[x3d-public] Negative scale and winding order

Fri Sep 22 09:02:12 PDT 2023

We could check behaviour of other existing X3D browsers, especially
freeWRL, Octaga and BSContact which are still available.

Since negative scaling has not been historically supported, it may be
worth deviating from established, but not well defined interpretations
in browsers. This would definitely require a spec. change/addition.

glTF compatibility can be accomplished by checking for negative scale
during import and adjusting ccw accordingly, or by global behaviour
affecting all geometres. If there are no backward compatibility
concerns, consistent behaviour for any affected geometry would be most
helpful for authoring.

glTF style, automatic adjustment of normals in case of negative
scaling presumably also applies to explicitly provided normals (in
normal maps) ? I think it must to support the mirroring use case.

https://github.com/KhronosGroup/glTF/issues/1697 has more discussion
about similar questions and points to

https://github.com/KhronosGroup/glTF-Asset-Generator/blob/master/Output/Positive/Node_NegativeScale/README.md

for a series of tests.

-Andreas

On Thu, Sep 21, 2023 at 7:55 PM Brutzman, Donald (Don) (CIV)
<brutzman at nps.edu> wrote:
>
> Negative scaling is certainly a special case and not something that has historically been supported in VRML or X3D. Thanks for careful consideration of any unexpected variations.
>
> Our design goals make it clear that we have no desire to vary from glTF in X3D 4.0. If there are any recommendations needed for authors to maximize and hopefully achieve full interoperability with glTF rendering, they are always welcome.
>
> v/r Don
> ________________________________
> From: x3d-public <x3d-public-bounces at web3d.org> on behalf of Holger Seelig <holger.seelig at yahoo.de>
> Sent: Thursday, September 21, 2023 2:56:48 PM
> To: Joe D Williams <joedwil at earthlink.net>
> Cc: Andreas Plesch <andreasplesch at gmail.com>; X3D <x3d-public at web3d.org>; holger.seelig at googlemail.com <holger.seelig at googlemail.com>
> Subject: Re: [x3d-public] Negative scale and winding order
>
> Unfortunately, I can't just change something that has been implemented this way since the beginning of X_ITE's existence. The way X_ITE handles negative scaling is exactly the way it is specified in glTF, which is not bad and is implemented in CGE under the term Robust Negative Scale. Until everything is thought over, I have to agree with Andreas with his advice to just set the solid field to false, as it is the easiest way to avoid this issue.
>
> Best regards,
> Holger
>
> --
> Holger Seelig
> Leipzig, Germany
>
> holger.seelig at yahoo.de
> https://create3000.github.io/x_ite/
>
> Am 21.09.2023 um 19:24 schrieb Joe D Williams <joedwil at earthlink.net>:
>
> So perhaps the question is how to apply negative scale transforms tonormals as intended by the spec.The Leif example has ccw=false and a negative scale (for z). Itappears as outside in on x_ite and outside out on view3dscene andx3dom.
>
>
> Of course I am interested in this because to me the X_ITE rendering is wrong, unless that is, the player is required to reverse the normals for scale z = -1.
> X_ITE is 'automagically' reversing the normals, so when I do the ccw=FALSE the  thing  is transparent from the front of the polygon. Of course if I use ccw=TRUE then the  other tools look like X_ITE, transparent from the front.
>
> Holger, can you please either convince the other players they are wrong, or fix yours to work like the rest. If the spec is not certain what to do about this, then please either fix the spec or X_ITE.
>
> I just want the model to show as intended and be correct, even if sort of a practical hack, so please l figure it out because I really need everyone to do the right thing on this, whatever that might be.
>
> . Thanks and Best Regards,
> Joe
>
>
> -----Original Message-----
> From: Joe D Williams <joedwil at earthlink.net>
> Sent: Sep 19, 2023 1:26 PM
> To: Andreas Plesch <andreasplesch at gmail.com>, X3D Graphics public mailing list <x3d-public at web3d.org>, <holger.seelig at googlemail.com>
> Subject: Re: [x3d-public] Negative scale and winding order
>
> 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.
>
>
> Sorry I sort of wrote this off, to both you and Holger.
> A good, reliable solution to this problem is needed, as learned or not from recent experience.
>
> One thing I think happened to me was seeing that common graphics authoring programs use as a 'Native' default or pre-selected +Zin with left-hand rule for axis rotations.
> For the Humanoid animation, we want to align ourselves in +Zout character space, so the character Front is looking out of the screen with +y up, +x to character left. In the case of x3d +Zout, the character is gaze is toward +z. In the case of +Zin, the character Front gaze is toward -z. Or, we may find the situation where the character is drawn in +Zin with gaze toward +z.
>
> For HAnim there are three main issues: skeleton and skin and animations. Well, actually these boil down into just one problem: They all have to work together. So to even consider importing stuffs drawn or animated in a different space a plan is needed. I used standard features to force display of skin because it was drawn in a different space. This almost worked everywhere.
>
> I think it should work everywhere, because the -1 for z scaling is going to operate just the same as if I had actually negated the coord points(?). Since the +Zin model with scale 1 1 1 will be seen as facing -z the character's back is toward the x3d default camera. Using scale 1 1-1 operates so that the character is now facing the x3d default camera.
> However, just guessing, since the polygon that was drawn ccw true is now being drawn ccw false and is now transparent from its front. Changing ccw to false seemed to get the normal aimed in the right direction.
>
> Thanks for the help,
> Joe
>
>
>
>
>
>
>
> -----Original Message-----
> From: Joe D Williams
> Sent: Sep 19, 2023 11:09 AM
> To: Andreas Plesch , X3D Graphics public mailing list
> Subject: Re: [x3d-public] Negative scale and winding order
>
>
> -----Original Message-----
> From: Andreas Plesch
> Sent: Sep 19, 2023 9:03 AM
> To: X3D Graphics public mailing list
> 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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-- 
Andreas Plesch
Waltham, MA 02453