Each tile is identified by a set of coordinates (x,y).

There are coordinates (x,y) which doesn't refer to a tile. These
coordinates are called masked.

The set of tiles can be viewed in various forms. For all forms the
relation "tile x has coordinates (x,y)" is the same. Each of the form
serves a certain function.

Properties:
  - compact: fills a rectangle (ignoring masking)
  - sparse: has gaps (ignoring masking)
  - tile-aligned: preserves the local geometry between tiles

Base Form
=========

This form is the only one which operates on coordinates. All the
others (as noted above) operate on tiles.

The coordinates form a rectangle which has angle 0. Dimensions
corresponds to map.[xy]size.

Used for: internal storage
Properties: compact, not sparse, maybe tile-aligned
Example: grid_compact.png or below


##########
##########
##########

Masked Form
===========

Made out of base form by removing all masked coordinates.

Used for: external storage (savegame, gamelog), whole_map
Properties: compact, not sparse, maybe tile-aligned
Example: below (-|+ is the bounding box, as defined by base form)

+----------+
|### ######|
|###### ###|
|######## #|
+----------+

but also 

+---------------+
|       #       | 
|    #######    | 
|   #########   | 
|  ###########  | 
| ############# | 
| ############# | 
| ############# | 
|###############|
| ############# | 
| ############# | 
| ############# | 
|  ###########  | 
|   #########   | 
|    #######    | 
|       #       | 
+---------------+

Effective Form
==============

Used for: wrapping, neighborhood (adjc_iterate, square_iterate), step
(DIRSTEP, MAPSTEP)
Properties: maybe compact, maybe sparse, tile-aligned

This form depends if the map is an iso-map or not.

Effective Form (non-iso map)
----------------------------

Equivalent to masked form.

Neighborhood:

    (x-1, y-1)    (x, y-1)   (x+1, y-1)
    (x-1, y)      (x, y)     (x+1, y)
    (x-1, y+1)    (x, y+1)   (x+1, y+1)

Effective Form (iso map)
------------------------

There are two variants depending on which coordinate you
compress.

Effective Form (iso map) variant 1
----------------------------------

Example: grid_iso_rot.png
Neighborhood: 

    (x,   y-2)    (x,   y-1)     (x+1, y)
    (x-1, y-1)    (x,   y)       (x,   y+1)
    (x-1, y)      (x-1, y+1)     (x,   y+2)

Effective Form (iso map) variant 2
----------------------------------

Example: grid_iso2_rot.png
Neighborhood: 

    (x,   y-1)     (x+1, y-1)    (x+2, y)
    (x-1, y-1)     (x,   y)      (x+1, y)
    (x-2, y)       (x-1, y)      (x,   y+1)

View Form
=========

Depends on the use of iso or non-iso tileset.

Used for: mapview

View Form (non-iso tileset)
---------------------------

Same as the effective form.
Example: grid_compact.png

View Form (iso tileset)
-----------------------

Effective form rotated 45° right (angle=pi/4).

Example: grid_iso.png, grid_iso2.png

Indexed Form
============

One possible form is to concat all lines together.

  base form:
  123456
  7890ab
  cdefgh

  indexed form:

  1234567890abcdefgh

Used for: currently nothing.


Implementation Details
======================

  Base Form
  ---------

  Used for: internal storage
No changes needed.

  Masked Form
  -----------

  Used for: external storage (savegame, gamelog), whole_map
No changes needed.

  Effective Form
  --------------

  Used for: wrapping

Changes needed (IMHO easy).

  step(DIRSTEP, MAPSTEP), simple neighborhood (adjc_iterate)

Changes needed (easy if we redefine DIR_D[XY] at runtime).

  extended neighborhood (square_iterate), 

Changes needed (fairly easy if DIR_D[XY] is used).