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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Fri, 8 Oct 1999 11:00:25 GMT
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Reply-To:
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Rui.Martins@link.(saynotospam)pt
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Viewed:
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431 times
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On Thu, 7 Oct 1999, Steve Bliss wrote:
> Lots of good stuff in your post, Rui!
Thanks!
> 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).
I knew that ! ;)
>
> > 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.
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 attempt
to represent translucency/transparency, by using dots.
If we can actually do transparency, we should to it to every polygon
that as transparent/translucid color (maybe these are not the correct
terms).
>
> <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)
You got the Ideia wright !
> 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.
Yes it will!
it will allow to keep backward compatibility with every file already
made, without changing anything. If the file is enhanced then it will
benefit a Faster Drawing and extract functionality (real transparency).
Remember that this is what is done in OpenGL, which was well though by
Silicon Graphics, after years in the 3D business.
To use this features, in a similar way, will simplify the making (or
appending) of a program written in OpenGL, and probably D3D (since they
use something similar).
>
> <SNIP About the stair case quads >
>
> > 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.
Maybe you are missing something here!
To invert a file, you simply have to change the matrix values in the
reference to the other file.
Remember that you can invert a file with any combination of 1 or 2
coord axis.
so, saying INVERT, wouldn't mean anything, if you didn't know how to
invert.
Since you have to define the matrix, the inversion information is
already in the matrix. The Tag would only be usefull to avoid
calculating the determinant.
> > 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.
Didn't grasp that one! :(
Referencing sub-files in subfiles, works like a stack! so what is you
point ? what do you mean by 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.
see above
Rui Martins
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Message has 3 Replies:
 | | Re: Clipping / CCW / CW / INVERT
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| Rui Martins wrote in message ... (...) You mean six of one, half a dozen of the other. :) (...) it (...) Using 0 FACE CW|CCW|DS|UNKNOWN will also keep backward compatibility. Either way though, some quadrilaterals will need to be manually tweaked (...) (25 years ago, 8-Oct-99, to lugnet.cad.dev)
| | | Re: Clipping / CCW / CW / INVERT
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| (...) No, thank you! (...) I don't know if I knew you knew that, but I was almost positive you knew that. And I knew most everyone else knew that, but I wasn't sure absolutely everyone knew that. ;) (...) Right. New programs may handle transparency (...) (25 years ago, 8-Oct-99, to lugnet.cad.dev)
| | | Re: Clipping / CCW / CW / INVERT
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| (...) No, when I look at the images I see real alpha-blended surfaces :) I've tried to turn backface culling in the transparent parts and they looked very ugly, it's better to draw everything in those cases. (...) Just a note to correct the (...) (25 years ago, 8-Oct-99, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: Clipping / CCW / CW / INVERT
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| Lots of good stuff in your post, Rui! (...) (Side note: there are not actually any solids in LDraw. Only lines and polygons). (...) Actually, since LDraw doesn't do shading, transparent surfaces *can* be clipped. Only edges, the top-most solid (...) (25 years ago, 7-Oct-99, to lugnet.cad.dev)
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