[Freeciv-Dev] Re: (PR#1180) [PATCH] cleanup to canvas_pos_to_map_pos

["Gregory Berkolaiko via RT" <rt@xxxxxxxxxxxxxx>]

Same patch but with even uglier DIVIDE.

G.


On Tue, 19 Nov 2002, Jason Short via RT wrote:

> 
> [glip - Tue Nov 19 20:56:43 2002]:
> 
> > So, skipping N_T_ stuff and cancelling the common factors, the
> > transformation is
> > 
> > x_map = [ x / W + y / H]
> > y_map = [ y / H - x / W]
> 
> Here's a new patch.  It still has what Ross calls the "DIVIDE ugliness",
> but the operations are made atomic and described as a whole.  It refers
> the your diagram by its URL right here in RT - alternately the diagram
> could be put into CVS (in /doc) or onto the main freeciv web site.
> 
> jason
> 
> 

Index: client/citydlg_common.c
===================================================================
RCS file: /home/freeciv/CVS/freeciv/client/citydlg_common.c,v
retrieving revision 1.11
diff -u -r1.11 citydlg_common.c
--- client/citydlg_common.c     2002/11/14 09:14:50     1.11
+++ client/citydlg_common.c     2002/11/21 13:22:08
@@ -55,35 +55,30 @@
 **************************************************************************/
 void canvas_pos_to_city_pos(int canvas_x, int canvas_y, int *map_x, int *map_y)
 {
+  int orig_canvas_x = canvas_x, orig_canvas_y = canvas_y;
+
   if (is_isometric) {
-    *map_x = -2;
-    *map_y = 2;
+    const int W = NORMAL_TILE_WIDTH, H = NORMAL_TILE_HEIGHT;
 
-    /* first find an equivalent position on the left side of the screen. */
-    *map_x += canvas_x / NORMAL_TILE_WIDTH;
-    *map_y -= canvas_x / NORMAL_TILE_WIDTH;
-    canvas_x %= NORMAL_TILE_WIDTH;
+    /* Shift the tile right so the top corner of tile (-2,2) is at
+       canvas position (0,0). */
+    canvas_y += H / 2;
 
-    /* Then move op to the top corner. */
-    *map_x += canvas_y / NORMAL_TILE_HEIGHT;
-    *map_y += canvas_y / NORMAL_TILE_HEIGHT;
-    canvas_y %= NORMAL_TILE_HEIGHT;
+    /* Perform a pi/4 rotation, with scaling.  See canvas_pos_to_map_pos
+       for a full explanation. */
+    *map_x = DIVIDE(canvas_x * H + canvas_y * W, W * H);
+    *map_y = DIVIDE(canvas_y * W - canvas_x * H, W * H);
 
-    assert(NORMAL_TILE_WIDTH == 2 * NORMAL_TILE_HEIGHT);
-    canvas_y *= 2;             /* now we have a square. */
-    if (canvas_x + canvas_y > NORMAL_TILE_WIDTH / 2)
-      (*map_x)++;
-    if (canvas_x + canvas_y > 3 * NORMAL_TILE_WIDTH / 2)
-      (*map_x)++;
-    if (canvas_x - canvas_y > NORMAL_TILE_WIDTH / 2)
-      (*map_y)--;
-    if (canvas_y - canvas_x > NORMAL_TILE_WIDTH / 2)
-      (*map_y)++;
+    /* Add on the offset of the top-left corner to get the final
+       coordinates (like in canvas_pos_to_map_pos). */
+    *map_x -= 2;
+    *map_y += 2;
   } else {
     *map_x = canvas_x / NORMAL_TILE_WIDTH;
     *map_y = canvas_y / NORMAL_TILE_HEIGHT;
   }
-  freelog(LOG_DEBUG, "canvas_pos_to_city_pos(pos=(%d,%d))=(%d,%d)", canvas_x, 
canvas_y, *map_x, *map_y);
+  freelog(LOG_DEBUG, "canvas_pos_to_city_pos(pos=(%d,%d))=(%d,%d)",
+         orig_canvas_x, orig_canvas_y, *map_x, *map_y);
 }
 
