area.cpp

#include <mitsuba/core/properties.h>
#include <mitsuba/core/warp.h>
#include <mitsuba/core/spectrum.h>
#include <mitsuba/render/emitter.h>
#include <mitsuba/render/medium.h>
#include <mitsuba/render/shape.h>
#include <mitsuba/render/texture.h>

NAMESPACE_BEGIN(mitsuba)
/**!

.. _emitter-area:

Area light (:monosp:`area`)
---------------------------

.. pluginparameters::

 * - radiance
   - |spectrum|
   - Specifies the emitted radiance in units of power per unit area per unit steradian.

This plugin implements an area light, i.e. a light source that emits
diffuse illumination from the exterior of an arbitrary shape.
Since the emission profile of an area light is completely diffuse, it
has the same apparent brightness regardless of the observer's viewing
direction. Furthermore, since it occupies a nonzero amount of space, an
area light generally causes scene objects to cast soft shadows.

To create an area light source, simply instantiate the desired
emitter shape and specify an :monosp:`area` instance as its child:

.. code-block:: xml
    :name: sphere-light

    <shape type="sphere">
        <emitter type="area">
            <spectrum name="radiance" value="1.0"/>
        </emitter>
    </shape>

 */

template <typename Float, typename Spectrum>
class AreaLight final : public Emitter<Float, Spectrum> {
public:
    MTS_IMPORT_BASE(Emitter, m_flags, m_shape, m_medium)
    MTS_IMPORT_TYPES(Scene, Shape, Texture)

    AreaLight(const Properties &props) : Base(props) {
        if (props.has_property("to_world"))
            Throw("Found a 'to_world' transformation -- this is not allowed. "
                  "The area light inherits this transformation from its parent "
                  "shape.");

        m_radiance = props.texture<Texture>("radiance", Texture::D65(1.f));

        m_flags = +EmitterFlags::Surface;
        if (m_radiance->is_spatially_varying())
            m_flags |= +EmitterFlags::SpatiallyVarying;
    }

    Spectrum eval(const SurfaceInteraction3f &si, Mask active) const override {
        MTS_MASKED_FUNCTION(ProfilerPhase::EndpointEvaluate, active);

        return select(
            Frame3f::cos_theta(si.wi) > 0.f,
            unpolarized<Spectrum>(m_radiance->eval(si, active)),
            0.f
        );
    }

    std::pair<Ray3f, Spectrum> sample_ray(Float time, Float wavelength_sample,
                                          const Point2f &sample2, const Point2f &sample3,
                                          Mask active) const override {
        MTS_MASKED_FUNCTION(ProfilerPhase::EndpointSampleRay, active);

        SurfaceInteraction3f si = zero<SurfaceInteraction3f>();
        si.t = math::Infinity<Float>;

        Float pdf = 1.f;

        // 1. Two strategies to sample spatial component based on 'm_radiance'
        if (!m_radiance->is_spatially_varying()) {
            PositionSample3f ps = m_shape->sample_position(time, sample2, active);

            // Radiance not spatially varying, use area-based sampling of shape
            si = SurfaceInteraction3f(ps, zero<Wavelength>());
            pdf = ps.pdf;
        } else {
            // Ipmortance sample texture
            std::tie(si.uv, pdf) = m_radiance->sample_position(sample2, active);
            active &= neq(pdf, 0.f);

            si = m_shape->eval_parameterization(Point2f(si.uv), active);
            active &= si.is_valid();

            pdf /= norm(cross(si.dp_du, si.dp_dv));
        }

        // 2. Sample directional component
        Vector3f local = warp::square_to_cosine_hemisphere(sample3);

