From 33783250932a78695c24ad32bdd35dde39eae7d9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Miloslav=20=C4=8C=C3=AD=C5=BE?= <tastyfish@seznam.cz>
Date: Mon, 3 Jun 2019 18:06:06 +0200
Subject: [PATCH] Fix disappearing triangles

---
 small3dlib.h | 12 ++++++++++++
 testSDL.c    |  6 +++---
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/small3dlib.h b/small3dlib.h
index 7336b41..83beaff 100644
--- a/small3dlib.h
+++ b/small3dlib.h
@@ -1531,6 +1531,18 @@ void S3L_drawTriangle(
                                                   loop */
 #endif
 
+      if (rXClipped < lXClipped)
+      {
+        /* This can sometimes happen because of numerical errors in sorting
+        left vs right triangle point, which are compared based on SLOPE, not
+        x coordinates. Here we swap the values to prevents not drawing the
+        triagle. */
+
+        S3L_ScreenCoord tmp = rXClipped;
+        rXClipped = lXClipped;
+        lXClipped = tmp;
+      }
+
       // draw the row -- inner loop:
 
       for (S3L_ScreenCoord x = lXClipped; x < rXClipped; ++x)
diff --git a/testSDL.c b/testSDL.c
index 1f75f8b..0ef8922 100644
--- a/testSDL.c
+++ b/testSDL.c
@@ -18,8 +18,8 @@
 
 #define S3L_PIXEL_FUNCTION drawPixel
 
-#define S3L_RESOLUTION_X 640
-#define S3L_RESOLUTION_Y 480
+#define S3L_RESOLUTION_X 110
+#define S3L_RESOLUTION_Y 88
 
 #include "small3dlib.h"
 
@@ -280,7 +280,7 @@ scene.camera.transform.rotation.x = -35;
 scene.camera.transform.rotation.y = 128;
 scene.camera.transform.rotation.z = 0;
 
-S3L_setTransform3D(-542,-449,3259,39,216,0,512,512,512,&(scene.camera.transform));
+S3L_setTransform3D(-542,-449,4000,39,216,0,512,512,512,&(scene.camera.transform));
 
   scene.modelCount = 2;
   scene.models = models;