From 11ee5c982808592c9fba540bf4a5804c494bb2af Mon Sep 17 00:00:00 2001
From: Zack Buhman <zack@buhman.org>
Date: Sun, 9 Feb 2025 14:04:21 -0600
Subject: [PATCH] silhouette triangulation

---
 main.cpp | 131 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 124 insertions(+), 7 deletions(-)

diff --git a/main.cpp b/main.cpp
index 551a12e..c1b9421 100644
--- a/main.cpp
+++ b/main.cpp
@@ -433,6 +433,7 @@ void render_quad(SDL_Renderer * renderer,
 
   assert(SDL_SetRenderDrawColorFloat(renderer, 1, 0, 0, 1));
 
+  /*
   vec3 origin = (position[quadrilateral->a.position] +
                  position[quadrilateral->b.position] +
                  position[quadrilateral->c.position] +
@@ -441,6 +442,57 @@ void render_quad(SDL_Renderer * renderer,
   vec3 origin_t = transform_vertex(origin, scale);
   vec3 origin_n_t = transform_vertex(origin + n, scale);
   render_line_vtx(renderer, origin_t, origin_n_t);
+  */
+}
+
+struct edge {
+  int a;
+  int b;
+};
+
+struct tri {
+  int a;
+  int b;
+  int c;
+};
+
+void render_tri(SDL_Renderer * renderer,
+                const vec3 * position,
+                const vec3 color,
+                struct tri tri
+                )
+{
+  assert(SDL_SetRenderDrawColorFloat(renderer, color.x, color.y, color.z, 1));
+
+  vec3 ap = position[tri.a];
+  vec3 bp = position[tri.b];
+  vec3 cp = position[tri.c];
+
+  float scale = 0.5f;
+  vec3 a = transform_vertex(ap, scale);
+  vec3 b = transform_vertex(bp, scale);
+  vec3 c = transform_vertex(cp, scale);
+
+  render_line_vtx(renderer, a, b);
+  render_line_vtx(renderer, b, c);
+  render_line_vtx(renderer, c, a);
+}
+
+struct tri generate_tri(struct edge * silhouette,
+                        int * order,
+                        int o0,
+                        int o1)
+{
+  struct tri tri;
+  tri.a = silhouette[order[o0]].a;
+  tri.b = silhouette[order[o0]].b;
+  if (silhouette[order[o0]].a == silhouette[order[o1]].a ||
+      silhouette[order[o0]].b == silhouette[order[o1]].a) {
+    tri.c = silhouette[order[o1]].b;
+  } else {
+    tri.c = silhouette[order[o1]].a;
+  }
+  return tri;
 }
 
 void render_silhouette(SDL_Renderer * renderer,
@@ -450,20 +502,85 @@ void render_silhouette(SDL_Renderer * renderer,
 {
   assert(SDL_SetRenderDrawColorFloat(renderer, 1, 0.5, 0, 1));
 
+  struct edge silhouette[6];
+  int ix = 0;
+
   for (int a = 0; a < edge_stride; a++) {
     for (int b = 0; b < edge_stride; b++) {
       uint8_t coloring = edge_coloring[a * edge_stride + b];
       if (coloring == 0b11) {
-        vec3 ap = position[a];
-        vec3 bp = position[b];
-
-        float scale = 0.5f;
-        vec3 av = transform_vertex(ap, scale);
-        vec3 bv = transform_vertex(bp, scale);
-        render_line_vtx(renderer, av, bv);
+        silhouette[ix++] = {a, b};
       }
     }
   }
+
+  assert(ix == 6);
+
+  int order[6];
+  order[0] = 0;
+  int order_ix = 1;
+  int vtx = silhouette[0].b;
+
+  // calculate edge ordering
+  while (order_ix < 6) {
+    for (int i = 1; i < 6; i++) {
+      if (i == order[order_ix - 1])
+        continue;
+
+      if (silhouette[i].a == vtx) {
+        vtx = silhouette[i].b;
+        order[order_ix++] = i;
+        break;
+      }
+      if (silhouette[i].b == vtx) {
+        vtx = silhouette[i].a;
+        order[order_ix++] = i;
+        break;
+      }
+    }
+  }
+
+  assert(order_ix == 6);
+
+  const vec3 colors[] = {
+    {1, 0, 0}, // red
+    {1, 0.5, 0}, // orange
+    {1, 1, 0}, // yellow
+    {0, 1, 0}, // green
+    {0, 1, 1}, // cyan
+    {0.5, 0, 1}, // purple
+  };
+
+  /*
+  for (int i = 0; i < 6; i++) {
+    assert(SDL_SetRenderDrawColorFloat(renderer, colors[i].x, colors[i].y, colors[i].z, 1));
+
+    vec3 ap = position[silhouette[order[i]].a];
+    vec3 bp = position[silhouette[order[i]].b];
+
+    float scale = 0.5f;
+    vec3 av = transform_vertex(ap, scale);
+    vec3 bv = transform_vertex(bp, scale);
+    render_line_vtx(renderer, av, bv);
+  }
+  */
+
+  // render triangulation
+  /*
+    {0.a, 0.b}, {1.a, 1.b}, {0._, 1._}
+    {2.a, 2.b}, {3.a, 3.b}, {2._, 3._}
+    {4.a, 4.b}, {5.a, 5.b}, {4._, 5._}
+    {0._, 1._}, {2._, 3._}, {4._, 5._}
+   */
+  struct tri q = generate_tri(silhouette, order, 0, 1);
+  struct tri r = generate_tri(silhouette, order, 2, 3);
+  struct tri s = generate_tri(silhouette, order, 4, 5);
+  struct tri t = {q.c, r.c, s.c};
+
+  render_tri(renderer, position, colors[1], q);
+  render_tri(renderer, position, colors[2], r);
+  render_tri(renderer, position, colors[3], s);
+  render_tri(renderer, position, colors[0], t);
 }
 
 void render_cube(SDL_Renderer * renderer, const vec3 light_vec)