From 0c893ea1232c96f43d6934bdce8f954ae05b1203 Mon Sep 17 00:00:00 2001
From: Jens Rottmann <30634967+JRottm@users.noreply.github.com>
Date: Fri, 4 Aug 2017 21:48:32 +0200
Subject: [PATCH] Fix player coordinate rounding in collisionMoveSimple()
 (#6197)

To determine the area (nodes) where a player movement took place
collisionMoveSimple() first took the old/new player coordinates and rounded
them to integers, then added the player character's collision box and
implicitely rounded the result. This has 2 problems:

Rounding the position and the box seperately, then adding the resulting
integers means you get twice the rounding error. And implicit rounding
always rounds towards 0.0, unlike floatToInt(), which rounds towards the
closest integer.

Previous (simplified) behavior: round(pos)+(int)box, for example player at
Y=0.9, body is 1.75m high: round(0.9)+(int)1.75 = 1+1 = 2.
==> A character's height of 1.75m always got rounded down to 1m, its width
of +/-0.3 even became 0.

Fixed by adding the floats first, then rounding properly: round(pos+box) =
round(0.9+1.75) = round(2.65) = 3.
---
 src/collision.cpp | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/src/collision.cpp b/src/collision.cpp
index 61972b2e8..9bd758995 100644
--- a/src/collision.cpp
+++ b/src/collision.cpp
@@ -257,20 +257,25 @@ collisionMoveResult collisionMoveSimple(Environment *env, IGameDef *gamedef,
 	//TimeTaker tt2("collisionMoveSimple collect boxes");
 	ScopeProfiler sp(g_profiler, "collisionMoveSimple collect boxes avg", SPT_AVG);
 
-	v3s16 oldpos_i = floatToInt(*pos_f, BS);
-	v3s16 newpos_i = floatToInt(*pos_f + *speed_f * dtime, BS);
-	s16 min_x = MYMIN(oldpos_i.X, newpos_i.X) + (box_0.MinEdge.X / BS) - 1;
-	s16 min_y = MYMIN(oldpos_i.Y, newpos_i.Y) + (box_0.MinEdge.Y / BS) - 1;
-	s16 min_z = MYMIN(oldpos_i.Z, newpos_i.Z) + (box_0.MinEdge.Z / BS) - 1;
-	s16 max_x = MYMAX(oldpos_i.X, newpos_i.X) + (box_0.MaxEdge.X / BS) + 1;
-	s16 max_y = MYMAX(oldpos_i.Y, newpos_i.Y) + (box_0.MaxEdge.Y / BS) + 1;
-	s16 max_z = MYMAX(oldpos_i.Z, newpos_i.Z) + (box_0.MaxEdge.Z / BS) + 1;
+	v3f newpos_f = *pos_f + *speed_f * dtime;
+	v3f minpos_f(
+		MYMIN(pos_f->X, newpos_f.X),
+		MYMIN(pos_f->Y, newpos_f.Y),
+		MYMIN(pos_f->Z, newpos_f.Z)
+	);
+	v3f maxpos_f(
+		MYMAX(pos_f->X, newpos_f.X),
+		MYMAX(pos_f->Y, newpos_f.Y),
+		MYMAX(pos_f->Z, newpos_f.Z)
+	);
+	v3s16 min = floatToInt(minpos_f + box_0.MinEdge, BS) - v3s16(1, 1, 1);
+	v3s16 max = floatToInt(maxpos_f + box_0.MaxEdge, BS) + v3s16(1, 1, 1);
 
 	bool any_position_valid = false;
 
-	for(s16 x = min_x; x <= max_x; x++)
-	for(s16 y = min_y; y <= max_y; y++)
-	for(s16 z = min_z; z <= max_z; z++)
+	for(s16 x = min.X; x <= max.X; x++)
+	for(s16 y = min.Y; y <= max.Y; y++)
+	for(s16 z = min.Z; z <= max.Z; z++)
 	{
 		v3s16 p(x,y,z);