From e97ea1c5cc3c86894d8a32460ef098338bdcf660 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Miloslav=20=C4=8C=C3=AD=C5=BE?= <tastyfish@seznam.cz>
Date: Thu, 16 May 2019 22:02:50 +0200
Subject: [PATCH] Fix z-divide

---
 s3l.h     | 15 +++++++++++++--
 testSDL.c | 13 +++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/s3l.h b/s3l.h
index e76ff0b..95a77c1 100644
--- a/s3l.h
+++ b/s3l.h
@@ -323,7 +323,7 @@ typedef S3L_Unit S3L_Mat4[4][4]; /**< 4x4 matrix, used mostly for 3D
                                       transforms. The indexing is this:
                                       matrix[column][row]. */
 #define S3L_writeMat4(m)\
-  printf("Mat4:\n  %d %d %d %d\n  %d %d %d %d\n  %d %d %d %d\n  %d %d %d %d\n"\
+  printfi"Mat4:\n  %d %d %d %d\n  %d %d %d %d\n  %d %d %d %d\n  %d %d %d %d\n"\
    ,(m)[0][0],(m)[1][0],(m)[2][0],(m)[3][0],\
     (m)[0][1],(m)[1][1],(m)[2][1],(m)[3][1],\
     (m)[0][2],(m)[1][2],(m)[2][2],(m)[3][2],\
@@ -1356,7 +1356,14 @@ void S3L_makeCameraMatrix(S3L_Transform3D cameraTransform, S3L_Mat4 *m)
 static inline void S3L_perspectiveDivide(S3L_Vec4 *vector,
   S3L_Unit focalLength)
 {
-  S3L_Unit divisor = S3L_nonZero(vector->z);
+  S3L_Unit divisor = vector->z > 0 ? vector->z : (-1 * (vector->z - 1));
+  /* ^ This has two purposes:
+
+    1. Prevent division by zero.
+    2. Prevent a "rapid flip" of the vertex, e.g.: having a vertex
+       [100,0,0.1] z-divides it to [1000,0], but when it shift a short
+       distance to [100,0,-0.1], it z-divides to [-1000,0], rapidly flipping
+       from right to the left. */
 
   vector->x = (vector->x * focalLength) / divisor;
   vector->y = (vector->y * focalLength) / divisor;
@@ -1398,8 +1405,12 @@ void S3L_drawModelIndexed(
       transformed##n.x = pointModel.x;\
       transformed##n.y = pointModel.y;\
       transformed##n.z = pointModel.z;\
+      transformed##n.w = S3L_FRACTIONS_PER_UNIT;\
       S3L_perspectiveDivide(&transformed##n,camera->focalLength);
 
+    /* TODO: maybe create an option that would use a cache here to not
+             transform the same point twice? */
+
     project(0)
     project(1)
     project(2)
diff --git a/testSDL.c b/testSDL.c
index f2272a7..b9a9ed8 100644
--- a/testSDL.c
+++ b/testSDL.c
@@ -127,6 +127,18 @@ S3L_DrawConfig conf;
 
 void draw()
 {
+/*
+  if (frame % 128 == 0)
+  {
+    printf("frame: %d\n",frame);
+    printf("camera:\n");
+    printf("  translation: ");
+    S3L_writeVec4(camera.transform.translation);
+    printf("  rotation: ");
+    S3L_writeVec4(camera.transform.rotation);
+  }
+*/
+
   offScreenPixels = 0;
 
   clearScreen();
@@ -198,6 +210,7 @@ int main()
   S3L_initCamera(&camera);
 
   camera.transform.translation.z = -S3L_FRACTIONS_PER_UNIT * 2;
+
 //  camera.transform.translation.x = S3L_FRACTIONS_PER_UNIT;
 //  camera.transform.translation.y = S3L_FRACTIONS_PER_UNIT;