Hi Michael,

I'm was a bit surprised by this shading error on simple loft or single curve sweep scenario.
Of course the revolve did the job to create that kind of shape, but I personnally feel more confortable to create them with the loft command.



(This 'not so nice shading' is also visible in the .lwo exported model)

Greetings from Montreal =)

- Edit

Looks like choosing Profiles / Exact resole the loft problem (Maybe 'auto' is a little bit too sensitive )
But single rail sweep is still giving weird result.

It's interesting when you Edit > Separate and Show control points, each method has a different control point arrangement, for me, I prefer the structure of Loft with exact fit, but then again if I made a cone from the Solid tools and chopped the top off to get the same result the control points are a square arrangement and the simplest, I guess these are different surfaces, maybe Michael can elaborate.

-

Hi PaQ, in some cases you may be seeing little teeny tiny micro bumps in the surface.

But in other cases you're probably seeing various kinds of polygon shading artifacts.

Keep in mind that shading a polygon in real time can take some shortcuts and result in various artifacts that are just basically part of the rendering process.

Mach banding is one fairly common problem: http://en.wikipedia.org/wiki/Mach_bands

I mean remember that if there were not tricks involved in rendering in general (not just for real time even) you would only see flat faceted objects all the time because polygons are not actually smooth, there are tricks involved to make them sort of appear as if they were smooth.

If you use more polygons, then the polygons will come closer and closer to the actual curved surface and those kinds of artifacts get reduced more and more.

- Michael

Hi PaQ, so another demonstration using your example file, using hemispheric lighting in MoI.

If you have a small number of polygons (here writing out to an OBJ file) like this:



Then you can more easily see the kinds of shading artifacts like this:



If you have a much denser number of polygons where each polygon is getting closer to the size of a pixel like this:




Then those shading glitches are greatly reduced - the image now comes closer to reproducing the actual NURBS surface:



If you want to have more 100% perfect shading without even little tiny artifacts you have to generate more polygons and not have ones that span larger rendered areas of the screen. For things like a cone you're probably getting some kind of longer skinnier ones by default unless you use the "Divide larger than" option to dice them up in the straight direction.

Or you could try to use a renderer that knows how to render NURBS surface data in a more direct manner like PRMan...


It seems to me that you're not liking how polygons work though - little flat things emulating curved surfaces just have some problems that go along with that...


You can actually get a pretty big improvement with just a bit of extra dicing actually - for example here with angle = 12:






Then with Divide larger than: 5 to dice long things up just a bit more:






But the other factor is that you may not want to obsess too much about making any real-time video card generated view perfect, a full renderer is going to take more care to do a more proper interpolation on things and can avoid some kinds of artifacts that are present in a real time view.


- Michael

Remember also, that a revolved surface is kind of...special. It becomes a "surface of revolution" and I believe the resulting surface(s) are lighter mathematically. And that's generally a good thing.

Hi Michael,

Thanks a lot for taking from you time.
It's quite amazing I didn't notice this 'low poly' shading limitation (probably because I'm a sds hires modeler), looks like I don't use cones that often :)

Again ... sorry

Michael,
Is there such a thing as a NURBS Video Card?

WHat I mean is, it's been explained that everything is actually polygons on a computer screen because the card translates it to that to present to the monitor. SOmetimes this is only because "everybody" started using 1 method. Is it possible, or does it exist? A video card that renders NURBS on the screen? No translation to poly's.

Hi PaQ, no problem.

> (probably because I'm a sds hires modeler)

Yeah, the most typical SDS usage tends to generate a lot of quite small evenly sized polygons being used which helps to avoid mach banding.

But also these artifacts are really increased quite a lot by real-time display techniques, which is just focused more on speed than quality.

A full render just does things in a more accurate (and also slower) manner and many things like that will simply disappear at actual render time.

Here's another example with your same file, exported with just angle = 12 from MoI with no extra subdivisions. In MoI the exported mesh looks like this:



Importing that semi-rough mesh into Cinema4D looks like this:




Note that real-time display actually looks even worse than things did in MoI - that's probably because the C4D real-time display is using only per-vertex gouraud shading which increases mach banding artifacts. When I updated the lighting engine in MoI in v2 to use light mapping instead of gouraud shading, it helped to reduce mach banding in MoI's display but not totally eliminate it.

