pbrt/src/lights/diffuse.rs

use super::{
    DenselySampledSpectrum, LightBase, LightBounds, LightLiSample, LightSampleContext, LightTrait,
    LightType, RGBIlluminantSpectrum, SampledSpectrum, SampledWavelengths, Spectrum, SpectrumTrait,
};
use crate::core::interaction::{
    Interaction, InteractionTrait, SimpleInteraction, SurfaceInteraction,
};
use crate::core::medium::MediumInterface;
use crate::core::pbrt::{Float, PI};
use crate::core::texture::{
    FloatTexture, FloatTextureTrait, TextureEvalContext, TextureEvaluator,
    UniversalTextureEvaluator,
};
use crate::geometry::{
    Bounds3f, Normal3f, Point2f, Point2fi, Point2i, Point3f, Point3fi, Ray, Vector3f, VectorLike,
};
use crate::image::Image;
use crate::shapes::{Shape, ShapeSample, ShapeSampleContext, ShapeTrait};
use crate::utils::color::RGB;
use crate::utils::colorspace::RGBColorSpace;
use crate::utils::hash::hash_float;
use crate::utils::transform::Transform;

use std::sync::Arc;

#[derive(Clone, Debug)]
pub struct DiffuseAreaLight {
    base: LightBase,
    shape: Shape,
    alpha: Option<FloatTexture>,
    area: Float,
    two_sided: bool,
    lemit: Arc<DenselySampledSpectrum>,
    scale: Float,
    image: Option<Image>,
    image_color_space: Option<Arc<RGBColorSpace>>,
}

impl DiffuseAreaLight {
    #[allow(clippy::too_many_arguments)]
    pub fn new(
        render_from_light: Transform<Float>,
        medium_interface: MediumInterface,
        le: Spectrum,
        scale: Float,
        shape: Shape,
        alpha: FloatTexture,
        image: Option<Image>,
        image_color_space: Option<Arc<RGBColorSpace>>,
        two_sided: bool,
    ) -> Self {
        let is_constant_zero = match &alpha {
            FloatTexture::FloatConstant(tex) => tex.evaluate(&TextureEvalContext::default()) == 0.0,
            _ => false,
        };

        let (light_type, stored_alpha) = if is_constant_zero {
            (LightType::DeltaPosition, None)
        } else {
            (LightType::Area, Some(alpha))
        };

        let base = LightBase::new(light_type, &render_from_light, &medium_interface);

        let lemit = LightBase::lookup_spectrum(&le);
        if let Some(im) = &image {
            let desc = im
                .get_channel_desc(&["R", "G", "B"])
                .expect("Image used for DiffuseAreaLight doesn't have R, G, B channels");

            assert_eq!(3, desc.size(), "Image channel description size mismatch");
            assert!(
                desc.is_identity(),
                "Image channel description is not identity"
            );

            assert!(
                image_color_space.is_some(),
                "Image provided but ColorSpace is missing"
            );
        }
        let is_triangle_or_bilinear = matches!(shape, Shape::Triangle(_) | Shape::BilinearPatch(_));
        if render_from_light.has_scale(None) && !is_triangle_or_bilinear {
            println!(
                "Scaling detected in rendering to light space transformation! \
                 The system has numerous assumptions, implicit and explicit, \
                 that this transform will have no scale factors in it. \
                 Proceed at your own risk; your image may have errors."
            );
        }

        Self {
            base,
            area: shape.area(),
            shape,
            alpha: stored_alpha,
            two_sided,
            lemit,
            scale,
            image,
            image_color_space,
        }
    }

    fn alpha_masked(&self, intr: &Interaction) -> bool {
        let Some(alpha_tex) = &self.alpha else {
            return false;
        };
        let ctx = TextureEvalContext::from(intr);
        let a = UniversalTextureEvaluator.evaluate_float(alpha_tex, &ctx);
        if a >= 1.0 {
            return false;
        }
        if a <= 0.0 {
            return true;
        }
        hash_float(&intr.p()) > a
    }
}

impl LightTrait for DiffuseAreaLight {
    fn base(&self) -> &LightBase {
        &self.base
    }

