Help - my cave is inside-out

[Wookey]

message re-sent as it seems to have got lost (same with the other two I sent
just now).

+++ Martin Budaj [05-01-03 12:59 +0100]:

>>> > Now that I've drawn about 8km of survey the last scrap (top right hand
>>> > corner - scrap bmb3_s3) has had 'pillar failure' again.
>>> >
>>> > There are 3 pillars which do not work (the insides are filled). They are
>>> > all walls with -outline in and I've tried reversing them, but it seems
>>> > to stay broken. What's going on?
>
>>
>> There is another problem. Even if scrap boundary has correctly specified
>> all -outline out/in/none options, there may be problem to determine which
>> area is inside of the scrap if the outline crosses itself (like figure 8).
>> In such a case you get a MetaPost warning
>>
>> Warning: scrap outline intersects itself in bmb3_s3 at fake


I have this warning in a few scrpas in that survey - I wondered how serious
it was, and what I should do about it. But without more details of _where_
it intersects itself (and why this is a problem), I could not do anything
but ignore it.

>> and it is than random if MetaPost determines the interior of scrap
>> correctly. The solution is to
>> 1) find where the loop occured -- this may be tricky, but start with the
>> endpoints where scraps should join. The loop may be very small, even
>> unnoticable. Even worse, it may not be visible at all, because it happens
>> anly after the joining of scraps -- which is the case of your example  :(
>> 2) correct the position of the control points (see the second picture)
>>
>> I hope the enclosed pictures make it more clear.


It does - thanks very much for taking the time to look at it.

I have several more scraps which I cannot include in the overall survey at
the moment because if I do I get cryptic metapost errors. We have to work
out a way of getting better feedback to the user about _where_ the problem
lies as it is currently extremely difficult to fix as you don't know which
of many lines is at fault.

How did you find the above problem. If I understood the process you used to
find this it might help me next time.

I'll add this answer to the FAQ too.


Wookey
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