But look what happens when I do an actual full render in C4D :




Poof - those artifacts are just gone... And that's even with long polygons, here is a look at the wireframe in C4D:




So much of the things you are referring to are artifacts in real-time display techniques, that won't actually apply at render time... (as long as you are getting the true vertex normals being used by the render)

The real-time artifacts tend to be more pronounced on long polygons, which you're getting more of with those cone shapes.

If the NURBS surface actually has little undulations and ripples in it, then that's another kind of problem, but that is not what is happening in the case you posted here though...


So I mean to make a long story short, you can just ignore those artifacts you are seeing in this case, they should not really be there when you do your final render.


- Michael

Hi Burr,

> Is there such a thing as a NURBS Video Card?

No, not for Trimmed NURBS surfaces which are complex to process.

There is some new stuff in the latest generation of Direct3D 11 cards to tessellate spline surfaces on the card, but it can only handle simple untrimmed surfaces, they don't have anything for processing trim curves.


> A video card that renders NURBS on the screen? No translation to poly's.

Well, one method for rendering NURBS "directly" can still actually involve polygons, it just dynamically produces enough polygons until each one is smaller than a pixel on the screen so you can't tell that they are polygons.

Another method is to do raytracing where you shoot out rays from the eye point and do an actual line/surface intersection for each pixel on the screen.

Either one of those involves a heavy amount of computations, so it tends to not be fast enough to do for a real-time viewport display.


- Michael

Hi Michael,

[So I mean to make a long story short, you can just ignore those artifacts you are seeing in this case, they should not really be there when you do your final render.]

Well looks like in modo it's an other story :(



I will try to post something about it on modo forum (using your screen proof), but I'm not sure if I have to post it as a bug, or as a feature request :P

Hi PaQ, hmm that is interesting... You have not deleted the normal map or anything like that have you?

Did you do any transforms on the object, or is it rendered directly after import without any kind of editing of the geometry?


Maybe we should make a standardized file for for testing - I took your Loft.3dm file and capped the middle shape, then exported it with Angle = 12 to make the attached ShadeTest.obj


Then here is the result rendered in Cinema4D v10:




- Michael

Hi Michael,

Opening the .obj, center the camera, F9 (rendering) ... I didn't edit anything else

And render in C4D10 with no specular looks like this:

Hi PaQ,

> Opening the .obj, center the camera, F9 (rendering) ... I didn't edit anything else

Well, looks like mach banding artifacts.

Is there any setting in materials to do full Phong shading?

Maybe it's not doing very careful interpolation of the normal or something like that.

As you can see in the Cinema4D renders, if the vertex normals are treated carefully by the renderer so that each individual pixel ends up getting shaded by a good normal, it makes for nice results even with a pretty rough mesh.

Maybe it is not as common to do that as I had thought though.

- Michael

Hi PaQ, quick question - can you switch to an orthographic/parallel type camera and then give it a try?

Does the render look different in that case?

- Michael

Hi Michael,

There are no shading option available in modo, there is a 'smooting angle option' in degree, to control the surface normal, but the vertex normal channel overwrite this setting anyway.
Same result whatever the camera mode (ortho or perspective)

Hi PaQ, well I guess for now if rendering in Modo you will need to force some dicing of long polygons into smaller pieces to reduce those artifacts.

It seems like it would be a good thing to report to them, because it doesn't really have to be that way, as you can see from the Cinema4D examples.

That kind of mach banding artifact usually happens in Gouraud real-time shading because in that shading method the normals are only evaluated against the full light sources just at the polygon's vertices, and then those colors blended throughout the polygon.

It kind of looks like you're getting that same kind of effect in the full render...

- Michael

I did some testing by building a cone in Modo with approx. same vert. count, and did one with double the verts. and it was
pretty smooth. Two cones on the right were created in modo. Similar faceting on the lower poly version.

Hi danperk, yes that is interesting that the same artifacts are present in natively created Modo geometry as well.

This kind of artifact gets reduced when polygons get smaller in size on the screen, so increasing the vertex count around the circular direction is one way to do that, but also doing a few vertical divisions is another way.

But you really should not need to go to such a high density to avoid these artifacts in a full render, it would be better if the full render did a more proper vertex normal interpolation so that you could get nice results with lower polygon counts.

Taking some shortcuts in interpolation can help to speed things up, maybe this is an area where some quality was sacrificed intentionally for a speed gain, but it would be good to have an option to control that if that was the case.

- Michael

A strait cylinder created in Modo seems to be pretty smooth beyond 12 sides, so it seems to be
the cones angle where the render has problems.