From 75ae399d389dd328baeec4674c36651378c0df8b Mon Sep 17 00:00:00 2001
From: Nico de Poel <ndepoel@gmail.com>
Date: Mon, 16 Jan 2023 22:03:16 +0100
Subject: [PATCH] Implemented quad strip topology for rendering faces, which is
 actually a good bit simpler than triangle strips.

---
 world.c | 47 +++++++++++++++++++++++++----------------------
 1 file changed, 25 insertions(+), 22 deletions(-)

diff --git a/world.c b/world.c
index ccfe05c..25fc3f1 100644
--- a/world.c
+++ b/world.c
@@ -63,30 +63,25 @@ void world_draw(const world_t *world)
             vecs[vertIdx].pad = vert->baseLight;
         }
 
-        // Draw the face as a triangle strip
-        const SVECTOR *v0, *v1, *v2;
+        // Draw the face as a quad strip
+        const SVECTOR *v0, *v1, *v2, *v3;
         const SVECTOR *head = vecs;
         const SVECTOR *tail = vecs + face->numFaceVertices;
-        u_char reverse = 0;
 
-        v2 = head++;    // Initialize first vertex to index 0 and set head to index 1
+        // Initialize the first two vertices
+        v2 = --tail;
+        v3 = head++;
 
-        for (u_char vertIdx = 0; vertIdx < face->numFaceVertices - 2; ++vertIdx)
+        // Normally a quad strip would have (N-2)/2 quads, but we might end up with a sole triangle at the end which will be drawn as a collapsed quad
+        u_char numQuads = (face->numFaceVertices - 1) / 2;
+        for (u_char quadIdx = 0; quadIdx < numQuads; ++quadIdx)
         {
-            if (reverse ^= 1)
-            {
-                v0 = v2;
-                v1 = head;
-                v2 = --tail;
-            }
-            else
-            {
-                v0 = v1;
-                v1 = ++head;
-                v2 = tail;
-            }
-
-            // Naively draw the triangle with GTE, nothing special or optimized about this
+            v0 = v2;
+            v1 = v3;
+            v2 = --tail;
+            v3 = head++;
+
+            // Naively draw the quad with GTE, nothing special or optimized about this
             gte_ldv3(v0, v1, v2);
             gte_rtpt();     // Rotation, translation, perspective projection
 
@@ -104,17 +99,25 @@ void world_draw(const world_t *world)
                 continue;
 
             // Draw a flat-shaded untextured colored triangle
-            POLY_G3 *poly = (POLY_G3*)mem_prim(sizeof(POLY_G3));
+            POLY_G4 *poly = (POLY_G4*)mem_prim(sizeof(POLY_G4));
             if (poly == NULL)
                 break;
 
-            setPolyG3(poly);
+            setPolyG4(poly);
+
+            gte_stsxy0(&poly->x0);
+            gte_stsxy1(&poly->x1);
+            gte_stsxy2(&poly->x2);
 
-            gte_stsxy3_g3(poly);
+            // Transform the fourth vertex to complete the quad
+            gte_ldv0(v3);
+            gte_rtps();
+            gte_stsxy(&poly->x3);
 
             poly->r0 = poly->g0 = poly->b0 = (uint8_t)v0->pad;
             poly->r1 = poly->g1 = poly->b1 = (uint8_t)v1->pad;
             poly->r2 = poly->g2 = poly->b2 = (uint8_t)v2->pad;
+            poly->r3 = poly->g3 = poly->b3 = (uint8_t)v3->pad;
 
             addPrim(curOT + depth, poly);
         }