From 3a29d4c679cf233be4df9f2ec5f89a3b8086711b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Miloslav=20=C4=8C=C3=AD=C5=BE?= <tastyfish@seznam.cz>
Date: Tue, 11 Jun 2019 15:37:40 +0200
Subject: [PATCH] Add correction funtion

---
 programs/demo.c |  4 +++-
 small3dlib.h    | 30 +++++++++++++++++++++++++++---
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/programs/demo.c b/programs/demo.c
index 419a775..87506af 100644
--- a/programs/demo.c
+++ b/programs/demo.c
@@ -11,7 +11,7 @@
 
 #define S3L_FLAT 0
 #define S3L_STRICT_NEAR_CULLING 0
-#define S3L_PERSPECTIVE_CORRECTION 1
+#define S3L_PERSPECTIVE_CORRECTION 2
 #define S3L_SORT 2
 #define S3L_STENCIL_BUFFER 1
 #define S3L_Z_BUFFER 0
@@ -80,6 +80,8 @@ void drawPixel(S3L_PixelInfo *p)
 {
 
 /*
+ S3L_correctBarycentricCoords(p->barycentric);
+
 if (p->barycentric[0] < 0 || p->barycentric[0] > 512)
   printf("%d %d %d\n",p->barycentric[0],p->barycentric[1],p->barycentric[2]);
 
diff --git a/small3dlib.h b/small3dlib.h
index 1acbd70..21b6558 100644
--- a/small3dlib.h
+++ b/small3dlib.h
@@ -448,9 +448,11 @@ typedef struct
                               triangle and interpolate values between it's
                               three points. Each one goes from 0 to
                               S3L_FRACTIONS_PER_UNIT (inclidung), but due to
-                              rounding error may fall outside this range. The
-                              sum of the three coordinates will always be
-                              exactly S3L_FRACTIONS_PER_UNIT. */
+                              rounding error may fall outside this range (you
+                              can use S3L_correctBarycentricCoords to fix this
+                              for the price of some performance). The sum of
+                              the three coordinates will always be exactly
+                              S3L_FRACTIONS_PER_UNIT. */
   S3L_Index triangleIndex;  ///< Triangle index.
   S3L_Index modelIndex;
   S3L_Unit depth;         ///< Depth (only if depth is turned on).
@@ -466,6 +468,12 @@ typedef struct
 
 static inline void S3L_initPixelInfo(S3L_PixelInfo *p);
 
+/** Corrects barycentric coordinates so that they exactly meet the defined
+  conditions (each fall into <0,S3L_FRACTIONS_PER_UNIT>, sum =
+  S3L_FRACTIONS_PER_UNIT). Note that doing this per-pixel can slow the program
+  down significantly. */
+static inline void S3LcorrectBarycentricCoords(S3L_Unit barycentric[3]);
+
 // general helper functions
 static inline S3L_Unit S3L_abs(S3L_Unit value);
 static inline S3L_Unit S3L_min(S3L_Unit v1, S3L_Unit v2);
@@ -1138,6 +1146,22 @@ S3L_Unit S3L_cos(S3L_Unit x)
   return S3L_sin(x - S3L_FRACTIONS_PER_UNIT / 4);
 }
 
+void S3L_correctBarycentricCoords(S3L_Unit barycentric[3])
+{
+  barycentric[0] = S3L_clamp(barycentric[0],0,S3L_FRACTIONS_PER_UNIT);
+  barycentric[1] = S3L_clamp(barycentric[1],0,S3L_FRACTIONS_PER_UNIT);
+
+  S3L_Unit d = S3L_FRACTIONS_PER_UNIT - barycentric[0] - barycentric[1];
+
+  if (d < 0)
+  {
+    barycentric[0] += d;
+    barycentric[2] = 0;
+  }
+  else
+    barycentric[2] = d;
+}
+
 void S3L_makeTranslationMat(
   S3L_Unit offsetX,
   S3L_Unit offsetY,