Subject:
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Re: Line in the Sand
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Newsgroups:
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lugnet.cad.dev
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Date:
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Sun, 12 Dec 1999 23:09:24 GMT
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Viewed:
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2595 times
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[ Still discussing
http://www.geocities.com/SouthBeach/Lagoon/8641/bfcspecv4.txt ]
Steve:
You can also put them on ldraw.org now that you have an
account here.
Here comes a commented/edited version. Lines starting with
"#" are my comments and lines starting with "J" are my
changes/additions.
------------------------------------------------------------------------------
[...]
7 revision: 7, 1999.12.08
[...]
4 Inversion is the one BFC option which accumulates globally. If the current
4 file is being rendered inverted, then any subfiles of the current file are
4 also rendered as inverted.
# In what sense is inversion more "global" than clipping?
[...]
4 INVERTNEXT
4 This option is used directly before a subfile command (linetype 1). It is
4 a flag, indicating to the renderer that the following subfile reference should
# ^^^^^^^^^^^^^^^^^^^^^^^^^^
# You might want to make it more clear that it only refers
# to the _immediately_ following line.
[...]
There will be a few requirements placed on the design of rendering programs,
# ^^^^^^^^^^^^
# What about reformulating this as "implementation advice"
# (since "Any program may violate...").
[...]
Matrix Inversions. Rendering engines will need to correct for transformation
1 matrices which inadvertently invert a subfile. They will also need to
1 allow for situations in which the matrix is deliberately given a negative
1 determinant, in order to mirror the subfile.
# I suppose that the meaning of this statement is that the
# (C)CW-ness of the polygons is compensated for the
# determinant of the accumulated orientation matrix, but it
# is not very clear.
[...]
7 Assumed inversions. Generally, it is not possible to assume that a subfile
7 is inverted or normal (which is the reason for the 0 BFC INVERTNEXT meta-
7 statement). One important special case is this: model files do not invert
7 part files. Parts are complex files which would be essentially useless if
7 they were inverted. Assuming that all part files used in a rendering allows
7 the rendering engine to apply BFC-processing on these parts (assuming the
J parts are certified), even if the calling files aren't certified.
[...]
7 If 32 <= Color And Color <= 47 Then // This restriction may or may not be
7 AccumClip = FALSE // required, depending on the style of
7 End If // rendering for transparent surfaces.
# Shouldn't the check on colour numbers rather be formulated
# "If Transparent (Colour) Then"? We should remember that
# people can redefine the meaning of the colour numbers, so
# other numbers than the range 32 to 47 can refer to
# transparent colours.
[...]
4 If Command.LineType != INVERTNEXT Then
4 InvertNext = FALSE
4 End If
# Shouldn't the check be:
J If Command.LineType != BFC Or Else no Option in Command is INVERTNEXT Then
J InvertNext = FALSE
J End If
[...]
6 --------------------------------------------------------------------------------
6 Guidelines for Part and Primitives authors
6 --------------------------------------------------------------------------------
6 - How to write double-sided and decoration sections
6 - Default values
6 0 BFC CERTIFY
6 =
6 0 BFC CLIP CCW
# ???
------------------------------------------------------------------------------
Play well,
Jacob
------------------------------------------------------------
-- E-mail: sparre@cats.nbi.dk --
-- Web...: <URL: http://hugin.ldraw.org/LEGO/Dyr/ > --
------------------------------------------------------------
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Message has 1 Reply:
 | | Re: Line in the Sand
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| (...) It's an admin role vs. personal role thing. I took it upon myself to write up a spec, I didn't want to imply that it was endorsed by the group at large. (...) Hmmm. You're right. Inversion is no more global than accumulated clipping. They are (...) (25 years ago, 14-Dec-99, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: Line in the Sand
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| (...) Seems Geocities is having some trouble, and the shortcut URLs won't work. Try these instead: (2 URLs) Steve (25 years ago, 6-Dec-99, to lugnet.cad.dev)
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