Subject:
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Re: Clipping / CCW / CW / INVERT
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Newsgroups:
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lugnet.cad.dev
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Date:
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Thu, 7 Oct 1999 19:14:23 GMT
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Viewed:
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372 times
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Lots of good stuff in your post, Rui!
On Wed, 6 Oct 1999 18:15:40 GMT, Rui Martins <Rui.Martins@link.pt> wrote:
> So if a solid object is Opaque, then from any view point there are some
> faces that are invisible, because they are facing away from the viewer.
(Side note: there are not actually any solids in LDraw. Only lines and
polygons).
> From this we now know that "Opaque solids" can have some faces "Clipped"
> and that transparent object (solid or not) CAN'T have any faces clipped.
Actually, since LDraw doesn't do shading, transparent surfaces *can* be
clipped. Only edges, the top-most solid surface, and the top-most
transparent surface will show up in the final rendered image.
<Snip description of WINDING and CLIPPING>
Here's a brief comparision of the proposed 0 FACE meta-statement, and the
WINDING/CLIPPING meta-statements:
0 FACE CW =
0 WINDING CW
0 CLIPPING ON
0 FACE CCW =
0 WINDING CCW
0 CLIPPING ON
0 FACE UNKNOWN =
0 CLIPPING OFF
0 WINDING UNKNOWN (if there was such a thing)
0 FACE DOUBLESIDED
0 CLIPPING OFF
(except FACE DS doesn't preserve any WINDING state)
Looks like half of one, six dozen of the other to me. Using
WINDING/CLIPPING would be slightly more powerful, but I don't know that it
would have any practical benefit.
> About the stair case quads
> --------------------------
> For Quads used has front and back:
>
> Correct the problem, by defining N subparts of the same quad (as much as
> needed) until each of the subparts winding can be defined. This probably
> must be done by hand.
Agreed.
> In the simple case, just disable CLIPPING, before the problematic quad and
> enable it again after. this way the quad is drawn as a two sided quad.
Easy patch, for the short-term.
> About the INVERT tag
> --------------------
> To know if the file should be inverted, we could look if the determinant
> is negative or not (Someone already said that).
>
> Why would we use a tag to inform the parser of something that he can find
> out?
To allow parts-authors to selectively invert subfiles.
> I propose that this flag scope is only valid until the next file
> reference, where it is used, after that (in this file, the one with the
> reference to another) it is disabled.
That sounds logical, but may be confusing, because of the potential hanging
references.
> The use of this tag will only SPEED THINGS UP, it's not a requirement!
Nope, what we talking about, it would be a required tag to acheive specific
functions.
Steve
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Message has 2 Replies:
 | | Re: Clipping / CCW / CW / INVERT
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| (...) I believe this is incorrect, no? ALL transparent surfaces should appear, if only to make a contribution to the color that underlying surfaces appear to be (consider a trans blue surface in front of a trans yellow in front of a white... ) (...) (25 years ago, 7-Oct-99, to lugnet.cad.dev)
| | | Re: Clipping / CCW / CW / INVERT
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| (...) Thanks! (...) I knew that ! ;) (...) LDraw doesn't, but this specification is for new Programs which are backwards compatible with Ldraw, but with a bunch of new enhancements. Check the instructions of any LEGO MODEL and you will see an (...) (25 years ago, 8-Oct-99, to lugnet.cad.dev)
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Message is in Reply To:
| | Clipping / CCW / CW / INVERT [DAT]
|
| First, let me apologise, if you get this message twice. It seams that the old e-Mail address (L-CAD@LISTSERV.UH.EDU) does not work completelly. Because I got the message back, but couldn't see it in www.lugnet.com/cad/dev note: i am subscribed in (...) (25 years ago, 6-Oct-99, to lugnet.cad.dev)
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