        Wavelength wavelength;
        Spectrum spec_weight;

        if constexpr (is_spectral_v<Spectrum>) {
            std::tie(wavelength, spec_weight) = m_radiance->sample_spectrum(
                si, math::sample_shifted<Wavelength>(wavelength_sample), active);
        } else {
            wavelength = zero<Wavelength>();
            spec_weight = m_radiance->eval(si, active);
        }

        return std::make_pair(
            Ray3f(si.p, si.to_world(local), time, wavelength),
            unpolarized<Spectrum>(spec_weight) * (math::Pi<Float> / pdf)
        );
    }

    std::pair<DirectionSample3f, Spectrum>
    sample_direction(const Interaction3f &it, const Point2f &sample, Mask active) const override {
        MTS_MASKED_FUNCTION(ProfilerPhase::EndpointSampleDirection, active);
        Assert(m_shape, "Can't sample from an area emitter without an associated Shape.");
        DirectionSample3f ds;
        Spectrum spec;

        // One of two very different strategies is used depending on 'm_radiance'
        if (!m_radiance->is_spatially_varying()) {
            // Texture is uniform, try to importance sample the shape wrt. solid angle at 'it'
            ds = m_shape->sample_direction(it, sample, active);
            active &= dot(ds.d, ds.n) < 0.f && neq(ds.pdf, 0.f);

            SurfaceInteraction3f si(ds, it.wavelengths);
            spec = m_radiance->eval(si, active) / ds.pdf;
        } else {
            // Importance sample the texture, then map onto the shape
            auto [uv, pdf] = m_radiance->sample_position(sample, active);
            active &= neq(pdf, 0.f);

            SurfaceInteraction3f si = m_shape->eval_parameterization(uv, active);
            si.wavelengths = it.wavelengths;
            active &= si.is_valid();

            ds.p = si.p;
            ds.n = si.n;
            ds.uv = si.uv;
            ds.time = it.time;
            ds.delta = false;
            ds.d = ds.p - it.p;

            Float dist_squared = squared_norm(ds.d);
            ds.dist = sqrt(dist_squared);
            ds.d /= ds.dist;

            Float dp = dot(ds.d, ds.n);
            active &= dp < 0;
            ds.pdf = select(active, pdf / norm(cross(si.dp_du, si.dp_dv)) *
                                        dist_squared / -dp, 0.f);

            spec = m_radiance->eval(si, active) / ds.pdf;
        }

        ds.object = this;
        return { ds, unpolarized<Spectrum>(spec) & active };
    }

    Float pdf_direction(const Interaction3f &it, const DirectionSample3f &ds,
                        Mask active) const override {
        MTS_MASKED_FUNCTION(ProfilerPhase::EndpointEvaluate, active);
        Float dp = dot(ds.d, ds.n);
        active &= dp < 0.f;

        Float value;
        if (!m_radiance->is_spatially_varying()) {
            value = m_shape->pdf_direction(it, ds, active);
        } else {
            // This surface intersection would be nice to avoid..
            SurfaceInteraction3f si = m_shape->eval_parameterization(ds.uv, active);
            active &= si.is_valid();

            value = m_radiance->pdf_position(ds.uv, active) * sqr(ds.dist) /
                    (norm(cross(si.dp_du, si.dp_dv)) * -dp);
        }

        return select(active, value, 0.f);
    }

    ScalarBoundingBox3f bbox() const override { return m_shape->bbox(); }

    void traverse(TraversalCallback *callback) override {
        callback->put_object("radiance", m_radiance.get());
    }

    std::string to_string() const override {
        std::ostringstream oss;
        oss << "AreaLight[" << std::endl
            << "  radiance = " << string::indent(m_radiance) << "," << std::endl
            << "  surface_area = ";
        if (m_shape) oss << m_shape->surface_area();
        else         oss << "  <no shape attached!>";
        oss << "," << std::endl;
        if (m_medium) oss << string::indent(m_medium);
        else         oss << "  <no medium attached!>";
        oss << std::endl << "]";
        return oss.str();
    }

    MTS_DECLARE_CLASS()
private:
    ref<Texture> m_radiance;
};

MTS_IMPLEMENT_CLASS_VARIANT(AreaLight, Emitter)
MTS_EXPORT_PLUGIN(AreaLight, "Area emitter")
NAMESPACE_END(mitsuba)