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

[Lars C. Hassing]

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