[x3d-public] Negative scale and winding order

[Joe D Williams]

-----Original Message-----
From: Andreas Plesch <andreasplesch at gmail.com>
Sent: Sep 19, 2023 9:03 AM
To: X3D Graphics public mailing list <x3d-public at web3d.org>
Subject: [x3d-public] Negative scale and winding order

> Holger made good points on why a negative scale in the parent
transform should not affect which side of a geomet is considered
facing outward and visible with solid=true.

OK, I thought I tried that with no effect. 

> While this approach is very practical, I am not sure what the spec.
actually requires in this respect. 

I did not care, it worked and I will look more later. 
The following is all good. I think the  players show it different because the user code is changed from what I sent.

> First, it does not explicitly single out the case of a negative scale.
Should it ? If so, it would probably have to deal with the effective
world transform.

https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-IS.proof/Part01/components/rendering.html#CommonGeometryFields

'The ccw field defines the ordering of the vertex coordinates of the
geometry with respect to user-given or automatically generated normal
vectors used in the lighting model equations. If ccw is TRUE, the
normals shall follow the right hand rule; the orientation of each
normal with respect to the vertices (taken in order) shall be such
that the vertices appear to be oriented in a counterclockwise order
when the vertices are viewed (in the local coordinate system of the
Shape) from the opposite direction as the normal. If ccw is FALSE, the
normals shall be oriented in the opposite direction. If normals are
not generated but are supplied using a Normal node, and the
orientation of the normals does not match the setting of the ccw
field, results are undefined.'

has the relevant ccw field language.

I think the specified process is:

- Compute the outward facing normal of a triangle using the ordering
of the vertices in such a way that the ordering appears
counterclockwise when viewed in the opposite direction of the computed
normal. This uses the local, untransformed (raw) coordinates.

- invert the normal if ccw=false.

- Apply the accumulated transforms to both the vertices and the
computed normal. The spec. has a strict definition on how to apply a
transform to vertices to transform from the child space to the parent
space. For normals transformations there is a computer graphics
standard approach and probably opengl based equations but the spec.
itself seems to be silent.

So perhaps the question is how to apply negative scale transforms to
normals as intended by the spec.

The Leif example has ccw=false and a negative scale (for z). It
appears as outside in on x_ite and outside out on view3dscene and
x3dom.

A workaround is solid=false but that may not always be possible or desired.
Another workaround is to not use negative scales/mirroring, eg.
preprocess geometry accordingly which seems like a major limitation.

-Andreas

On Tue, Sep 19, 2023 at 10:37 AM wrote:
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> 2. Negative Scale
>
> A negative scale will not affect what is front face and what is back face. This means that a Text node with solid true, mirrored with negative scale (0 -1 0), also shows its front face to the camera. This makes it possible to create a text with solid true which has a mirrored counterpart which is a clone of the Text node.
>
> A negative scale will not turn the model inside out, it stays as it is. The main advantage is as described above: we can use solid true models in many situations where a negative scale is used without flipping inside out.
>
> This effect is achieved by detecting if there is any negative scale, which can be determined if the determinant of the scale-rotation matrix is less than zero. If this is the case, ccw must be exchanged with cw and vice versa.

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