 /**************************************************************************
Index: client/mapview_common.c
===================================================================
RCS file: /home/freeciv/CVS/freeciv/client/mapview_common.c,v
retrieving revision 1.15
diff -u -r1.15 mapview_common.c
--- client/mapview_common.c     2002/11/19 20:13:38     1.15
+++ client/mapview_common.c     2002/11/21 13:22:08
@@ -187,34 +187,42 @@
                                  int map_view_topleft_map_y)
 {
   if (is_isometric) {
-    *map_x = map_view_topleft_map_x;
-    *map_y = map_view_topleft_map_y;
+    /* The basic operation here is a simple pi/4 rotation; however, we
+     * have to first scale because the tiles have different width and
+     * height.  Mathematically, this looks like
+     *   | 1/W  1/H | |x|    |x`|
+     *   |          | | | -> |  |
+     *   |-1/W  1/H | |y|    |y`|
+     *
+     * Where W is the tile width and H the height.
+     *
+     * In simple terms, this is
+     *   map_x = [   x / W + y / H ]
+     *   map_y = [ - x / W + y / H ]
+     * where [q] stands for integer part of q.
+     *
+     * Here the division is proper mathematical floating point division.
+     *
+     * A picture demonstrating this can be seen at
+     * http://rt.freeciv.org/Ticket/Attachment/16782/9982/grid1.png.
+     *
+     * The calculation is complicated somewhat because of two things: we
+     * only use integer math, and C integer division rounds toward zero
+     * instead of rounding down.
+     *
+     * For another example of this math, see canvas_pos_to_city_pos().
+     */
+    const int W = NORMAL_TILE_WIDTH, H = NORMAL_TILE_HEIGHT;
 
-    /* first find an equivalent position on the left side of the screen. */
-    *map_x += canvas_x / NORMAL_TILE_WIDTH;
-    *map_y -= canvas_x / NORMAL_TILE_WIDTH;
-    canvas_x %= NORMAL_TILE_WIDTH;
-
-    /* Then move op to the top corner. */
-    *map_x += canvas_y / NORMAL_TILE_HEIGHT;
-    *map_y += canvas_y / NORMAL_TILE_HEIGHT;
-    canvas_y %= NORMAL_TILE_HEIGHT;
-
-    /* We are inside a rectangle, with 2 half tiles starting in the
-       corner, and two tiles further out. Draw a grid to see how this
-       works :). */
-    assert(NORMAL_TILE_WIDTH == 2 * NORMAL_TILE_HEIGHT);
-    canvas_y *= 2;             /* now we have a square. */
-    if (canvas_x > canvas_y) {
-      *map_y -= 1;
-    }
-    if (canvas_x + canvas_y > NORMAL_TILE_WIDTH) {
-      *map_x += 1;
-    }
+    *map_x = DIVIDE(canvas_x * H + canvas_y * W, W * H);
+    *map_y = DIVIDE(canvas_y * W - canvas_x * H, W * H);
   } else {                     /* is_isometric */
-    *map_x = map_view_topleft_map_x + canvas_x / NORMAL_TILE_WIDTH;
-    *map_y = map_view_topleft_map_y + canvas_y / NORMAL_TILE_HEIGHT;
+    *map_x = canvas_x / NORMAL_TILE_WIDTH;
+    *map_y = canvas_y / NORMAL_TILE_HEIGHT;
   }
+
+  *map_x += map_view_topleft_map_x;
+  *map_y += map_view_topleft_map_y;
 
   /*
    * If we are outside the map find the nearest tile, with distance as
Index: common/shared.h
===================================================================
RCS file: /home/freeciv/CVS/freeciv/common/shared.h,v
retrieving revision 1.105
diff -u -r1.105 shared.h
--- common/shared.h     2002/11/13 19:44:25     1.105
+++ common/shared.h     2002/11/21 13:22:08
@@ -65,6 +65,13 @@
 
 #define BOOL_VAL(x) ((x)!=0)
 
+/*
+ * DIVIDE() divides and rounds down, rather than just divides and
+ * rounds toward 0.  It is assumed that the divisor is positive.
+ */
+#define DIVIDE(n, d) \
+    ( (n) / (d) - ( (n) < 0 && (n) % (d) < 0 ) )
+
 /* Deletes bit no in val,
    moves all bits larger than no one down,
    and inserts a zero at the top. */