[mkgmap-dev] Area splitting

Sat Jan 29 22:29:52 GMT 2011

Hi

It would be good if we had a tool to visualise the strategies used to 
allocate elements to the subdivisions. Something that displays the 
subdivision boundaries and highlights the elements that belong to a
subdivision in some way.

I really have no idea of the best way to do this.

> 1. Class Area definition
> An area is defined with minLat, minLong, maxLat, maxLong. It is not
> possible to define an area with minLat==maxLat or minLong==maxLong. Is
> there any reason for this?

Perhaps it is but then it would not be able to contain anything except 
perhaps a vertical or horizontal line. Why do you think it is important 
that it should be possible.

> 2. Class Area split
> The split method divides the area into a given number of (rather) equal
> sized subareas.
> An area [(0,0) to (100,100)] would be split (xsplit==2, ysplit==1) into
> [(0,0) to (50,100)]
> [(50,0) to (100,100)]
> I think this is wrong, because the two areas are overlapping at x==50.
> Does anyone know why the split method does not split into distinct areas?

This is just a grid used to allocate elements to subdivision.  In 
reality the subdivision is expanded to contain its elements. So they 
always overlap by quite a bit.

When a division is split, then each element is allocated in one of the 
two new divisions. When the splitting is finished the real subdivision 
bounds are calculated based on the actual contents. The only important 
thing is that each element can be allocated into exactly one of the new 
divisions.

> 3. Class MapSplitter
> As far as I understand lines and shapes are put to the subdivision that
> contain their center points.

I can't remember the whole history here, I think at one time I used the 
first point in the line, but it didn't produce particularly uniform results.

> Assuming the situation (I describe a line example but the same applies
> to shapes)
>
>            E
>    ------- +
>    |     | +
>    |  x  | +
>    ------- +
>            +
> S+++++++++
>
> S = start point of a line
> E = end point of a line
> + = line
> x = center point
> -| = borders of the subdivision
>
> The line is defined in the subdivision although it does not intersect
> the subdivision. But its center point is located in the subdivision.
> Is that correct?

Because the grid is only used to allocate elements to subdivisions, it 
doesn't matter that no point is actually in the grid section.  The 
subdivision will always be sized to contain its contents.

I'm not saying that its a good thing though. Possible room for 
improvement there if it happens often.

> The coordinates of the line points are stored as diffs to the center of
> the subdivision as a kind of compression. The diffs have a higher and
> lower limit. If a line is quite long and one point is far away from the
> center of the subdivision it could not be stored. What happens in such a
> case? I think a warning message is printed in the constructor of the
> Subdivision class but what happens to the line point?
> Would it be better to split a line in such a case? Or would it be better
> to split a line so that the complete line fits into a subdivision?

Well this is the main problem.  We split lines earlier on.  Originally I 
split lines so that in the worst case they would never be too big to fit 
into a subdivision. Perhaps we should split them after we know roughly 
where the subdivision boundaries are going to be.

> 4. Subdivision splitting
> In case a subdivision is too full it is splitted into equal sized areas.
> Would it be a good first approach to improve the splitting so that the
> algorithm tries to split the subdivision into rather equal filled
> subdivisions (e.g. each splitted subdivision should contain the same
> number of points plus center points of lines/shapes?

Probably yes!

I think our existing approach gives sub-divisions that overlap a lot 
more than Garmin or cgpsmapper produced maps.

It is certainly something that needs looking at again and I encourage 
you to do so.

..Steve