[OSM-dev] Advice sought on polygon-with-hole drawing

Andy Robinson (blackadder) blackadderajr at googlemail.com
Mon Mar 17 14:28:21 GMT 2008

Andy Allan wrote:
>Sent: 17 March 2008 12:33 PM
>To: Robert (Jamie) Munro
>Cc: dev
>Subject: Re: [OSM-dev] Advice sought on polygon-with-hole drawing
>On Fri, Mar 14, 2008 at 10:43 AM, Robert (Jamie) Munro
><rjmunro at arjam.net> wrote:
>>  Yes. That is what I am saying. 1 simple rule:
>>  *All* areas should be colour on the right (i.e. clockwise)
>No chance. This is a completely terrible idea. It makes it harder for
>newbies, who have to know to check and change the "direction" of the
>ways of an area, which is an entirely pointless process from their
>point of view and will probably make no sense to them.
>Direction is important for one-way roads but shoudn't be for areas.
>Coastline is an understandable exception.

And even that's easy for a user to swap around without realising the affect.
In reality though, since it's a wiki, when an error is spotted it gets
corrected, so we could take the same view with areas, if it renders the
wrong way around its going to get fixed pretty quickly. A new user is
probably going to find it a lot easier to understand that an area has to
point in the clockwise direction than to have to add some complicated
tagging to non closing areas to render properly, so for this approach I
don't feel its too much to ask. For closed areas then it will always be
possible to overrule if the rendering engine so decides.



>>  It's simple, it's validatable (albeit the current JOSM validator get's
>>  it wrong), it means that coastline is not an exception, it makes the
>>  maths simpler. It might even mean that you don't need relationships to
>>  associate inner and outer -  Any system that gets 1 segment of an area
>>  should be able to know which side of that segment the feature is on.
>>  | Also, my idea would allow a way to serve as an "inner" member of one
>>  | multipoly at the same time as as an "outer" member of another; I think
>>  | you couldn't get that with evenodd.
>>  That's really ugly. There should be 2 ways. They can share nodes if that
>>  ~ is wanted. If you really want to use only one way, then you could put
>>  direction=-1 tag or something in the relationship that defines the
>>  tagging for the inner area, but I still don't like that. I think that if
>>  the edge of an area crosses through something you should know what is on
>>  each side of it without having to consider special tags for exceptional
>>  cases.
>>  Robert (Jamie) Munro
>>  Version: GnuPG v1.4.6 (Darwin)
>>  Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
>>  iD8DBQFH2lboz+aYVHdncI0RAgafAKCJBEW2LG9F6Rczm4gp+EU8/8Qt+gCgpMqE
>>  M1p5oF0jvynuW31P8KNnu7o=
>>  =c5+U
>>  -----END PGP SIGNATURE-----
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