[x3d-public] TwoSidedMaterial

[Michalis Kamburelis]

Andreas Plesch <andreasplesch at gmail.com> wrote:
>
> There is movement around TwoSidedMaterial. See
>
> https://github.com/castle-engine/view3dscene/issues/23
>
> and
>
> http://web3d.org/pipermail/x3d-public_web3d.org/2019-December/011594.html
>
> I would vote to keep a version of it since the alternative of manually doubling the geometry with inverted normals is too awkward for normal users.
>

If we keep TwoSidedMaterial (with sepatateBackColor=true possibility),
then we also need a solution for PBR for this. So my proposed
TwoSidedAnyMaterial from https://github.com/Web3DConsortium/X3D/pull/8
would be relevant. It allows to make two-sided Phong Material or
two-sided PhysicalMaterial.

( I don't have an opinion here, whether we should keep
TwoSidedMaterial, or remove it and instead have a simple boolean field
"twoSided", I just note that *if* we decide that TwoSidedMaterial is
useful, then necessarily also new TwoSidedAnyMaterial is useful, to
have the same capabilities for Phong and physical materials. )

Personally, I never saw real use-cases for TwoSidedMaterial with
separateBackColor=true, but maybe my needs were specific :) If many
people use with separateBackColor=true, then we should reconsider
keeping it indeed.

> There is a need, however, to clear up some confusion. As far as I can judge there is no need for separateBackColor. It should always be true since this case appears to be the main point of the node.
>

As per my comments in
https://github.com/castle-engine/view3dscene/issues/23 -- I think that
right now the specification says something different (solid=false with
one-sided Material means that back faces are black). It's also
somewhat consistent with implementation (using OpenGL[ES] etc.):

- one thing determines backface culing ("solid"), like glEnable(GL_CULL_FACE)

- another, independent thing controls whether the "back" faces have
lighting calculated correctly (using TwoSidedMaterial instead of
Material). This is typically a code in GLSL shader that inverts a
normal if gl_FrontFacing==false.

But I could be persuaded to your approach :) I see your point -- right
now we have some combinations that aren't really useful:

- Using solid=false with one-sided Material is hardly useful, as
authors don't really want to see pure black faces.

- Using solid-true with TwoSidedMaterial isn't very useful, since you
activate two-sided lighting calculation, but then don't render back
faces.

As I understand, you would prefer to simplify it.

- Connect whether "the back faces are visible" with "the back faces
have correct lighting calculated" into a single feature, activated in
one place (by "solid" field).

- And then the only purpose of TwoSidedMaterial would be to achieve
*different* colors on front vs back faces.

This would actually be somewhat aligned with current CGE
implementation too. In CGE I actually *sometimes* realize Material as
two-sided (in Phong shading),
https://castle-engine.io/x3d_implementation_shape_extensions.php#section_ext_shading
.

> Also, there needs to be clarification how solid=true and TwoSidedMaterial interact. I think x3dom lets TwoSidedMaterial win over solid=true in the intent to render back faces. Most other browser probably let solid=true win, eg. will not render back faces at all even with a TwoSidedMaterial.

The current X3D specification is clear here, IMHO:

The "solid" field determines whether back faces are visible. It is
unrelated to Material/TwoSidedMaterial. So the X3DOM behaviour is not
correct, IMHO.

I'm not saying that we cannot change it :)

Regards,
Michalis