From 4ed5e5fd842322a39e6394318a25a4321e775b18 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Miloslav=20=C4=8C=C3=AD=C5=BE?= <tastyfish@seznam.cz>
Date: Sat, 22 Jun 2019 14:41:30 +0200
Subject: [PATCH] Add screen projection function

---
 programs/level.c | 28 +++++++++++++++++++++
 small3dlib.h     | 63 ++++++++++++++++++++++++++++++++++++++++--------
 todo.txt         |  2 +-
 3 files changed, 82 insertions(+), 11 deletions(-)

diff --git a/programs/level.c b/programs/level.c
index 0dc38b7..65b2f1c 100644
--- a/programs/level.c
+++ b/programs/level.c
@@ -29,6 +29,8 @@
 S3L_Model3D models[3];
 S3L_Scene scene;
 
+S3L_Vec4 teleportPoint;
+
 uint32_t pixels[S3L_RESOLUTION_X * S3L_RESOLUTION_Y];
 
 uint32_t frame = 0;
@@ -80,6 +82,20 @@ void sampleTxture(S3L_Unit u, S3L_Unit v, uint8_t *r, uint8_t *g, uint8_t *b)
   *b = texture[index];
 }
 
+void drawTeleport(int16_t x, int16_t y, S3L_ScreenCoord size)
+{
+  int16_t halfSize = size / 2;
+
+  S3L_ScreenCoord x0 = S3L_max(0,x - halfSize);
+  S3L_ScreenCoord x1 = S3L_min(S3L_RESOLUTION_X - 1,x + halfSize);
+  S3L_ScreenCoord y0 = S3L_max(0,y - halfSize);
+  S3L_ScreenCoord y1 = S3L_min(S3L_RESOLUTION_Y - 1,y + halfSize);
+
+  for (S3L_ScreenCoord j = y0; j < y1; ++j)
+    for (S3L_ScreenCoord i = x0; i < x1; ++i)
+      setPixel(i,j,255,0,0);
+}
+
 void drawPixel(S3L_PixelInfo *p)
 {
   if (p->triangleID != previousTriangle)
@@ -142,6 +158,13 @@ void draw()
 
   S3L_drawScene(scene);
 
+  S3L_Vec4 screenPoint;
+
+  project3DPointToScreen(teleportPoint,scene.camera,&screenPoint);
+
+  if (screenPoint.z < S3L_zBufferRead(screenPoint.x,screenPoint.y)) 
+    drawTeleport(screenPoint.x,screenPoint.y,screenPoint.w);
+
   clock_t nowT = clock();
 
   double timeDiff = ((double) (nowT - nextT)) / CLOCKS_PER_SEC;
@@ -167,6 +190,11 @@ int main()
   SDL_Surface *screenSurface = SDL_GetWindowSurface(window);
   SDL_Event event;
 
+  teleportPoint.x = 6 * S3L_FRACTIONS_PER_UNIT;
+  teleportPoint.y = -3 * S3L_FRACTIONS_PER_UNIT;
+  teleportPoint.z = 2 * S3L_FRACTIONS_PER_UNIT;
+  teleportPoint.w = S3L_FRACTIONS_PER_UNIT;
+
   nextT = clock();
 
   S3L_initCamera(&scene.camera);
diff --git a/small3dlib.h b/small3dlib.h
index 44a67db..b553e7d 100644
--- a/small3dlib.h
+++ b/small3dlib.h
@@ -512,6 +512,15 @@ static inline S3L_Unit S3L_cos(S3L_Unit x);
 S3L_Unit S3L_vec3Length(S3L_Vec4 v);
 S3L_Unit S3L_sqrt(S3L_Unit value);
 
+/** Projects a single point from 3D space to the screen space (pixels), which
+  can be useful e.g. for drawing sprites. The w component of input and result
+  holds the point size. If this size is 0 in the result, the sprite is outside
+  the view. */
+void project3DPointToScreen(
+  S3L_Vec4 point,
+  S3L_Camera camera,
+  S3L_Vec4 *result);
+
 /** Computes a normalized normal of given triangle. */
 void S3L_triangleNormal(S3L_Vec4 t0, S3L_Vec4 t1, S3L_Vec4 t2,
   S3L_Vec4 *n);
@@ -1419,6 +1428,50 @@ void S3L_initTransoform3D(S3L_Transform3D *t)
   t->scale.w = 0;
 }
 
+/**
+  Performs perspecive division (z-divide). Does NOT check for division by zero.
+*/
+static inline void S3L_perspectiveDivide(S3L_Vec4 *vector,
+  S3L_Unit focalLength)
+{
+  vector->x = (vector->x * focalLength) / vector->z;
+  vector->y = (vector->y * focalLength) / vector->z;
+}
+
+void project3DPointToScreen(
+  S3L_Vec4 point,
+  S3L_Camera camera,
+  S3L_Vec4 *result)
+{
+  S3L_Mat4 m;
+  S3L_makeCameraMatrix(camera.transform,&m);
+
+  S3L_Unit s = point.w;
+
+  point.w = S3L_FRACTIONS_PER_UNIT;
+
+  S3L_vec3Xmat4(&point,&m);
+
+  point.z = S3L_nonZero(point.z);
+
+  S3L_perspectiveDivide(&point,camera.focalLength);
+
+  S3L_ScreenCoord x, y;
+
+  S3L_mapProjectionPlaneToScreen(point,&x,&y);
+
+  result->x = x;
+  result->y = y;
+  result->z = point.z;
+
+  result->w =
+    (point.z < 0) ? 0 :
+    (
+      (s * camera.focalLength * S3L_RESOLUTION_X) /
+        (point.z * S3L_FRACTIONS_PER_UNIT)
+    );
+}
+
 void S3L_lookAt(S3L_Vec4 pointTo, S3L_Transform3D *t)
 {
   S3L_Vec4 v;
@@ -2296,16 +2349,6 @@ void S3L_makeCameraMatrix(S3L_Transform3D cameraTransform, S3L_Mat4 *m)
   S3L_mat4Xmat4(m,&r);
 }
 
-/**
-  Performs perspecive division (z-divide). Does NOT check for division by zero.
-*/
-static inline void S3L_perspectiveDivide(S3L_Vec4 *vector,
-  S3L_Unit focalLength)
-{
-  vector->x = (vector->x * focalLength) / vector->z;
-  vector->y = (vector->y * focalLength) / vector->z;
-}
-
 int8_t S3L_triangleWinding(
   S3L_ScreenCoord x0,
   S3L_ScreenCoord y0, 
diff --git a/todo.txt b/todo.txt
index f344893..289348c 100644
--- a/todo.txt
+++ b/todo.txt
@@ -16,7 +16,7 @@ features:
 - function to set/clear stencil buffer -- can be useful
 
 - function to map one point in 3D space to screen, along with size (for mapping
-  billboards/sprites etc.)
+  billboards/sprites etc.)                                DONE
 
 - Optimize persp. correction!