </shape>

    <!-- Shapes -->

    <shape type="obj" id="floor">
        <string name="filename" value="meshes/cbox_floor.obj"/>
        <ref id="white"/>
    </shape>

    <shape type="obj" id="ceiling">
        <string name="filename" value="meshes/cbox_ceiling.obj"/>
        <ref id="white"/>
    </shape>

    <shape type="obj" id="back">
        <string name="filename" value="meshes/cbox_back.obj"/>
        <ref id="white"/>
    </shape>

    <shape type="obj" id="greenwall">
        <string name="filename" value="meshes/cbox_greenwall.obj"/>
        <ref id="green"/>
    </shape>

    <shape type="obj" id="redwall">
        <string name="filename" value="meshes/cbox_redwall.obj"/>
        <ref id="red"/>
    </shape>

    <shape type="sphere" id="mirrorsphere">
        <transform name="to_world">
            <scale value="0.5"/>
            <translate x="-0.3" y="-0.5" z="0.2"/>
        </transform>
        <ref id="mirror"/>
    </shape>

    <shape type="sphere" id="glasssphere">
        <transform name="to_world">
            <scale value="0.25"/>
            <translate x="0.5" y="-0.75" z="-0.2"/>
        </transform>
        <ref id="glass"/>
        <sensor type="irradiancemeter">
            <film type="hdrfilm">
                <string name="pixel_format" value="rgba"/>
                <integer name="width" value="1920"/>
                <integer name="height" value="1080"/>
            </film>
        </sensor>
    </shape>
</scene>

[aantg]

I'm not expert with mitsuba, but obviously your image was rendered with perspective sensor. To render with particular sensor you should pass it as 'sensor' parameter to render() method or just remove all other sensors from scene description.
Also irradiancemeter 'is used with films of 1 by 1 pixels.', you can find this out by careful reading about irradiancemeter
Therefore there's no point in saving an image rendered with this sensor as you only get 1 pixel. This sensor is meant to retrieve numerical irradiance value (in for ex. W/m^2) on a shape it is attached to.