[LSC] Ambiguity in the draft BFC spec - changing winding in a file

[William Howard]

OK,

If I've followed everyones input, there is no ambiguity on changing winding
in a file - it is currently permitted by the standard and not a problem.

However, we may need make it more explicit in the standard that it is NOT an
error to change winding within a file.

W

-----Original Message-----
From: lsc-bounces at ldraw.org [mailto:lsc-bounces at ldraw.org] On Behalf Of Ross
Crawford
Sent: 12 October 2006 23:40
To: LDraw Standards Committee
Subject: Re: [LSC] Ambiguities in the draft BFC spec




Lars C. Hassing wrote:
> William Howard wrote:
>> To address point 1, I would like to see the syntax definition changed to
>>     0 BFC NOCERTIFY
>>     0 BFC CERTIFY [CW|{CCW}]
>>     0 BFC [CLIP|NOCLIP] [CW|CCW] [INVERTNEXT]
>> where [] indicates an optional item and {} the default value
> 
> We might event change it to
>     0 BFC NOCERTIFY
>     0 BFC CERTIFY [CW|{CCW}]
>     0 BFC CLIP|NOCLIP
>     0 BFC CW|CCW
>     0 BFC INVERTNEXT
> since I don't think it gives much sense specifying the last three
together.
> Maybe "CLIP CW" would make sense, but NOCLIP doesn't fit with any.
> Even if "CLIP CW" could be fine, it would be no big deal to specify them 
> on two lines.

Could say 0 BFC CLIP [CW|CCW]|NOCLIP. Would have to say that omitting CW 
and CCW causes the current winding direction to be maintained.

I dunno if ldview etc accept such without warning, but I can't see 
anything wrong with allowing it in the spec.

>> What does "0 BFC" mean (it's valid syntax, so it must mean something)?
> 
> No longer an issue with the above change.
> 
> 
>> Do we need to explicity state that the CERTIFY/NOCERTIFY variants of 
>> the BFC
>> meta-commands  must occur before any "Operational Command-Lines"?
> 
> I think it already say so in the first section,
> but no harm to explicitly state it.

Agreed.

>> And what to do about point 2?  If rendering tools can cope with 
>> changes in
>> winding within a file should we care if parts are singularly wound?
> 
> I think it should be allowed to change winding.
> Some part of a part may be easier to write in a certain winding,
> e.g. if you do some hand-crafted mirroring.

I see no problem allowing it, as all the tools currently seem to support 
it. I generally try to get all the winding in the same direction in my 
parts (LDAO editor has a very useful "swap winding" function) but I can 
see cases like Lars says where keeping mirrored section with opposite 
winding could make maintenance easier.

ROSCO
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