Subject:
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Re: BFC: LITS 2
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Newsgroups:
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lugnet.cad.dev
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Date:
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Fri, 31 Mar 2000 13:16:08 GMT
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Second in a series, collect them all...
In <http://www.lugnet.com/cad/dev/?n=3159>, Lars wrote:
> I think you should make it clear what "default" means:
> 1) absence of "0 CERTIFY", or
> 2) "0 CERTIFY" with no (or other) tags
I believe the language around the 'defaults' has been cleared up
sufficiently.
> > 0 WINDING [ CW | CCW | UNKNOWN ]
> > default: CCW
>
> I don't think there should be a default. You should always specify either
> CW, CCW or UNKNOWN.
Hmm. I don't think there was much more discussion about whether every
file should have an explicit 0 BFC CCW CLIP statement, or not. The
current proposal provides for default values.
Here's my take on defaults: assume the BFC standard *required* explicit
declaration of winding and clipping, so no defaults were specified.
Sooner or later, someone would make a mistake, and mis-declare their
file. Any file which blew the declaration by omitting one of these
parameters would be anamolous. In a case like that, the renderer would
have to choose whether to report the error or swallow it, and whether to
allow culling or not. That would establish defacto defaults. But not
all renderers would necessarily handle the problem in the same way.
Which would lead to conflicting standards.
So why not avoid the (minor) problem, and specify the defaults in the
BFC standard?
At the least, the standard should say, "the BFC options must be
explicitly declared in each file. If a file has an invalid declaration,
the rendering engine should assume X and Y."
In <http://www.lugnet.com/cad/dev/?n=3195>, Rui wrote:
> check the following two statements:
> ---------------------------------------------------------------------------
> > To make this standard useful and effective, the LDraw parts library
> > must be updated to follow the new standard. Since it would be difficult to
> > rewrite the entire library in one update, the standard will allow for a
> > mix of extended and unextended files in one rendering.
>
> ---------------------------------------------------------------------------
> > 2 Allowable Clipping. A subfile can only have clipping applied when the
> > 2 following conditions apply:
> > - All superfiles are certified.
> > - The current file is certified.
> > 2 - No superfile has disabled clipping prior to referencing this
> > 2 subfile.
>
> ---------------------------------------------------------------------------
>
> Allows a mix of extended and unextended files in one rendering, but
> to allow clipping, all files, have to be certified !?!?
This is an issue of language in the document. The second quoted passage
was updated to read:
|4 Clipping. It must be possible for a file to enable and disable clipping. But
|4 even when clipping is enabled locally, it may not be possible to perform
|4 clipping on the file in some circumstances. Any file will have clipping
|4 applied only when the following conditions apply:
|4 - All superfiles (in the current reference branch) are certified.
|4 - The current file is certified.
|4 - No superfile has disabled clipping prior to referencing this
|4 subfile.
|4 Unless all of these conditions are met at the time a subfile is rendered,
|4 no clipping is possible.
> > In some cases, it may be desirable to assume that a file is
> > right-sideout, (and therefore clippable) even though not all superfiles
> > are certified. One obvious example is files in the ldraw\parts
> > directory.
>
> This paragraph is another one, sorry to say this, but IMHO
> this paragraph it's a mess. (Could someone explain better !?)
This section was rewritten as:
|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
|8 they were inverted. Assuming part files are never inverted allows
|9 the rendering engine to apply BFC-processing on these parts (assuming the
|9 parts are certified), even if the calling files aren't certified.
|7
|7 No assumptions can be made about models which make direct use of primitives
|7 or polygon commands, so a rendering engine should not simply treat uncertified
|7 model files as certified.
> In my opinion the language extensions should be written with curly
> braces '{' '}' instead of '[' ']', because the later means optional
> parameter and the previous means compulsory parameter with possible
> options if the pipe '|' is used.
The syntax-spec is currently written as:
|4 Syntax:
|4 0 BFC [CERTIFY|NOCERTIFY] [CLIP|NOCLIP] [CW|CCW|NOWIND] [INVERTNEXT]
Each of the parameters is optional. There are no required parameters.
If a BFC meta-statement carried no parameters, then it would have no
effect.
> > UNKNOWN = winding direction is unknown or variable. This setting will
> > disable clipping, until the winding is reset.
>
> Using someones expression, the UNKNOWN is 'syntatic sugar', because this
> will be implemented internally as CLIPPING OFF.
> If the user want's to inform that the 'winding' is not known,
> then he can make a regular comment, and use CLIPPING OFF
In <http://www.lugnet.com/cad/dev/?n=3196>, Jacob responded to this
point:
> This sounds correct, but do we want to eliminate this
> syntactic sugar?
And, in <http://www.lugnet.com/cad/dev/?n=3208>, I also provided:
> Yes. WINDING UNKNOWN allows a DAT author to specify what is happening in the
> file more precisely than CLIPPING OFF. Adding WINDING DOUBLE-SIDED would allow
> even more author-precision, but there is no practical difference between
> DOUBLE-SIDED and UNKNOWN, AFAIK.
>
> There is a slight practical difference between WINDING UNKNOWN and CLIPPNG OFF
> -- WINDING is a file-specific setting; the file's author can assume that they
> always know the current WINDING setting. CLIPPING is passed down the reference
> branch, and the current file should make *as few* assumptions about the CLIPPING
> setting as possible.
In <http://www.lugnet.com/cad/dev/?n=3211>, Lars wrote:
> Also there should be some words about using 0 CLIPPING OFF for a
> double-sided
> section of a file.
This is an open issue. It's noted at the end of the document. That
should be moved to the top.
Steve
--
27 out of 83 messages scanned
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Message has 2 Replies:
 | | Re: BFC: LITS 2
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| (...) I agree with the default from the start, I proposed them, somewhere ! (...) Agreed! but that's NOT currently written that way, is it ? (...) NO it is NOT ! read on. (...) I know that the branch only would be required, but I don't agree with (...) (24 years ago, 2-Apr-00, to lugnet.cad.dev)
| | | Re: BFC: LITS 2
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| I read the current proposal, and I have this to say: I don't agree with the following paragraph when an entire branch has to have cliping on to be able to be culled, don't think so. ( I know about the invert case, solution required ) 4 Control of (...) (24 years ago, 2-Apr-00, to lugnet.cad.dev)
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Message is in Reply To:
| | BFC: LITS 2
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| Rui has stated that there are outstanding issues with the BFC proposal that are not included in the issues list at the start of that proposal. With this message, I am going to review the messages from the "Line in the Sand" thread, which started at (...) (24 years ago, 30-Mar-00, to lugnet.cad.dev)
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