[x3d-public] Nurbs patch coloring

Fri Apr 17 08:22:17 PDT 2020

Hi Michail,

I noticed that in asymptote the 3d renderings have a more shiny
effect, presumably to emphasize smoothness. Here is your example with
enhanced shininess ,eg. tighter highlights:

https://5e99c5bcc7a1cd0006474cd3--x3dom.netlify.app/examples/functional/nurbs/bezierpatches

(doubleclick to recenter and zoom in, rotate)

Looking through the discussion on

https://sourceforge.net/p/asymptote/discussion/409349/thread/9900c3a8ea/

I would say there is room for both, a X3D export (like the PDF
export), and an optimized webgl renderer which does dynamic
retesselation when required. Presumably the role of a V3D asymptote
format would be to allow non-asymptote V3D generators use the
asymptote renderers. Although the asymptote renderers seem very
targeted, there may some demand for that.

The X3D export could even keep the original asy receipt/program as
metainformation. One also could think about including optional static,
prerendered views, or/and pretesselated meshes (for NURBS incapable
browsers).

By the way, most X3D browsers, understand both xml and VRML syntax,
and it is easy to generate XML from VRML if an authors prefers the
more concise syntax.

Andreas

On Thu, Apr 16, 2020 at 11:16 AM Andreas Plesch <andreasplesch at gmail.com> wrote:
>
> Dear Michail,
>
> thanks for your thoughtful response and going through the trouble of
> applying the nurbs "vertex" color additions to your example case. Let
> me first provide a full build of x3dom with these changes here:
>
> https://5e986ab62dd47c00063c419a--x3dom.netlify.app/
> https://5e986ab62dd47c00063c419a--x3dom.netlify.app/x3dom-full.debug.js
>
> This should make it easier to test other examples.
>
> Of course, smooth Nurbs surfaces are very useful. The question about
> use cases was more about if vertex coloring is really that useful if
> the necessary interpolation between control point colors is harder to
> predict. But if you agree that this is the case for your use case of a
> patchwork of small Bezier patches, then I agree that it is worth
> considering as an addition to the standard. I think we will provide
> the changes to x3dom soon with the dev version.
>
> The CAD working group would be the body to involve in standardization.
> Typically there are a lot of details to consider. For example, how
> would vertex coloring apply to other Nurbs geometries ? Leave the
> color interpolation method up to the specific browser or define it
> more tightly ? Transparency ? Presumably, the idea would be to just
> follow vertex coloring rules for vertex based geometries but this
> needs some checking.
>
> I took the liberty to add your example here for easier access:
>
> https://5e986ab62dd47c00063c419a--x3dom.netlify.app/examples/functional/nurbs/bezierpatches.html
>
> There is now a quick preview which uses the control points directly
> without Nurbs interpolation. Since the control points are already
> quite dense, the following incremental smoothing is a bit harder to
> make out. It ends up with more than 700K triangles.
>
> Let me also point to another technique for elevation coloring, eg.
> coloring by one the coordinates. The idea is to use the coordinate
> value directly as a texture coordinate, after appropriate scaling and
> shifting.
>
> Here is the example adopted to color by elevation, the y-axis:
>
> https://5e986ab62dd47c00063c419a--x3dom.netlify.app/examples/functional/nurbs/bezierpatchesuvcoord.html
>
> This uses TextureCoordinateGenerator mode='COORD' which makes the
> local x and y coordinates directly u and v texture coordinates.
> A TextureTransform shifts these then to the appropriate [0,1] range.
> The texture is a (very small) 1x3 pixel image but could be any color bar.
>
> You could swap the y and z control point coordinates to get the
> intended elevation coloring.
>
> Sincerely,
>
> -Andreas
>
>
> > Date: Wed, 15 Apr 2020 21:50:33 +0300
> > From: Michail Vidiassov <master at iaas.msu.ru>
> > To: X3D Graphics public mailing list <x3d-public at web3d.org>
>
> > Dear Andreas,
> >
> > On Tue, Apr 7, 2020 at 5:50 PM Andreas Plesch <andreasplesch at gmail.com> wrote:
> > > So it would make sense to add "vertex" coloring if there is a convincing use case.
> >
> > > I think the idea to compute a color surface in color space using the
> > > same Nurbs parameters as the geometric surface makes the most sense.
> > > In fact, I cannot think of another solution which is both general and
> > > easy to implement. But it remains to be seen if this would be
> > > practical since the resulting colors may be hard to predict/control.
> >
> > What is required to make the use case convincing?
> > Wikipedia in its ultimate wisdom tells us "B?zier patch meshes are
> > superior to triangle meshes as a representation of smooth surfaces".
> > Using patchwork of Bezier triangular or rectangular pieces seems to be
> > a quite common technique.
> > Other forms of curved pieces are also used.
> > Lots of references can be produced, but since NurbsSet is in X3D since
> > v 3.0, it looks like it is already accepted  that handling objects
> > composed from NURBS pieces is a thing to do.
> >
> > Surfaces composed of flat triangles or quads may have variable color
> > applied to show some forth attribute that could not be mapped to
> > coordinates, or highlight one coordinate (height) or some surface
> > properties (curvature). Examples also abound.
> > Since Bezier (NURBS) patch approximation is an improvement upon flat
> > patch approximation I do not see reason for it to lack the option of
> > getting smooth colors.
> >
> > > I think the idea to compute a color surface in color space using the
> > > same Nurbs parameters as the geometric surface makes the most sense.
> > > In fact, I cannot think of another solution which is both general and
> > > easy to implement. But it remains to be seen if this would be
> > > practical since the resulting colors may be hard to predict/control.
> >
> > In my case of cubic Bezier patches and linear interpolation of 4
> > colors given at vertices (corners) of a rectangle this solution works
> > nicely.
> > I have to give color information as 4x4 matrix instead of 2x2, but
> > that is a small price.
> > As to the general case - my guess is that  the resulting colors on
> > NURBS may be as hard to predict/control as the resulting texture
> > coordinates, that are already in the standard.
> >
> > As to a (sort of) live test case:
> > What am I trying to achieve:
> > http://media.iaas.msu.ru/smoothelevation.html
> > What I was able to do with your modified x3dom (rough and ready, just
> > a proof it works for now)
> > http://media.iaas.msu.ru/mytest.htm
> > Where the example comes from:
> > https://asymptote.sourceforge.io/gallery/3Dgraphs/
> >
> > Asymptote - program that started with drawing in 2D with cubic Bezier
> > curves (as they are native to TeX and PostScript, it was a "MetaPost
> > replacement") and later moved to 3D, where Bezier patches are used as
> > building blocks for most surfaces.
> >
> > Sincerely, Michail
> >
> >
> >
> > ------------------------------
> >
> > Subject: Digest Footer
> >
> > _______________________________________________
> > x3d-public mailing list
> > x3d-public at web3d.org
> > http://web3d.org/mailman/listinfo/x3d-public_web3d.org
> >
> >
> > ------------------------------
> >
> > End of x3d-public Digest, Vol 133, Issue 63
> > *******************************************
>
>
>
> --
> Andreas Plesch
> Waltham, MA 02453

-- 
Andreas Plesch
Waltham, MA 02453