euc 0.5.3

A software rendering crate that lets you write shaders with Rust
Documentation 
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use super::*;
use crate::{Interpolate, Pipeline, Target};
use core::marker::PhantomData;
#[cfg(not(feature = "std"))]
use num_traits::Float;
use vek::{Mat3, Vec2, Vec3};

/// A rasterizer that produces filled triangles from groups of 3 consecutive vertices.
///
/// Use the BackfaceCullingEnabled type parameter to enable backface culling.
/// Use the BackfaceCullingDisabled type parameter to disable backface culling.
pub struct Triangles<'a, D, B: BackfaceMode = BackfaceCullingEnabled> {
    phantom: PhantomData<(&'a D, B)>,
}

impl<'a, D: Target<Item = f32>, B: BackfaceMode> Rasterizer for Triangles<'a, D, B> {
    type Input = [f32; 3]; // Vertex coordinates
    type Supplement = Option<&'a mut D>; // Depth buffer

    fn draw<P: Pipeline, T: Target<Item = P::Pixel>>(
        pipeline: &P,
        vertices: &[P::Vertex],
        target: &mut T,
        mut depth: Self::Supplement,
    ) {
        if let Some(depth) = depth.as_ref() {
            assert_eq!(
                target.size(),
                depth.size(),
                "Target and depth buffers are not similarly sized!"
            );
        }

        let size = Vec2::from(target.size());
        let half_scr = size.map(|e: usize| e as f32 * 0.5);

        let to_ndc = Mat3::from_row_arrays([
            [2.0 / size.x as f32, 0.0, -1.0],
            [0.0, -2.0 / size.y as f32, 1.0],
            [0.0, 0.0, 1.0],
        ]);

        let (depth_test, depth_less, depth_write) = match pipeline.get_depth_strategy() {
            DepthStrategy::IfLessWrite => (true, true, true),
            DepthStrategy::IfLessNoWrite => (true, true, false),
            DepthStrategy::IfMoreWrite => (true, false, true),
            DepthStrategy::IfMoreNoWrite => (true, false, false),
            DepthStrategy::None => (false, false, false),
        };

        vertices.chunks_exact(3).for_each(|verts| {
            // Compute vertex shader outputs
            let (a_hom, a_vs_out) = pipeline.vert(&verts[0]);
            let (b_hom, b_vs_out) = pipeline.vert(&verts[1]);
            let (c_hom, c_vs_out) = pipeline.vert(&verts[2]);

            // Convert homogenous to euclidean coordinates
            let a = Vec3::new(a_hom[0], a_hom[1], a_hom[2]) / a_hom[3];
            let b = Vec3::new(b_hom[0], b_hom[1], b_hom[2]) / b_hom[3];
            let c = Vec3::new(c_hom[0], c_hom[1], c_hom[2]) / c_hom[3];

            // Backface culling
            let ((a, a_hom, a_vs_out), (c, c_hom, c_vs_out)) =
                    // Back face?
                    if (b - a).cross(c - a).z < 0.0 {
                        // If backface culling is enabled, just return: we're done with this tri.
                        if B::ENABLED {
                            return;
                        } else {
                            // Reverse the vertex order
                            ((c, c_hom, c_vs_out), (a, a_hom, a_vs_out))
                        }
                    } else {
                        // Maintain vertex order
                        ((a, a_hom, a_vs_out), (c, c_hom, c_vs_out))
                    };

            let fb_to_weights = {
                let c = Vec3::new(c_hom[0], c_hom[1], c_hom[3]);
                let ca = Vec3::new(a_hom[0], a_hom[1], a_hom[3]) - c;
                let cb = Vec3::new(b_hom[0], b_hom[1], b_hom[3]) - c;
                let n = ca.cross(cb);
                let rec_det = if n.magnitude_squared() > 0.0 {
                    1.0 / n.dot(c)
                } else {
                    1.0
                };
                // Compute matrix inverse
                Mat3::from_row_arrays([
                    cb.cross(c).into_array(),
                    c.cross(ca).into_array(),
                    n.into_array(),
                ]) * rec_det
                    * to_ndc
            };

            debug_assert!(fb_to_weights.into_row_array().iter().all(|e| e.is_finite()));

            // Convert to framebuffer coordinates
            let a_scr = half_scr
                * Vec2 {
                    x: a.x + 1.0,
                    y: a.y.mul_add(-1.0, 1.0),
                };
            let b_scr = half_scr
                * Vec2 {
                    x: b.x + 1.0,
                    y: b.y.mul_add(-1.0, 1.0),
                };
            let c_scr = half_scr
                * Vec2 {
                    x: c.x + 1.0,
                    y: c.y.mul_add(-1.0, 1.0),
                };

            let a_px = a_scr.map(|e| e as i32);
            let b_px = b_scr.map(|e| e as i32);
            let c_px = c_scr.map(|e| e as i32);

            let min = a_px
                .map2(b_px, |e, b| e.min(b))
                .map2(c_px, |e, c| e.min(c))
                .map(|e| e.max(0))
                .map2(size, |e, sz| (e).min(sz as i32) as usize);
            let max = a_px
                .map2(b_px, |e, b| e.max(b))
                .map2(c_px, |e, c| e.max(c))
                .map(|e| e.max(0))
                .map2(size, |e, sz| (e + 1).min(sz as i32) as usize);

            for y in min.y..max.y {
                for x in min.x..max.x {
                    // Where is the centre of the fragment?
                    let p = Vec3::new(x as f32 + 0.5, y as f32 + 0.5, 1.0);

                    // Calculate vertex weights
                    let weights_hom = fb_to_weights * p;
                    let wa = weights_hom.x / weights_hom.z;
                    let wb = weights_hom.y / weights_hom.z;
                    let wc = 1.0 - wa - wb;

                    if (wa - 0.5).abs() > 0.5 || (wb - 0.5).abs() > 0.5 || (wc - 0.5).abs() > 0.5 {
                        continue;
                    }

                    // Calculate the interpolated depth of this fragment
                    let z_lerped =
                        f32::lerp3(a_hom[2], b_hom[2], c_hom[2], wa, wb, wc) * weights_hom.z;

                    let should_draw = if depth_test {
                        if depth_less {
                            depth.as_ref().map(|depth| z_lerped <= unsafe { depth.get([x, y]) }).unwrap_or(true)
                        } else {
                            depth.as_ref().map(|depth| z_lerped >= unsafe { depth.get([x, y]) }).unwrap_or(true)
                        }
                    } else {
                        true
                    };

                    if should_draw {
                        // Calculate the interpolated vertex attributes of this fragment
                        let vs_out_lerped = P::VsOut::lerp3(
                            a_vs_out.clone(),
                            b_vs_out.clone(),
                            c_vs_out.clone(),
                            wa,
                            wb,
                            wc,
                        );

                        unsafe {
                            // Write depth
                            if depth_write {
                                depth.as_mut().map(|depth| depth.set([x, y], z_lerped));
                            }

                            target.set([x, y], pipeline.frag(&vs_out_lerped));
                        }
                    }
                }
            }
        });
    }
}