From 86c9c009b09ed690a6b9d58f36a394f9fedfcceb Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Sat, 26 Oct 2019 19:07:48 -0400
Subject: render: model: Support faces with 5 or more elements in OBJ loader.

---
 chickadee/render/model.scm | 39 +++++++++++++++++++++++++++++++++------
 1 file changed, 33 insertions(+), 6 deletions(-)

diff --git a/chickadee/render/model.scm b/chickadee/render/model.scm
index 42bfc9a..cf8b917 100644
--- a/chickadee/render/model.scm
+++ b/chickadee/render/model.scm
@@ -330,12 +330,12 @@
                             (deduplicate-face-element e)))
         (define (parse-face args material)
           (match args
-            ;; Simple triangle
+            ;; A single triangle.  Ah, life is so simple...
             ((a b c)
              (push-face material a)
              (push-face material b)
              (push-face material c))
-            ;; Quadrilateral, need to split into 2 triangles
+            ;; A quadrilateral.  Needs to be split into 2 triangles.
             ;;
             ;; d-------c
             ;; |      /|
@@ -343,18 +343,45 @@
             ;; |  /    |
             ;; |/      |
             ;; a-------b
-            ;;
-            ;; triangle 1: a b c
-            ;; triangle 2: a c d
             ((a b c d)
+             ;; triangle 1: a b c
              (push-face material a)
              (push-face material b)
              (push-face material c)
+             ;; triangle 2: a c d
              (push-face material a)
              (push-face material c)
              (push-face material d))
+            ;; 3 or more triangles.  Interpret as a strip of triangles
+            ;; moving from right to left (because counter-clockwise
+            ;; winding) like this:
+            ;;
+            ;; h-------f-------d-------c
+            ;; |      /|      /|      /|
+            ;; |    /  |    /  |    /  |
+            ;; |  /    |  /    |  /    |
+            ;; |/      |/      |/      |
+            ;; g-------e-------a-------b
+            ;;
+            ;; ... and so on for however many face elements there are.
+            ;; Every other triangle is flipped over, hence the 'flip?'
+            ;; flag in the loop below.
+            ((a b . rest)
+             (let loop ((a a)
+                        (b b)
+                        (args rest)
+                        (flip? #f))
+               (match args
+                 (() #t)
+                 ((c . rest)
+                  (push-face material a)
+                  (push-face material b)
+                  (push-face material c)
+                  (if flip?
+                      (loop c a rest #f)
+                      (loop a c rest #t))))))
             (_
-             (parse-error "unsupported number of face elements" args))))
+             (parse-error "invalid face" args))))
         ;; Register default material
         (hash-set! material-map "default" default-phong-material)
         ;; Parse file.
-- 
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