[x3d-public] TwoSidedTexture node

[Andreas Plesch]

Duplicating the geometry is also what was suggested as a quick solution for
the x3dom issue.

While this is certainly possible, a dedicated node would allow for more
optimal implementations and would mean better ergonomics.

An initial BackAppearance or TwoSidedTexture implementation could just use
naive internal duplicating of geometry but still provide the ease of use.
Later it could be optimized if necessary.

Some thoughts,

Andreas



On Nov 3, 2017 6:08 PM, "Michalis Kamburelis" <michalis.kambi at gmail.com>
wrote:

> 2017-11-03 18:36 GMT+01:00 Andreas Plesch <andreasplesch at gmail.com>:
> > There was a question a while ago on x3dom issues if there is a
> > TwoSidedTexture node similar to the TwoSidedMaterial node:
> >
> > http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/
> components/shape.html#TwoSidedMaterial
> >
> > There is MultiTexture so it is not unreasonable to expect a
> TwoSidedTexture.
> >
> > Is there a way currently to have a different texture on the backside
> > apart from duplicating the shape ?
> >
> > What could be good way to generalize such that different material,
> > texture, or common surface shader nodes can be used simultaneously on
> > backs and fronts ?
> >
> > Perhaps a BackAppearance node ?
> >
>
> 1. BackAppearance seems a bit "too flexible to implement" for me ---
> it would also allow for different "shaders" for front and back faces,
> since I guess "Appearance.shaders" can be different than
> "BackAppearance.shaders". And I have no idea how to implement such
> thing (with a single draw call :) ). Typically only the fragment
> shader really knows which side you're looking at.
>
> 2. In fact, even TwoSidedTexture seems a bit uneasy to implement for
> me, if the children of TwoSidedTexture could be any texture node,
> including MultiTexture. You could then specify that
>
> - front side has MultiTexture e.g. with 3 textures inside (e.g. some
> cubemap multiplied by some 2d texture multiplied by some 3d texture)
> - back side has MultiTexture e.g. with 2 textures inside (e.g. some 2d
> texture added to some other 2d texture)
>
> IOW, you could have different texture logic/equations for front and
> for back side. I can implement this :), but I'm not sure is this
> acceptable to all implementors.
>
> In contrast, the existing TwoSidedMaterial has much simpler
> implementation: both front and back sides have lighting calculated
> using the exact same equations, only the parameters to these equations
> are different depending on which side you're looking at.
>
> Following this, I think a safer version of TwoSidedTexture would be
> something that cannot have MultiTexture as a children, and maybe even
> that requires that both front and back textures are the same kind
> (both 2d, or both 3d, or both cubemaps...). And MultiTexture can have
> (as a children) a TwoSidedTexture. So then you could do (in classic
> encoding):
>
> Appearance {
>   texture TwoSidedTexture {
>     front ImageTexture { url "front.png" }
>     back ImageTexture { url "back.png" }
>   }
> }
>
> or
>
> Appearance {
>   texture MultiTexture {
>     texture [
>       ImageTexture { url "first_layer.png" }
>       TwoSidedTexture {
>         front ImageTexture { url "second_layer_front.png" }
>         back ImageTexture { url "second_layer_back.png" }
>       }
>     ]
>   }
> }
>
> I think this would be OK to implement :)
>
> 3. I'm not sure how much is such TwoSidedTexture needed, in practice.
> You can already achieve a similar effect by reusing the same geometry
> but with different "ccw" value. E.g.
>
> ----
> Shape {
>   geometry IndexedFaceSet {
>     DEF MyCoord Coordinate { ... }
>     ccw TRUE
>     solid TRUE # this is the default, I just emphasise it here
>     coordIndex ...
>   }
>   appearance Appearance {
>     # configure Material, Texture, etc. for ccw=true side
>   }
> }
> Shape {
>   geometry IndexedFaceSet {
>     USE MyCoord
>     ccw FALSE
>     solid TRUE # this is the default, I just emphasise it here
>    coordIndex ... # unfortunately, you have to repeat coordIndex here
>   }
>   appearance Appearance {
>     # configure Material, Texture, etc. for ccw=false side
>   }
> }
> ----
>
> The above is not perfect (a renderer will make 2 draw calls, and you
> need to duplicate coordIndex contents). But it already works, in all
> browsers, and you can change any Appearance parameter this way :)
>
> Regards,
> Michalis
>
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