Pixar Photorealistic RenderMan - Artist tools (RAT) Tutorials

Displacement power in Reyes mode.
 

	Introduction
	Examples of True Displacement
	How to create Displacement maps?
	Conclusion

Softwares used: Alias Maya 7.0.1, Pixar RAT 6.5.2, Pixar Renderman server pro 12.5.2

Note: Although this article is written with PRman displacement capabilities in mind, displacement method is not limited to PRman and can be used in many other renderers as well.

Introduction

One of the things that the Renderman power lies behind is True Displacement. Using Displacement we can add geometric surface details, those ones that are difficult or impractical to build and work on in the 3D scene. Displacement shaders move vertices in render time based on parameters you can define in shader. These can be as complex as shaping and forming an entire object to simple things like bumps. However, the algorithm is not yet implemented completely in Raytrace mode, thus we need to stick to Reyes mode if we want to get good results in reasonable render time. Fortunately it is possible to bake indirect illumination in Raytrace mode to be used later in Reyes mode. Apart from speed, Reyes mode seems to still give out a higher quality image than Raytrace mode. Pixar has offered a new way to cache indirect illumination called brick maps to use it later in Reyes mode. Mr. Eric Hanson the author of Maya Killer Tips book has informed me that they used this technique in the movie The Day After Tomorrow. Another payoff of using True Displacement is that it will be much easier to have LOD (Level of details) set for simple polygonal objects. The displacement shader has some kind of built-in LOD itself since it uses mipmaps.

Examples of True Displacement

Once I was working on an underwater scene. In such scenes objects have so much fine details that it's unwise to work on all of those tedious details in the 3D scene, even if you're using a 3D painting tool like ZBrush. So I decided to go with True Displacement. The modeled objects  were very light polygonal objects with Pixar SubDivision applied on. Keep in mind that although objects in 3D scene were very light and render time was pretty fast, but True Displacement consumes lots of memory if the displacement bounds are not set properly.

The scene here contains one spot light with shadow on, one directional light, one MTOR coordinate system object and the sample object with displacement shader assigned.
I've used a dual Xeon 2.6 GHz with 3 GB memory here for rendering.

Picture 1: Render time 18 seconds.

Map 1: One of the displacement maps which is used in Picture 1

Picture 2: A rusted metal column rendered in 33 seconds.

Picture 3: Sample sea-bed portion rendered in 28 seconds.

Picture 4: A stone rendered in 31 second. An incredible render time for such an object.
Actually two stone objects are defined as CSG (Constructive Solid Geometry) primitives and then "difference" method is used.

Picture 5: Render time only 63 seconds.

How to create Displacement maps?

For very fine details I suggest to stick to 2D painting softwares like Photoshop. One thing to remember is that Displacement uses Grayscale image or one channel image. Usually images have 3 RGB channels and each channel has 8 bits which means every channel can be an integer number between 1 & 256. Since Displacement uses only one channel of your map (or grayscale map which contains only one channel), you can have up to 256 different levels in displacement of vertices. In other words in render time, every micro-polygon on the object can be displaced for a maximum of 256 times of the smallest possible displacement which is set.
This is enough for most cases but in some cases you need much more. For example in the cases that there are both very big displacements and also very fine ones used on the same object, (i.e. A flat surface is displaced into a cliff and then you also want to add fine details like small cracks, bumps & etc.). In such cases that 256 level is not enough.

• Procedure1: You can either use 16 bit per channel pictures

• Procedure2: You can have several layers of displacement and several maps, some for big displacements and some for fine details.

Unfortunately Photoshop does not handle 16bit per channel pictures very well and most of the effects are not available in this mode. I personally stick to the second procedure when possible.

The other way around creating displacement maps is 3D painting the displacement details directly on the object in a program like ZBrush, extracting the produced map and then using it as displacement map for render time true displacement in Renderman or other renderers. However you can not paint very fine details with current 3D painting methods since the finer you paint, the heavier model becomes till you run out of memory/cpu.  Current solution to this problem can be to 3D painting the big displacements and then refining the extracted map later in a 2D editing program.

Conclusion

As you can see the usage possibilities of True Displacement is virtually unlimited. Although displacement is not a new tool, it's usually forgotten by new comers to the 3D world. I hope this article can encourage people to explore the exciting vast usage possibilities of True Displacement which can make life easier for modelers!

Sepehr Dehpour
Oct 13 2006

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