Draw water surfaces as unlit textured polygons

draw.h

@@ -223,4 +223,65 @@ static INLINE void draw_quad_strip_tex(STVECTOR *verts, u_char numVerts, u_short
     }
 }
 
+static INLINE void draw_quad_strip_water(STVECTOR *verts, u_char numVerts, u_short tpage)
+{
+    int p;
+
+    // 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 = (numVerts - 1) / 2;
+    if (!mem_checkprim(sizeof(POLY_FT4), numQuads))
+        return;
+
+    // Draw the face as a quad strip
+    const STVECTOR *v0, *v1, *v2, *v3;
+    const STVECTOR *head = verts;
+    const STVECTOR *tail = verts + numVerts;
+
+    // Initialize the first two vertices
+    v2 = --tail;
+    v3 = head++;
+
+    for (u_char quadIdx = 0; quadIdx < numQuads; ++quadIdx)
+    {
+        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
+
+        // Average Z for depth sorting and culling
+        gte_avsz3();
+        gte_stotz(&p);
+        short depth = p >> 2;
+        if (depth <= 0 || depth >= OTLEN)
+            continue;
+
+        // Draw a flat-shaded untextured colored quad
+        POLY_FT4 *poly = (POLY_FT4*)mem_prim(sizeof(POLY_FT4));
+        setPolyFT4(poly);
+        gte_stsxy0(&poly->x0);
+        gte_stsxy1(&poly->x1);
+        gte_stsxy2(&poly->x2);
+
+        // Transform the fourth vertex to complete the quad
+        gte_ldv0(v3);
+        gte_rtps();
+        gte_stsxy(&poly->x3);
+
+        // Texture UVs
+        setUV4(poly, v0->u, v0->v, v1->u, v1->v, v2->u, v2->v, v3->u, v3->v);
+        poly->clut = quake_clut;
+        poly->tpage = tpage;
+
+        // Unlit
+        poly->r0 = poly->g0 = poly->b0 = 255;
+
+        addPrim(curOT + depth, poly);
+        ++polyCount;
+    }
+}
+
 #endif  // __DRAW_H__

world.c

@@ -128,7 +128,10 @@ static void world_drawface(const world_t *world, const ps1bsp_face_t *face)
                 curVert->pad = polyVertex->light;
             }
 
-            draw_quad_strip_tex(verts, poly->numPolyVertices, faceTexture->tpage);
+            if (face->flags & SURF_DRAWWATER)
+                draw_quad_strip_water(verts, poly->numPolyVertices, faceTexture->tpage);
+            else
+                draw_quad_strip_tex(verts, poly->numPolyVertices, faceTexture->tpage);
         }
     }
 }