Rendering Concepts

The Attributes Editor lets you define both materials and lights, the two components necessary to create rendered images. Materials described the surface attributes of a rendered object, while lights provide illumination. This chapter summarizes some basic rendering concepts.

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What is Rendering?

To generate a realistic picture of the object, or render it, you need to:

• create a material and assign it to the object (this defines the surface appearance of the object)
• create one or more lights to illuminate the object

Both materials and lights have a number of attributes, which can be modified using the Attributes Editor. Depending on the attributes you specify, rendered objects can have almost an infinite variation in their appearance:

Figure 1.1 Left, wireframe object, middle, rendered with single color, right, rendered with texture map

Some of these attributes correspond to visual characteristics found in the physical world (such as color or opacity), while others have no such counterpart but help simulate a realistic appearance (such as reflection maps).

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Rendering a Scene in N·World

• (CLICK-L) on the Render button at the bottom of the Attributes Editor window.

Depending on your preference settings, the rendered frame will be displayed in the Nichimen Render window, or on the Paint Canvas (see "Output Window," on page 3-6).

From N-Geometry

• (CLICK-L) on GeoMenus>Utilities>Render.

From N-Dynamics

Render Performance

If you plan on rendering on more than one machine, set up the render output directory to be local to each machine. Again, if you don't localize the output, render operations may be noticeably slowed as a result of network overhead. You'll also want to periodically remove files from this directory to conserve disk space.

For additional speed up suggestions, see Appendix C, "Increasing Render Speed," on page C-1.

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Render Domains

• view the objects interactively on the SGI screen
• generate high-quality 2D images for full motion video
• use the objects as 3D characters on a 3D game machine

For each use, you'll want to specify a different render domain when defining render attributes. A render domain is essentially a "template" that includes all the possible attributes you can use when assigning attributes for a given target.

N·World currently supports the following render domains:

• GL Shade defines attributes for an object so that it can be rendered interactively on the SGI.
• Render is the default domain for N-Render, Nichimen's renderer. This domain is intended to produce realistic, high-quality images, with features like opacity and reflection maps, as well as ray tracing. The Render domain has more attributes than, for example, the GL Shade domain, offering more variety-however, Render domain attributes cannot be displayed interactively. Use the Render domain to create full motion video sequences or for other non-real time uses.
• The SonyPSX domain lets you define materials appropriate for objects that you'll export and render in real time on the Sony Playstation.
• The N64 domain lets you define materials appropriate for objects that you'll export and render in real time on the Nintendo 64.
• The Sega domain lets you define materials appropriate for objects that you'll export and render in real time on the Sega Saturn.

The Sony PSX, N64, and Sega domains may or may not be available to you, depending upon which Nichimen Express products are installed on your system.

The Sony PlayStation, Nintendo 64, and Sega Saturn all render 3D objects interactively-this means that you assign colors and textures to your objects using the Attributes Editor, then export those object to the target game platform. Different platforms support different features, and the different express domains let you create attributes that take advantage of the special features of each platform.

One Material Supports Multiple Domains

Figure 1.2 Top, material with attributes defined for one domain; middle, with attributes copied to a second domain; bottom, with attributes for all domains. Custom domains (*) can be created using Nichimen's Wide Open World developer kit

For example, let's suppose you created a GL Shade material called red. After designing your object, you decide that you also want to send the object to the N64. Rather than creating a separate red material for each platform, you can copy the attributes from any domain to any other within the same material. You can modify attributes in each domain independently of one another-for example, you might need to change the Diffuse Color in the N64 domain slightly to match the red in the GL Shade domain.

Assigning Materials

Inheriting Materials

Suppose you were working several objects, then discovered that certain characteristics were shared by more than one of those objects. You might, for example, have a number of buildings with a stucco finish, which are each a different color.

If you defined a different material for each building, you'd probably create one material, make a copy of it, change an attribute or two, then repeat the process until you had a number of materials whose only difference was their diffuse color.

Figure 1.3 Materials without inheritance

However, what if you decided upon rendering the scene that the "stucco" didn't jump out quite enough, and you wanted to modify the bump maps used to create the stucco feel of the surface? You'd have to open each material and modify the bump map value by hand.

As a better, you could define a set of inherited materials. Instead of creating a new material for each building, you could create a base material called Stucco. For each building, you could create a new material that inherited most of its characteristics from Stucco.

Figure 1.4 Materials inheriting from another material

In the relationship shown in Figure 1.4, any changes that you made to the Stucco material would automatically be reflected in Red, Blue, and Pink. If you wanted to deepen the bump map for all the stucco buildings, you would only have to modify the values in Stucco; when the scene was re-rendered, Red, Blue, and Pink which all inherit from Stucco, would now use the modified bump map values too.

Inheritance between materials can be defined in either the Attributes Editor or the Browser.

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Rendering with N-Render or GL Shade

1. Create your objects in N-Geometry.

Your scene may contain any number of objects.

You can create lights in N-Geometry or in the Attributes Editor using the Lights Editor.

Materials are sets of attributes that determine the basic surface appearance of an object. A material also specifies which lights affect the current material.

Materials are assigned in N-Geometry the Browser, Nichimen's visual hierarchy manager.

