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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Tue, 14 Dec 1999 17:06:28 GMT
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Viewed:
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2300 times
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In lugnet.cad.dev, Jacob Sparre Andersen wrote:
> [ 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.
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.
> 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?
Hmmm. You're right. Inversion is no more global than accumulated clipping.
They are identically global. This paragraph is now:
9 Inversion accumulates down the reference branch. If the current
4 file is being rendered inverted, then any subfiles of the current file are
4 also rendered as inverted.
> 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.
OK. The paragraph does have some verbiage in that direction, but I will add to
it:
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 subfile reference should
4 have its orientation inverted, i.e. the inside/outside definition should be
4 swapped. Inverted subfiles must be preceeded by a 0 BFC INVERTNEXT statement,
4 and the 0 BFC INVERTNEXT statement may not appear anywhere else in a file.
9 The INVERTNEXT flag applies *only* to the one subfile which immediately
9 follows. Any subfile commands afterward are processed normally.
> 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...").
OK:
9 Rendering Engine Guidelines
--------------------------------------------------------------------------------
9 This section gives some suggestions for the design of rendering programs,
3 in order to achieve correct renderings. Any program may violate
9 these guidelines, if there is another way to acheive a valid rendering.
> 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.
It's not very clear, because I was trying to avoid telling people specifically
how to code their programs. It might make sense to write a sample program to
illustrate these hard-to-explain points.
Try this:
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
9 determinant, in order to mirror the subfile.
9
9 The typical method of determining that the file has been manually inverted is
9 to calculate the determinate of the transformation matrix. If the determinate
9 is negative, then the subfile has been inverted. This document does not
9 attempt to address the details of 3D graphics algorithms and issues.
9
9 The typical way to adjust for matrix inversions is to switch the winding of
9 the polygon vertices. That is, if the DAT specifies the winding as CW, and
9 the current subfile has been inverted, the rendering program would proceed as
9 if the winding is CCW.
> 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.
OK.
> 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.
Good idea. But that entire test belongs in the BFC() function.
I changed every occurrence of Color to Colour. Any other American-isms which
need to be changed to Australia-isms?
> 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
I changed it to:
9 If Command.LineType != BFC Then
4 InvertNext = FALSE
9 ElseIf No Option in Command is INVERTNEXT Then
9 InvertNext = FALSE
4 End If
... to avoid any problems with evaluating tests in compound boolean expressions.
> 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
>
> # ???
Ties back to the issue at the top of the file:
6 - The authors guidelines section is completely undeveloped.
Steve
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Message has 1 Reply:
 | | Re: Line in the Sand
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| [ Still discussing (URL) ] Steve: (...) That makes sense. (...) I'll reuse the notation. ---...--- (...) # Fine. (...) # I'll try a complete reformulation: 4 INVERTNEXT J This option is used to swap the definitions of "inside" and "outside" (i.e. J (...) (24 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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| [ Still discussing (URL) ] 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. ---...--- (...) (24 years ago, 12-Dec-99, to lugnet.cad.dev)
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