From b87a79148d6041cc8ff217d0b6751363218813dc Mon Sep 17 00:00:00 2001
From: Zack Buhman <zack@buhman.org>
Date: Sat, 8 Feb 2025 16:28:43 -0600
Subject: [PATCH] detect angle between light and square

---
 main.cpp | 77 ++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 67 insertions(+), 10 deletions(-)

diff --git a/main.cpp b/main.cpp
index 2dcea64..445a2b7 100644
--- a/main.cpp
+++ b/main.cpp
@@ -189,6 +189,30 @@ struct state state = {
   .normal = { -1, 0, 0 },
 };
 
+struct edge_normal {
+  struct line edge;
+  vec3 normal;
+};
+
+struct edge_normal quad[4] = {
+  {
+    .edge = {{-1, -1, 0}, { 1, -1, 0}},
+    .normal = {0, -1, 0},
+  },
+  {
+    .edge = {{ 1, -1, 0}, { 1,  1, 0}},
+    .normal = {1, 0, 0},
+  },
+  {
+    .edge = {{ 1,  1, 0}, {-1,  1, 0}},
+    .normal = {0, 1, 0},
+  },
+  {
+    .edge = {{-1,  1, 0}, {-1, -1, 0}},
+    .normal = {-1, 0, 0},
+  },
+};
+
 static int window_width = 1;
 static int window_height = 1;
 
@@ -276,6 +300,47 @@ void render_basis(SDL_Renderer * renderer)
   // cyan: X
   assert(SDL_SetRenderDrawColorFloat(renderer, 0, 1, 1, 1));
   render_line(renderer, transform_line({{0, 0, 0}, {1, 0, 0}}, 1.0f));
+
+  vec3 z = transform_vertex({0, 0, 1}, 1.1);
+  vec3 y = transform_vertex({0, 1, 0}, 1.1);
+  vec3 x = transform_vertex({1, 0, 0}, 1.1);
+
+  render_text(renderer, ((z.x - 30) * 64), ((z.y + 10) * 64), "+z", 2);
+  render_text(renderer, ((y.x - 30) * 64), ((y.y + 10) * 64), "+y", 2);
+  render_text(renderer, ((x.x - 30) * 64), ((x.y + 10) * 64), "+x", 2);
+}
+
+static float ltheta = 0;
+
+void render_quad(SDL_Renderer * renderer)
+{
+  vec3 light_origin = {0, 0, 0};
+  vec3 light_pos = {1, 1, 1};
+  mat3x3 rot = {
+    cos(ltheta), -sin(ltheta), 0,
+    sin(ltheta), cos(ltheta), 0,
+    0, 0, 1,
+  };
+  light_pos = rot * light_pos;
+  ltheta += deg / 4;
+  vec3 light_vec = light_origin - light_pos;
+
+  for (int i = 0; i < 4; i++) {
+    float d = dot(light_vec, quad[i].normal);
+
+    if (d > 0)
+      assert(SDL_SetRenderDrawColorFloat(renderer, 1, 1, 1, 1));
+    else
+      assert(SDL_SetRenderDrawColorFloat(renderer, 0, 0, 1, 1));
+    render_line(renderer, transform_line(quad[i].edge, 0.25f));
+
+    vec3 origin = (quad[i].edge.a + quad[i].edge.b) / 2.0f;
+    assert(SDL_SetRenderDrawColorFloat(renderer, 1, 0, 0, 1));
+    render_line(renderer, transform_line({origin, origin + quad[i].normal}, 0.25f));
+  }
+
+  assert(SDL_SetRenderDrawColorFloat(renderer, 0, 1, 0, 1));
+  render_line(renderer, transform_line({light_origin, light_pos}, 0.5f));
 }
 
 mat3x3 rotate_to(vec3 old_normal, vec3 new_normal)
@@ -332,8 +397,6 @@ mat4x4 look_at(vec3 eye, vec3 center, vec3 up)
 
 void render_lines(SDL_Renderer * renderer)
 {
-  render_basis(renderer);
-
   // line
   assert(SDL_SetRenderDrawColorFloat(renderer, 1, 0.5, 0.5, 1));
   for (int i = 0; i < num_lines; i++) {
@@ -393,13 +456,6 @@ void render_text_state(SDL_Renderer * renderer)
   int len = snprintf(buf, 64, "%.03f", d);
   x_advance = render_text(renderer, x_advance, y_advance, buf, len);
   */
-  vec3 z = transform_vertex({0, 0, 1}, 1.1);
-  vec3 y = transform_vertex({0, 1, 0}, 1.1);
-  vec3 x = transform_vertex({1, 0, 0}, 1.1);
-
-  render_text(renderer, ((z.x - 30) * 64), ((z.y + 10) * 64), "+z", 2);
-  render_text(renderer, ((y.x - 30) * 64), ((y.y + 10) * 64), "+y", 2);
-  render_text(renderer, ((x.x - 30) * 64), ((x.y + 10) * 64), "+x", 2);
 }
 
 static float theta = 0;
@@ -465,7 +521,8 @@ int main()
     assert(success == true);
 
     render_text_state(renderer);
-    render_lines(renderer);
+    render_basis(renderer);
+    render_quad(renderer);
 
     while (SDL_GetTicks() - ticks < (1000 / 60)) { SDL_Delay(1); }
     SDL_RenderPresent(renderer);