Level of Detail

The animations on this page demonstrate the support offered by RenderMan for level-of-detail (LOD). Three models of a short length of Spanish Moss were made in Maya. Using LOD the renderer selects the appropriate model to render based on the area of screen space occupied by the model.


The spanish moss geometry was created by using CV curves as guides to extrude poly cylinders into a moss-like form. By adjusting the taper values of the extrusions I was able to get the cylinders to diminish in size as they approached the end of their guiding curves. The low resolution mesh consists only of curves since Renderman is able to render curves unlike Maya. The medium resolution mesh is the initial extrusion of cylinders, and the high resolution mesh is the medium resolution subdivided once.

This modeling technique allowed me to maintain continuity between all three levels of detail. When all three models are composited on top of each other their geometry follows the same basic shape minimizing the amount of popping as the models transition between levels of detail.

Poly Counts
Low Resolution: curves
Medium Resolution: 1687 polygons
High Resolution: 6,680 polygons

flash content will replace this text

Level of Detail Ranges

To determine which model should appear visible to the renderer, renderman must know the ranges of screen space (amount of pixels occupied on screen) each object will exist at. I decided to calibrate these ranges with a screen space of 640x480, resulting in a pixel space of 307,200 pixels.

After entering different ranges of numbers I was able to hone down a set of values that blended the models at appropriate distances from the camera making use of three levels of detail. Below is an animation of the camera approaching the model from thirty units away. Notice how the model changes from red (low res) to green (medium res) to blue (high res).

flash content will replace this text

Detail ranges of each model:
Low: DetailRange [ 1 1 2000 4000 ]
Medium: DetailRange [2000 4000 7000 50000]
High: DetailRange [7000 50000 3000000 400000000]

flash content will replace this text

Dynamics Simulations

To animate the moss I decided to use Maya's nCloth to dynamically simulate motion. I started with creating a poly plane that had four height divisions with one proxy box attached to each division. The proxy boxes reference a rib file of the spanish moss models with level of detail ranges programmed in. When the scene is rendered the proxy boxes are replaced with moss models resulting in one continuous strand of spanish moss. My goal with the nCloth was to have the proxy boxes animate dynamically so I would not have to manually keyframe each one.

Initial Test

Each proxy box was appended to the nCloth poly plane by using a mel script called parentToSurfrace which creates a follicle node at a point on the plane closest to the center of the proxy box. The proxy box is then parented to the follicle allowing it to follow the motion of the nCloth plane.

In this initial test all values are set to default. I created a cylinder and set it as a passive nCloth object allowing it to collide with the plane as it passes through. This animation only required two keyframes to be set for the motion of the cylinder collision object.

flash content will replace this text

Wind Test

NCloth also has wind attributes allowing for subtle animation perfect for strings of spanish moss. Inside the nucleus node created with the nCloth plane are wind speed and gravity values. In this test I set the wind speed to 12.

I also had to tweak the properties of the nCloth plane so it would not blow out of control in the wind. By adjusting the mass to 0.5 and the tangential drag to 0.05 I was able to dampen the effect of the wind.

flash content will replace this text

Collision Strings

The end result of this project will have multiple string of moss clumped together to form a clump of moss as seen in my reference images. I decided to test multiple strings of the moss colliding with an object to foresee how a group of nCloth objects will interact together

In this test I strung together 10 proxy boxes per each nCloth poly plane. The increased length caused a problem that I did not notice before on my first collision test. Notice how some the proxy boxes jitter wildly as they collide with the sphere. If the bottom and top of the boxes don't align then the moss strand will appear to be broken at render time instead of flowing as one solid piece.

The collision values in this test are set at the defaults.

flash content will replace this text

Adjusted Collision Values

To fix this problem I decided to adjust the nCloth properties of the poly plane. By adjusting the collision flag, mass, dampening, and tangential drag I was able to soften the collision effects however the boxes still jolt enough to break the continuity of the moss.

Damp: 1
Collision Flag: Edge
Point Mass: 50
Tangential Drag: 1

The real problem resides where the proxy box is pinned to the poly plane. Because the follicle is located at the center of the proxy box, the box rotates around the follicle which pulls the top and bottom faces of the box from lining up with the next box in line. To fix this problem the follicle would have to be located at the top of the box.

flash content will replace this text

Proper Ncloth Collision

After a series of tests I discovered that I had to freeze transforms on the poly planes before making them nCloth. Initially I froze the transforms on both the poly planes and proxy boxes but that caused the referenced rib files to lose their positional data when instantiated through readMossRI making them appear at the origin of world space.

I used the nCloth simulations to animate the moss in demonstrating the level of detail model swapping as can be seen above in the "Level of Detail Ranges" section of this page.

flash content will replace this text

Moss Grove

I wanted to use the nCloth dynamics to allow a camera to move through a scene full of dangling moss with a collision sphere around the camera pushing the moss out of the way as it progresses, however, I was unable to get the dynamics to work properly.

This scene was also my demonstration of models changing level of detail as they get closer to the camera but I encountered a problem with Maya that would not allow me to batch render an animation with Renderman. The renderer would render proxy boxes instead of the moss rib files as seen below. The error within maya allowed me to render single frames properly using renderman with the in view renderer but batch rendering ignored my readMossRI(); preshape mel command.

To solve this problem I had to export each frame of the animation as a rib file and then render each one within the cutter environment. A mel script provided by Malcolm Kesson named batchRenderRI automated this process within Maya.

flash content will replace this text

flash content will replace this text

Final Render

To create this clump of moss I duplicated many strings of proxy boxes of various sizes and heights to create a tapered form. Two spot lights were used as key and rim lights with subtle hues of warm and cold colors. The shader used is lambert with a green/grey diffuse color.