fixed half-tile shifting when teleporting between playfield borders

When relocating the player without centering the screen and without
scrolling ("teleporting" to a different playfield area, but keeping
the screen position), shifting the visible part of the playfield by
half a tile may happen if the screen width or height is even.

This is especially true when teleporting from and to an area that is
directly aligned with the border of the playfield (regardless of the
tiles at the playfield border being only half or fully visible, which
only happens with even sized screen width or height, depending on the
current player scroll position).

This change makes sure that both the source and destination area of
the playfield are displayed either aligned by half a tile or a full
tile when doing a "teleporting" player relocation in such a case.

diff --git a/src/game.c b/src/game.c
index 88fa0a6e3e19a3d80126795e35459f6d0f3d167e..94b74f786cdc2c7d211af21d484a3cec17a22eb0 100644 (file)
--- a/src/game.c
+++ b/src/game.c
@@ -5687,6 +5687,40 @@ static void DrawRelocateScreen(int old_x, int old_y, int x, int y,
     // make sure that shifted scroll position does not scroll beyond screen
     new_scroll_x = SCROLL_POSITION_X(shifted_scroll_x + MIDPOSX);
     new_scroll_y = SCROLL_POSITION_Y(shifted_scroll_y + MIDPOSY);
+
+    // special case for teleporting from one end of the playfield to the other
+    // (this kludge prevents the destination area to be shifted by half a tile
+    // against the source destination for even screen width or screen height;
+    // probably most useful when used with high "game.forced_scroll_delay_value"
+    // in combination with "game.forced_scroll_x" and "game.forced_scroll_y")
+    if (quick_relocation)
+    {
+      if (EVEN(SCR_FIELDX))
+      {
+       // relocate (teleport) between left and right border (half or full)
+       if (scroll_x == SBX_Left && new_scroll_x == SBX_Right - 1)
+         new_scroll_x = SBX_Right;
+       else if (scroll_x == SBX_Left + 1 && new_scroll_x == SBX_Right)
+         new_scroll_x = SBX_Right - 1;
+       else if (scroll_x == SBX_Right && new_scroll_x == SBX_Left + 1)
+         new_scroll_x = SBX_Left;
+       else if (scroll_x == SBX_Right - 1 && new_scroll_x == SBX_Left)
+         new_scroll_x = SBX_Left + 1;
+      }
+
+      if (EVEN(SCR_FIELDY))
+      {
+       // relocate (teleport) between top and bottom border (half or full)
+       if (scroll_y == SBY_Upper && new_scroll_y == SBY_Lower - 1)
+         new_scroll_y = SBY_Lower;
+       else if (scroll_y == SBY_Upper + 1 && new_scroll_y == SBY_Lower)
+         new_scroll_y = SBY_Lower - 1;
+       else if (scroll_y == SBY_Lower && new_scroll_y == SBY_Upper + 1)
+         new_scroll_y = SBY_Upper;
+       else if (scroll_y == SBY_Lower - 1 && new_scroll_y == SBY_Upper)
+         new_scroll_y = SBY_Upper + 1;
+      }
+    }
   }
 
   if (quick_relocation)