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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Thu, 11 Nov 1999 18:45:56 GMT
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Viewed:
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3620 times
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[ Still discussing http://www.geocities.com/partsref/bfcspec.txt ]
Steve:
> On Wed, 10 Nov 1999 00:44:17 GMT, "Lars C. Hassing"
> <lch@ccieurope.com> wrote:
[...]
> > 0 INVERT
> > (or maybe "0 INVERTNEXT" to stress that it is only the
> > following line, which must be of type 1, that is turned
> > inside-out)
>
> Hmm. Interesting thought. I like it. Anyone else want
> to ( yay | nay ) this?
"INVERTNEXT" is good. It makes the effect much more clear.
> > The CERTIFY section:
> > > operational command-line in the file. No other
> > > statements are required for backface culling to be
> > > applied to a file.
> >
> > Then please add:
> > 0 CERTIFY BFC implies 0 CLIPPING ON and 0 WINDING CCW.
>
> CERTIFY BFC doesn't imply CLIPPING ON -- the clipping
> mode/setting comes from the superfile (for the main
> model-file, clipping is set by the rendering engine or an
> explicit CLIPPING ON statement).
It gets much too messy when you mix the states of a
parameter and the setting of that parameter.
CERTIFY BFC does imply CLIPPING ON, but it will not
necessarily mean that any clipping will be done, because
that depends on the super-file as well.
When processing a LDraw file you have three _local_
variables:
- "bfc-certified" is initialised to false and should at
most be changed once by a LDraw file.
- "winding" is initialised to "ccw".
- "clipping" is initialised to "on".
The statement "0 CERTIFIED ( BFC | NOBFC )" will modify the
variable "bfc-certified", the statement "0 WINDING ( CW |
CCW | UNKNOWN )" will modify the variable "winding", and the
statement "0 CLIPPING ( ON | OFF )" will modify the variable
"clipping".
Two parameters are passed to the processing routine:
- "accumulated_clipping" is true when processing the root
file, and will otherwise only be true if both
"accumulated_clipping" and "clipping" were true in the
super file at the calling point.
- "inverted" is false when processing the root file, and
will otherwise depend on whether the calling statement
was preceded by a "0 INVERTNEXT" line. If it was, the
value of "inverted" will be the opposite of that in the
super file. If it wasn't tha value will be the same as
that in the super file.
Back-face-culling will only be applied if all of the
following apply:
- "bfc-certified" is true
- "accumulated_clipping" is true
- "clipping" is true
- "winding" is "cw" or "ccw"
The winding used for back-face-culling will be that of the
variable "winding", if "inverted" is false. Otherwise it
will be the opposite ("unknown" being neutral).
> 0 WINDING CW
> 0 CERTIFY BFC
>
> ... the winding should be CW, right?
Maybe. It depends on the definition of "operational
command-line".
I understand why you sometimes use specialised "programming"
languages for writing specifications.
> If the CERTIFY statement 'implied' the winding, then this
> example would not be clear (at least to the human reader)
> if the winding for this file is CW or CCW.
> Does CERTIFY override the earlier WINDING or not?
If "0 WINDING CW" is an operational command-line, it doesn't
matter. Otherwise the result is somewhat unclear. I would
prefer that that sequence wasn't allowed.
There isn't much point in allowing the use of BFC
meta-commands before you have notified the renderer that you
intend to follow the rules for BFC meta-commands.
> That's why the WINDING setting has a default value, and
> CERTIFY doesn't affect the WINDING setting in any way.
>
> BUT, I'll see what I can do with the verbiage, to make
> things more clear.
> > The WINDING section:
> > > 0 WINDING [ CW | CCW | UNKNOWN ]
> > > default: CCW
The default value is not for the WINDING _statement_, but
for the "winding" variable.
> CLIPPING doesn't need a default, because it is passed down
> from the superfile. It is up to the rendering engine to
> provide the initial value of CLIPPING.
The "clipping" variable does need to have a default value.
Otherwise you would be unable to distinguish between the
local setting and the clipping _status_ passed from the
super file (or you would break the clipping rules).
> Maybe it would be better to drop WINDING UNKNOWN and
> specify that the default value for CLIPPING is ON.
I don't think so. If you could translate my explanation
above to proper English, then the problem should be solved,
with the effect I understand you intended.
Play well,
Jacob
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Message has 2 Replies:
 | | Re: Line in the Sand
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| (...) OK, I'll change this in the document. Changes from the last few days will be uploaded to GeoCities in the next hour or so. Steve (25 years ago, 12-Nov-99, to lugnet.cad.dev)
| | | Re: Line in the Sand
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| (...) Did you mean you=Steve or you=anyone? (...) I agree, the sequence should be illegal. My point was, does CERTIFY BFC change the value of the internal local_clipping variable, or not? My intention was that it does not. From a practical (...) (25 years ago, 12-Nov-99, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: Line in the Sand [DAT]
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| On Wed, 10 Nov 1999 00:44:17 GMT, "Lars C. Hassing" <lch@ccieurope.com> wrote: Still discussing (URL) (...) Yes, but the 0 CERTIFY ( BFC | NOBFC ) format is more common. And it emphasizes that is one statement with various parameters. And it's less (...) (25 years ago, 10-Nov-99, to lugnet.cad.dev)
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