[LSC] Call for votes: Back Face Culling (BFC) Specification 1.0

[Travis Cobbs]

Inverted is intended in this statement to mean "invertnext", NOT mirrored
via the matrix.  All mirror inversions are required to be tracked and
accounted for by the renderer anyway.  In other words, parts that are
referenced mirrored should still be BFC-processed even when the model isn't
BFC-certified.  It's never valid to use an invertnext BFC command
immediately before a part, though.

--Travis

On 11/11/06, Lars C. Hassing <lars at hassings.dk> wrote:
>
> William Howard wrote:
> >> I think it should be made more clear that renderers can treat models
> >> as implicitly certified and apply BFC to the parts they reference,
> >> either directly or via submodels.
> >> If models contains line type 3 or 4, or references primitives,
> >> these cannot have BFC applied (unless the model is explicitly
> certified).
> >
> > But's that's exactly the problem.  Because a renderer does not know if a
> > model contains polygons as well as parts it *cannot* just assume that
> the
> > file is clippable.
>
> Well, a renderer *do* know if a model contains polygons
> (it can easily check the model's line types).
> So, the polygons cannot be clipped, but the parts can.
>
> >  However, as Travis added the bit about parts being
> > closed objects that can be clipped (culled) provided they are certified
> > (even if the (sub-)model itself is not certified), everything you have
> > pointed out is already covered by the spec.
>
> Yes, I know, but I was aiming at the paragraph:
>
>     Assuming part files are never inverted allows the rendering engine
>     to apply BFC-processing on certified parts, even if the calling
> file(s)
>     (for example, the main model file or sub-models of the main model)
>     aren't certified.
>
> The way I read it tells me that parts can not be inverted and still have
> BFC applied.
> But parts *can* be inverted (the renderer will keep track of the negative
> determinant)
> and still have BFC applied. I.e. you can make a left wing as a mirrored
> right wing,
> and all certified parts in both wings will be clipped.
> /Lars
>
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