From c96625b8eb81ed49e14ba2b8cf958e7bc8ebb034 Mon Sep 17 00:00:00 2001
From: hheik <4469778+hheik@users.noreply.github.com>
Date: Fri, 15 Dec 2023 17:18:02 +0200
Subject: [PATCH] Added software shadows

---
 src/game/darkness.rs | 30 +++++++++++++++++-------------
 src/util.rs          | 17 +++++++++++++++++
 2 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/src/game/darkness.rs b/src/game/darkness.rs
index 4147a04..4342144 100644
--- a/src/game/darkness.rs
+++ b/src/game/darkness.rs
@@ -11,7 +11,7 @@ use bevy_ecs_ldtk::{LdtkLevel, LevelEvent};
 use bevy_prototype_debug_lines::DebugLines;
 use bevy_rapier2d::prelude::{Collider, QueryFilter, RapierContext};
 
-use crate::debug::DebugMode;
+use crate::{debug::DebugMode, util::vec2_intersection};
 
 mod material;
 
@@ -349,6 +349,9 @@ fn calculate_shadow_map(
     for x in 0..SHADOWMAP_RESOLUTION {
         let angle = (x as f32 / SHADOWMAP_RESOLUTION as f32) * 2.0 * std::f32::consts::PI
             - std::f32::consts::PI;
+        while angle >= vertices[next_index % vertices.len()].angle && next_index < vertices.len() {
+            next_index += 1;
+        }
 
         let prev_vertex = if next_index == 0 {
             vertices[vertices.len() - 1]
@@ -356,19 +359,20 @@ fn calculate_shadow_map(
             vertices[(next_index - 1) % vertices.len()]
         };
         let next_vertex = vertices[next_index % vertices.len()];
-        if angle >= next_vertex.angle && next_index < vertices.len() {
-            next_index += 1;
-        }
 
-        let mut relative_angle = angle - prev_vertex.angle;
-        // Technically not correct, but works
-        if relative_angle < 0.0 {
-            relative_angle =
-                (std::f32::consts::PI - prev_vertex.angle) + (angle + std::f32::consts::PI);
-        }
-
-        // TODO: Calculate between-points
-        let distance = (next_vertex.point - global_position).length();
+        let distance = match vec2_intersection(
+            global_position,
+            global_position
+                + Vec2 {
+                    x: angle.cos(),
+                    y: angle.sin(),
+                },
+            prev_vertex.point,
+            next_vertex.point,
+        ) {
+            Some(point) => global_position.distance(point),
+            None => global_position.distance(prev_vertex.point),
+        };
         shadow_map[x] = distance;
     }
     shadow_map
diff --git a/src/util.rs b/src/util.rs
index 3f52d4c..9c6704a 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -124,6 +124,23 @@ pub fn move_towards_vec3(from: Vec3, to: Vec3, amount: f32) -> Vec3 {
     from + diff.normalize() * length.min(amount)
 }
 
+/// Get the intersection point (if any) of 2d lines a and b.
+/// Lines are defined by 2 points on the line
+pub fn vec2_intersection(a1: Vec2, a2: Vec2, b1: Vec2, b2: Vec2) -> Option<Vec2> {
+    let a_dir = a2 - a1;
+    let b_dir = b2 - b1;
+    let determinant = a_dir.perp_dot(b_dir);
+    if determinant.abs() <= f32::EPSILON {
+        return None;
+    }
+    Some(
+        Vec2 {
+            x: a_dir.x * (b1.x * b2.y - b1.y * b2.x) - (a1.x * a2.y - a1.y * a2.x) * b_dir.x,
+            y: (a1.x * a2.y - a1.y * a2.x) * -b_dir.y + a_dir.y * (b1.x * b2.y - b1.y * b2.x),
+        } / determinant,
+    )
+}
+
 pub fn loop_value(from: f32, to: f32, value: f32) -> f32 {
     let range = to - from;
     if !range.is_normal() {