# Last edited on 2012-08-28 22:54:35 by stolfilocal

Wikimapia proposed feature:
POLYGONS AS CONCATENATED POLYLINES

Wikimapia should allow polygons to be defined as concatenation of independent 'polyline' objects.

The main motivation is that many polygons share boundaries with other
polygons. Representing these shared parts only once would save editor's
work and maintain consistency automatically.

Another motivation is that polygons with many points, too large to be
edited as a single objects, could be segmented into several smaller
polylines which could be edited individually.

Namely, in this proposal, the 'outline' of a Wikimapia object Z has a new
binary attribute OPEN or CLOSED, and may be

  (TYPE0) empty (missing)
  
  (TYPE1) a list of one or more points p[0],..p[n-1]

  (TYPE2) a list of one or more objects X[0],..X[m-1] with signs s[0],..s[m-1]

In TYPE1, the outline of Z consists of the polygonal line with corners
p[0],.. p[n-1]; namely, the n-1 segments p[i]<--->p[i+1], for i in
0..n-2. If Z is CLOSED, then n must be at least 3, and there
is also the segment p[n-1]<--->p[0].

In TYPE2, the outlines of the objects X[0],.. X[m-1] (the /children/ of Z) are
implicitly concatenated, in that order, with one extra segment between
the last point of of X[i] and the first point of X[i+1], for i in
0..m-2. If Z is CLOSED then the total number of points must
be at least 3, and there is also a segment from the last point of X[m-1] and
the first point of X[0]. Each sign s[i] is either +1 or -1; if the latter, the corresponding child 
X[i]} is implicitlly reversed.

In the current Wikimapia, objects with oultines are always TYPE1 CLOSED.

Under the new proposal, creating a new polygonal object from scratch
works pretty much as in the current Wikimapia. The user first draws a
polyline, and the object is implicitly OPEN. After adding three points,
the user may click on the initial point, which makes the outline CLOSED.
However, under the new proposal, the user shall also have the option of
saving the object while still in the OPEN state. Either way, the new
object is TYPE1.

There will be little change also when editing the outline of an existing
object Z. In both TYPEs, the user will see a yellow polygon or polyline
with draggable red dots at the corners. If Z is CLOSED, the closing
segment is drawn and the interior is filled, as now.

Dragging a point will behave just as now.  However, if Z is TYPE2, the 
action will actually modify a point of the TYPE1 descendant Y of Z 
that contains that point; and it will affect every other TYPE2 outline
that uses (is an ancestor of) object Y.

Inserting a new corner in Z has some restrictions. Suppose the corners 
or Z are p[0]...p[n-1] in order. We define J(Z,r,i) as i if r is +1,
and as n-i-1 if r is -1. We will denote B(Z,r,i) 
the operation of inserting a new corner in Z before point p[J(Z,r,i)] if
r is +1, and after that point if r is -1. Then prepending a new point at the 
beginning B(Z,+1,0) and appending at the end is B(Z,-1,0).
If Z is CLOSED then these last two operations are visually indistinguishable; the interface
may use the same gesture for both and choose arbitrarily between them.
(However, if Z is then made OPEN the system's choice will become obvious).

If Z is TYPE1 then the action B(Z,r,i) is performed on its
point list in the obvious way.  Now suppose Z is TYPE2.
let Y be the child of Z that owns u, and k the absolute
difference |J(Z,r,i) - J(Z,r,0)|  *ETC*
of 
such that corner j of Y is corner i of Z.

are applied to its first child X[0] and to its last child X[m-1],
respectively; except that I(Y) becomes F(Y), and vice-versa, if the
sign s[0] and 


equivalent to an I() operation on its firts Z Let U,V be the two TYPE1 descendants of Z that contain the
two corners.  The action   If U=V (in particular, when Z is TYPE1) 
then insertion is allowed and works as now.

If the two corners belong the same TYPE1 eleting a point from Z also 

The editor shall have some way of toggling the OPEN/CLOSED attribute of
Z while editing, as long as Z is a top-level object (not the child of
any other object).

If the object being edited is TYPE2, dragging a point will modify the point of some object