    fn phi(&self, lambda: SampledWavelengths) -> SampledSpectrum {
        let mut l = SampledSpectrum::new(0.);
        if let Some(image) = &self.image {
            for y in 0..image.resolution().y() {
                for x in 0..image.resolution().x() {
                    let mut rgb = RGB::default();
                    for c in 0..3 {
                        rgb[c] = image.get_channel(Point2i::new(x, y), c);
                    }
                    l += RGBIlluminantSpectrum::new(
                        self.image_color_space.as_ref().unwrap(),
                        RGB::clamp_zero(rgb),
                    )
                    .sample(&lambda);
                }
            }
            l *= self.scale / (image.resolution().x() * image.resolution().y()) as Float;
        } else {
            l = self.lemit.sample(&lambda) * self.scale;
        }
        let two_side = if self.two_sided { 2. } else { 1. };
        PI * two_side * self.area * l
    }

    fn sample_li(
        &self,
        ctx: &LightSampleContext,
        u: Point2f,
        lambda: &SampledWavelengths,
        _allow_incomplete_pdf: bool,
    ) -> Option<LightLiSample> {
        let shape_ctx = ShapeSampleContext::new(ctx.pi, ctx.n, ctx.ns, 0.0);
        let ss = self.shape.sample_from_context(&shape_ctx, u)?;
        let mut intr: SurfaceInteraction = ss.intr.as_ref().clone();

        intr.common.medium_interface = Some(self.base.medium_interface.clone());
        let p = intr.p();
        let n = intr.n();
        let uv = intr.uv;

        let generic_intr = Interaction::Surface(intr);
        if self.alpha_masked(&generic_intr) {
            return None;
        }

        let wi = (p - ctx.p()).normalize();
        let le = self.l(p, n, uv, -wi, lambda);

        if le.is_black() {
            return None;
        }

        Some(LightLiSample::new(le, wi, ss.pdf, generic_intr))
    }

    fn pdf_li(&self, ctx: &LightSampleContext, wi: Vector3f, _allow_incomplete_pdf: bool) -> Float {
        let shape_ctx = ShapeSampleContext::new(ctx.pi, ctx.n, ctx.ns, 0.);
        self.shape.pdf_from_context(&shape_ctx, wi)
    }

    fn l(
        &self,
        p: Point3f,
        n: Normal3f,
        mut uv: Point2f,
        w: Vector3f,
        lambda: &SampledWavelengths,
    ) -> SampledSpectrum {
        if self.two_sided && n.dot(w.into()) < 0. {
            return SampledSpectrum::new(0.);
        }
        let intr = Interaction::Surface(SurfaceInteraction::new_minimal(
            Point3fi::new_from_point(p),
            uv,
        ));

        if self.alpha_masked(&intr) {
            return SampledSpectrum::new(0.);
        }
        if let Some(image) = &self.image {
            let mut rgb = RGB::default();
            uv[1] = 1. - uv[1];
            for c in 0..3 {
                rgb[c] = image.bilerp_channel(uv, c);
            }

            let spec = RGBIlluminantSpectrum::new(
                self.image_color_space.as_ref().unwrap(),
                RGB::clamp_zero(rgb),
            );

            self.scale * spec.sample(lambda)
        } else {
            self.scale * self.lemit.sample(lambda)
        }
    }

    fn le(&self, _ray: &Ray, _lambda: &SampledWavelengths) -> SampledSpectrum {
        todo!()
    }

    fn preprocess(&mut self, _scene_bounds: &Bounds3f) {
        unimplemented!()
    }

    fn bounds(&self) -> Option<LightBounds> {
        let mut phi = 0.;
        if let Some(image) = &self.image {
            for y in 0..image.resolution.y() {
                for x in 0..image.resolution.x() {
                    for c in 0..3 {
                        phi += image.get_channel(Point2i::new(x, y), c);
                    }
                }
            }
        } else {
            phi = self.lemit.max_value();
        }

        let nb = self.shape.normal_bounds();
        Some(LightBounds::new(
            &self.shape.bounds(),
            nb.w,
            phi,
            nb.cos_theta,
            (PI / 2.).cos(),
            self.two_sided,
        ))
    }
}