Problem stepping from Soft renderer to Mental Ray

[janina-overley]

I have in a scene a flat circular NURBS disc made by trimming a grid with a circle. Step size for the disc is 3x3, and 10 for the trim curve.

This object displays correctly, as a perfect circle, in all viewports, and renders as a perfect circle when rendered using the Softimage renderer. However switch to Mental Ray, and it renders very jagged - maybe 10 or 12 sides. This number is unaffected by increasing or decreasing the various step sizes

I don't get it - could someone explain this to me? I'd be very greatful as I really can't think what the problem is

snowctrl

[devdoka]

Just tried it here and works as expected. no problems.

Why are you creating a disc by trimming rather than revolving a curve or the like?

Matt

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Matt Lind
Animator / Technical Director
Softimage certifiedi instructor
Softimage|3D
Softimage|XSI
speye_21@hotmail.com

[fredc]

In reply to:
I have in a scene a flat circular NURBS disc made by trimming a grid with a circle. Step size for the disc is 3x3, and 10 for the trim curve.

This object displays correctly, as a perfect circle, in all viewports, and renders as a perfect circle when rendered using the Softimage renderer. However switch to Mental Ray, and it renders very jagged - maybe 10 or 12 sides. This number is unaffected by increasing or decreasing the various step sizes

I don't get it - could someone explain this to me? I'd be very greatful as I really can't think what the problem is

snowctrl

Sounds like there is something odd about the geometry. MR is more accurate than the Soft renderer and catches stuff like this when the other doesn't. Anyway, how did you creat your disks? Step by step.

Urp

[janina-overley]

Hi Matt, thanks for your reply

First of all the reason I'm making the disc by trimming a grid is that the grid is already present in the scene - it's a floor, and the disc is to be a platform that raises out of it, leaving a hole beneath.

I'm using Soft 3.9 with Mental Ray 2.1 on a DEC Alpha - earlier today I went back to Soft 3.8SP2 which I still have loaded so as to try Mental Ray 1.9, and the scene rendered fine! So the problem is with Mental Ray 2.1

Are there extra render options for 2.1 that I may have missed? I'm familiar with Info>Mental Ray and with the Render Options dialogue box - is there more elsewhere?

I've also been unable to get Mental Ray motion blur working correctly (it doesn't recognize transparency) - does this sound like a faulty install?

I'd be greatful for your advice

snowctrl

[devdoka]

@First of all the reason I'm making the disc by trimming a grid is that the grid is already present in the scene - it's a floor, and the disc is to be a platform that raises out of it, leaving a hole beneath.

If you can, I suggest rebuilding that segment of the floor - there's no need for trims as it just takes up resources. Remember, trims are "selective hiding", they don't actually cut the geometry. The hidden surface is still there at all times.

@I'm using Soft 3.9 with Mental Ray 2.1 on a DEC Alpha - earlier today I went back to Soft 3.8SP2 which I still have loaded so as to try Mental Ray 1.9, and the scene rendered fine! So the problem is with Mental Ray 2.1. Are there extra render options for 2.1 that I may have missed? I'm familiar with Info>Mental Ray and with the Render Options dialogue box - is there more elsewhere?

I did all my tests in mr 2.1 and everything worked fine. A couple of things to check:

• Do the trim surface and trim curves consist of the same NURBS geometry type? (ie - cubic, linear, quadratic, etc..)

• Did you by chance activate adaptive tessellation of the surface? If so, a simple tweak of the parameters will smooth the surface out to the desired smoothness.

• If you start a fresh session of v3.9 and perform a trim on a NURBS grid as you do the floor, do you still get the artifacts?

@I've also been unable to get Mental Ray motion blur working correctly (it doesn't recognize transparency) - does this sound like a faulty install?

What have you done so far to set it up?

Matt

---

Matt Lind
Animator / Technical Director
Softimage certified instructor:
Softimage|3D
Softimage|XSI
speye_21@hotmail.com

[janina-overley]

What would be the best way to rebuild the floor? (sorry if this is a little basic). It is simply a large square with a circular hole in it's centre

In a freshly loaded Soft 3.9.1 Build 10.41 (incase that helps), I got a Cubic Nurbs grid of default size, and a Cubic Nurbs circle of radius 2.5, step 8. Create Modelling Relations is turned off. After rotating the circle through 90 degrees in the X and Freezing Transformations, I used Trim Nurbs Surface to Crop the grid in global Y, leaving me with a circular disc in the XZ plane. Info>Selection shows Step 3x3, and Step 10 for the Trim curve. Surface Approximation is Parametric 3x3 (not Adaptive) by default.

This previews perfectly with the SoftImage renderer, but is horribly jaggy with Mental Ray 2.1. Reducing the Trim curve step size changes the appearance of the disc in viewports and in SoftImage renders, but in 2.1 nothing seems to have changed - the same jaggy render is produced.

I've repeated this test in 3.8SP2 - the Soft and MR1.9 renders are both fine, while the 2.1 render remains jaggy.

As for the motion blur (using 3D, Exact), I've pinned down the exact nature of the problem (I had thought it was more general) - Mental Ray doesn't seem to recognize texture movement. I have a scene in which there are both moving objects on which the blur works fine, and static objects with moving transparency maps, on which the blur has no effect. This is the same in 1.9, 2.1 and both versions of Soft. Are there texture settings I need to use?

Thanks

snowctrl

[devdoka]

@@What would be the best way to rebuild the floor?

Since there seems to be a bug with the trimming - use polygons. The other option is to draw a curve and revolve it into a disc. The texture can be an XZ planar projection, or you can use Rendermap to convert to UV. The revolve option will allow you to retain control over the smoothness of the surface.

The trimming bug is not present in v3.9.2 if that helps.

@@As for the motion blur (using 3D, Exact), I've pinned down the exact nature of the problem (I had thought it was more general) - Mental Ray doesn't seem to recognize texture movement. I have a scene in which there are both moving objects on which the blur works fine, and static objects with moving transparency maps, on which the blur has no effect. This is the same in 1.9, 2.1 and both versions of Soft. Are there texture settings I need to use?

Motion blur is computed from motion vectors stored in the frame buffer. Motion vectors are only generated from vertices of geometry where motion is detected. A texture animated across a surface will not blur unless the surface it is mapped to is moving and has motion vectors generated for it.

So the quick answer is to animate the geometry rather than the texture cropping to get the desired motion blurred texture map.

Matt

---

Matt Lind
Animator / Technical Director
Softimage certified instructor:
Softimage|3D
Softimage|XSI
speye_21@hotmail.com

[janina-overley]

OK

I'm glad the bug is with the trimming and not with Mental Ray 2.1 as a whole

Thanks for your help Matt

snowctrl