From 06ac67fd332c3be6ea9793809d393675b3156de0 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Fri, 25 Oct 2019 08:49:31 -0400
Subject: render: model: Correctly handle faces with 4 elements in OBJ loader.

---
 chickadee/render/model.scm | 50 ++++++++++++++++++++++++++++++++--------------
 1 file changed, 35 insertions(+), 15 deletions(-)

diff --git a/chickadee/render/model.scm b/chickadee/render/model.scm
index 1a9356d..42bfc9a 100644
--- a/chickadee/render/model.scm
+++ b/chickadee/render/model.scm
@@ -317,24 +317,44 @@
               (let ((new-indices (make-array-list)))
                 (hash-set! face-indices-map material new-indices)
                 new-indices)))
+        (define (deduplicate-face-element e)
+          ;; Faces are often redundant, so we deduplicate in order to
+          ;; make the VBOs we build later as small as possible.
+          (or (hash-ref face-map e)
+              (let ((i (array-list-size faces)))
+                (array-list-push! faces (parse-face-element e))
+                (hash-set! face-map e i)
+                i)))
+        (define (push-face material e)
+          (array-list-push! (indices-for-material material)
+                            (deduplicate-face-element e)))
         (define (parse-face args material)
           (match args
-            ((and (_ _ _ . _) args)
-             (for-each (lambda (e)
-                         ;; Faces may very well be redundant, so we
-                         ;; deduplicate in order to make the VBOs we
-                         ;; build later as small as possible.
-                         (let ((cached-index (hash-ref face-map e))
-                               (face-indices (indices-for-material material)))
-                           (if cached-index
-                               (array-list-push! face-indices cached-index)
-                               (let ((i (array-list-size faces)))
-                                 (array-list-push! faces (parse-face-element e))
-                                 (array-list-push! face-indices i)
-                                 (hash-set! face-map e i)))))
-                       args))
+            ;; Simple triangle
+            ((a b c)
+             (push-face material a)
+             (push-face material b)
+             (push-face material c))
+            ;; Quadrilateral, need to split into 2 triangles
+            ;;
+            ;; d-------c
+            ;; |      /|
+            ;; |    /  |
+            ;; |  /    |
+            ;; |/      |
+            ;; a-------b
+            ;;
+            ;; triangle 1: a b c
+            ;; triangle 2: a c d
+            ((a b c d)
+             (push-face material a)
+             (push-face material b)
+             (push-face material c)
+             (push-face material a)
+             (push-face material c)
+             (push-face material d))
             (_
-             (parse-error "wrong number of face arguments" args))))
+             (parse-error "unsupported number of face elements" args))))
         ;; Register default material
         (hash-set! material-map "default" default-phong-material)
         ;; Parse file.
-- 
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