[mkgmap-dev] Tile boundary artefacts

Tue Apr 6 21:16:23 BST 2010

Am 06.04.2010 12:33, schrieb Steve Ratcliffe:
> On 05/04/10 18:35, WanMil wrote:
>> I started to make very basic checks to get the source of the tile bounds
>> problems.
>>
>> I created the attached bbox1.osm and bbox2.osm. Both contain one
>> rectangular polygon identical to the bounds. After compiling it with the
>> default style I see in MapSource that the underlying land polygon
>> (yellow) does not match to the data (blue) (see land_diff.png).
>>
>> Any ideas why this happens? Do you have any hints how I could proceed
>> and where I should continue my investigations?
>> Up to now I tried to decompile the img file with the Display project but
>> failed on the RGN files (will need some time to investigate this).
>
> I looked at bbox2.  Both the background and polygon rectangles in the
> map RGN are the same size at every level.
>
> The issue is probably therefore in the overview map. As this has a
> very low resolution it is not possible to accurately represent the map
> bounds unless the bounds happen to fall on a low resolution
> coordinate.  If the tile is produced by splitter it should be
> correctly aligned, but for randomly produced tiles there is likely to
> be some misalignment.
>
> When the area in the overview map has to be rounded there are a number
> of ways of doing it, each of which work in different cases.  The last
> change in this area was r1532, a change made by Mark for a particular
> use case.  I think that something a bit more complex is needed, for
> example detecting the outside boundaries of a tile set
> and rounding outward just on those, leaving the internal boundaries
> rounded as they are (they have to fit without overlapping).
>
> Once we have a proper overview map then its resolution could be
> increased and this would make the problem a lot less noticeable even if
> nothing else changed.
>
> ..Steve

Now I start to understand the reason for splitter alignments.

I also managed to have a look on the rgn file. I compiled both bbox1.osm 
and bbox2.osm to one gmapsupp.img. I do not fully understand the rgn 
file but found one difference between the background polygon and the 
other polygon.

The RGN file (see below) in level 2, subdiv 4 uses a start point for the 
orchard (type 0x4e) and 4 coordinates. The background polygon (type 
0x4b) only uses a start point plus 3 coordinates.

Is that a desired behaviour? (Sorry if that's a silly question but I am 
on rookie level regarding the file format)

WanMil

RGN excerpt:

--------- Level 2, Subdiv 4 
----------------------------------------------------
          |        |                         | final at ca, (end 4d)
          |        |                         | line start 9f to ca
0000009f | 000000 | 4e                      | Type 4e
000000a0 | 000001 | 00 00 00                | Label offset: 0x000000
000000a3 | 000004 | dd fe                   | Long delta -291
000000a5 | 000006 | 96 fc                   | Lat delta -874
          |        |                         | Start point 55,99993,8,80009
000000a7 | 000008 | 0e                      | Bitstream length 14
000000a8 | 000009 | a9                      | Base a9
000000a9 | 00000a | 00 c0 b4 61 24 00 00 00 | Bitstream (len: 14, 0xe)
000000b1 | 000012 | b4 e4 ba 0d 00 00       |
          |        |                         | xsame false, xneg false
          |        |                         | ysame false, yneg false
          |        |                         | xbase 12, ybase 14
          |        |                         | Coord 56,59972,8,80009 
2637738/410114
          |        |                         | Coord 56,59972,8,99991 
2637738/419426
          |        |                         | Coord 55,99993,8,99991 
2609786/419426
          |        |                         | Coord 55,99993,8,80009 
2609786/410114
          |        |                         | 6 bits left, 0
          |        |                         |
000000b7 | 000018 | 4b                      | Type 4b
000000b8 | 000019 | 00 00 00                | Label offset: 0x000000
000000bb | 00001c | dd fe                   | Long delta -291
000000bd | 00001e | 96 fc                   | Lat delta -874
          |        |                         | Start point 55,99993,8,80009
000000bf | 000020 | 09                      | Bitstream length 9
000000c0 | 000021 | 99                      | Base 99
000000c1 | 000022 | 32 12 00 00 c0 b4 75 1b | Bitstream (len: 9, 0x9)
000000c9 | 00002a | 00                      |
          |        |                         | xsame false, xneg false
          |        |                         | ysame true, yneg false
          |        |                         | xbase 12, ybase 11
          |        |                         | Coord 55,99993,8,99991 
2609786/419426
          |        |                         | Coord 56,59972,8,99991 
2637738/419426
          |        |                         | Coord 56,59972,8,80009 
2637738/410114
          |        |                         | 0 bits left, 0