Grass Displacement Problem

[IceAged]

Hi

I have followed Jeff Pattons grass displacement tutorial, with much success up to now...

The grass is 100mm and 300mm high, max displacement at 300mm, max level at 5, edge length 0.5 pixels.

The problems begin when i start to use final gather - mental ray starts to throw up numerous warning messages saying:

'angle between normal and geometry normal >90 degrees'

...and eventually crashes during final gather computation, or bucket rendering, (although what it does manage to render before crashing looks absolutely fine).

Has anyone else had a similar problem with displacement??

[pixelperfectg]

Are you using the height map displacement or the regular displacement? What type of map are you using for the displacement? Displacement settings?

 

I have encountered problems like this when the displacement intersects itself (same holds true for bump mapping). The buckets that do not render (when it crashes) are where the problem area is (or atleast where it starts)

 

I haven't ran into the problem when using the height map displacement since it's rare for the displacement to intersect itself. Are you putting the displacement on an object that has hills/bumps where the displacement could possibly intersect itself on the sides of the hills/bumps? If so, you'll need to change that or reduce the height of the displacement to keep it from intersecting.

[IceAged]

Hi Jeff!

To answer your first question, yes, i am using the height map displacement. I followed your tutorial step by step, making only a few minor tweaks to adjust the length of the grass, etc.

I'm using multi/sub object with material ID's, slots 3 and 5 accomodate a mental ray material, DGS applied to the surface, with a grass jpeg in the diffuse slot. 'Height Map Displacement' in the displacement slot, with cellular in the height map, tiling all set to 1, cell sizes 1, spread 0.4. One of the slots has grass at 100mm heigh, and the other at 300mm.

You mentioned displacement on the sides of the object, however - and i have applied the displacement material to 3D objects as opposed to the 2D mesh that you worked with, (most of the model was originally created by someone else in ADT).

I've tried the 3D meshes and testing the materials on a 2D mesh, but still the error messages and crashes occur.

 

I'm totally stuck here... the grass looks great when it works, but it is dangerously unstable to render...

[pixelperfectg]

Try it with just one displacement material instead of two. If I could see a render of the mesh and where the displacement is applied (by color if nothing else), then I may be able to offer specific ideas...but right now all I can offer is still the thought that the displacement is intersecting itself and creating the error. Take a close look at the mesh where the displacement will be applied and try to visualize if the displacement would intersect itself (pay close attention to avoid angles where it would intersect itself or even another mesh)

[IceAged]

Hi Jeff

I have already tried with just one displacement map, but the crashes were still taking place. I did manage to render this image (attached), although the 'normal and geometry normal' warning still came up a lot of times...

Certainly, the displacement intersects with other geometry in the scene, but even when i try hiding it the messages and crashes still occur. If you're interested and have a spare moment, i could send the file itself for you to take a look at...?

[pixelperfectg]

Wow, I'm surprised to see that it's giving you that error on those flat meshes. Yes, I'd like to take a look at it to see what's going on if you don't mind. You can send it to me at: jeff [AT] jeffpatton [dot] net