[LSC] File Format Spec - Quads (winding, concave, T-junctions, etc)

[Travis Cobbs]

On 7/11/07, William Howard <william at howard-family.fsworld.co.uk> wrote:

>

> Lars C. Hassing wrote:To: "LDraw Standards Committee" <lsc at ldraw.org>

> >Travis Cobbs wrote:

> >> Quads (line type 4) should be required to be in clockwise or

> >> counterclockwise order.  In my opinion, the current "recommendation" to

> >> do this is insufficient.

> I agree that quads should be wound correctly, but what is "correctly".  If

> we allow concave quads (which the spec, by virtue of not disallowing them,

> currently does), a bow-tie rectangle **could** actually be meant to

> represent a bow-tie (or hour-glass) shape!



Sorry about my confusing wording.  I didn't mean that we should specify the
order, but that bow-tie quads should be forbidden by the spec.  Currently,
bow-tie quads are simply discouraged.  The BFC spec is sufficient for
winding order, as far as I'm concerned.

Personally, I would explicitly disallow concave quads, which then by

> implication disallows non-wound quads.  But I'd also take the extra step

> and

> explicitly disallow non-wound quads.



I'd go either way on the concave ones.  As I said, they should at least be
discouraged.  If other people think they should be forbidden, that's fine by
me.



>> There was some disagreement in the community in my most recent thread on

> >> T-junctions here:

> >> http://news.lugnet.com/cad/dat/parts/?n=6086

> >>

> >> I think my opinion on the matter is fairly clear in that thread, and

> I'm

> >> all for including information in the spec about the problem, along with

> >> instructions that they should be avoided when practical.

>

> While I feel just as strongly on T-junctions as you do, with the

> proliferation of box primitives they are becomming more common (where a

> polygon touches a box face).  So I agree that we should mention the

> problem,

> but not forbid it.



Can you provide an example?  Since I'm not a part author, and I rarely even
review parts, I'm not up on the common practices used when authoring parts.
The reason I'm asking for an example is because you should be able to
arrange the geometry that connects to the box primitive to not have
T-junctions with the box.  I know this might not always be practical or
possible, but I'd just like to see what you mean.



>> However, given some of the comments from part authors, I doubt we can

> >> outright forbid them in the spec.

> >[Lars] I agree with all your points.

> I suspect there'd be a virtual riot if we did!

>


Are there any provisions for ldraw.org voting members firing an entire LSC?

:-)


--Travis
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