Note. You can also assign a material to face parts of objects.

5. Render the scene.

You can render the scene directly from N-Geometry, from the Attributes Editor, or from an N-Dynamics script. You can output the resulting image to a file, a separate render window, or directly to the N-Paint canvas.

Rendering scenes is typically an iterative process-you may modify the materials and lights used to render a scene any number of times in the Attributes Editor until the scene looks just right.

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Rendering with the Game Domains

1. Create your objects in N-Geometry.

Your scene may contain any number of objects.

You can create lights in N-Geometry or in the Attributes Editor using the Lights Editor.

Materials are sets of attributes that determine the basic surface appearance of an object. A material also specifies which lights affect the current material.

Materials are assigned in N-Geometry the Browser, Nichimen's visual hierarchy manager.

Note. You can also assign a material to face parts of objects.

5. Preview your objects on the target platform.

Objects can be previewed interactively on the appropriate target platform by the artist.

Once objects look correct, you can write them out in target specific formats, using tools like the N64 Express, PlayStation Express, and Saturn Express.

Assigning materials for use on the various game platforms is described in more detail in the appropriate N64 Express, PlayStation Express, and Saturn Express documentation.

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Rendering: Some Concepts

Scan Line vs. Ray Trace Rendering

N-Render supports two basic rendering techniques for calculating the value of each pixel in the 2D image:

• scan line rendering
• ray trace rendering

Scan line rendering is the simpler of the two. In this model, light rays are emitted by a light source and travel through the scene. Each interaction with an object in the scene affects the color value of the pixel that the ray maps to. Rays travel until one of two things happens:

• They encounter a completely opaque object
• They travel beyond the clipping planes defined for the scene.

Figure 1.7 Scan line rendering

The color value for each pixel is calculated based on the surface color of the "ray" encounters. Attributes associated with these objects determine the result of the color calculation.

Ray trace rendering, on the other hand, uses a more realistic model which allows for rays to "bounce" off of objects in the scene. Rays begin with a certain amount of "energy" and lose some of the energy with every bounce. These interactions can result in the creation of new rays, which must be traced and their effect on pixels in the finished image calculated. Rays travel until they "bounce" a predetermined number of times, or expend their energy.

Figure 1.8 Ray trace rendering

Mappers & Maps

The mapper object provides a visual cue of the relationship of the map image to the object. Mappers can be in one of two states:

• Frozen mappers fix the relationship between the map and a point on the object surface. You typically use frozen mappers on objects whose surface will change shape.
• Dynamic mappers can be animated in N-Dynamics. You could, for example, project a texture map of the globe onto a sphere, then create a script to rotate the mapper (which would simulate the globe spinning)

The types of maps supported vary by domain, but include the following:

• Texture maps apply images to the surface of an object.
• Opacity maps define how opaque an object is.
• Bump maps give an object the illusion of a bumpy texture by using dark and light areas in the map to modify the surface color of the object.
• Reflection maps simulate how elements in a scene would be reflected onto the object.
• Refraction maps simulate how elements inside the object would be reflected onto the object's surface (usually for a semi-transparent object).

Figure 1.9 Map types: from left to right, (1) texture, (2) bump+texture, (3) texture+reflection, (4) texture+reflection+refraction, (5) texture+opacity

Shading Models

N-Render uses five different shading models:

• Flat
• Lambert
• Phong
• Blinn
• Cook

Figure 1.10 Shading models: from left to right, Flat, Lambert, Phong, Blinn, and Cook

All of these models are described in more detail in the next chapter.

Opacity

• Fully opaque objects completely obscure objects behind them.
• Objects that are semi-transparent reveal a percentage of the background scene that is inversely proportional to the object's opacity.
• Objects using opacity maps reveal parts of the objects behind them based on the characteristics of the map

In other words, the more opaque an object, the less you can see of what's behind it. To render an object as a brick, you'd want to use a high opacity setting; for a sheet of glass, you'd use a lower opacity setting.

Figure 1.11 The sphere in the foreground of the scene has an opacity map applied to it, making some areas of the object transparent

Visibility

Figure 1.12 Left, original object; object with 75% visibility; right, with 25% visibility. In all three cases, the sphere is in front of the pyramid

To grasp the difference between visibility and opacity, compare an object whose opacity is decreasing until the object is completely transparent with an object whose visibility is decreasing until the object becomes invisible. The specular gleam from the completely transparent object remains clearly visible, but the invisible object gives off no specular gleam at all. You can illustrate the difference for yourself by animating a sequence in N-Dynamics, first animating visibility, then trying the same animation animating opacity.

Lights

N-Render supports the following types of lights:

• point
• infinite
• ambient
• spot

Just as you specify attributes for a material, you also specify attributes for a light (such as the light's color, relative brightness, shadow casting characteristics, and so forth. Working with lights is discussed in more detail in "Lights," on page 4-1.

Oblique Effects

A soda bottle with thick glass walls serves as a good example of such an object. More light is obscured toward the edges because the light must travel farther through the glass at the silhouette edges. (The inverse effect is also possible in N-Render, with the result that opacity fades out toward the edges. This is useful in producing "glow" effects.)

Figure 1.13 Oblique effects

For example, line of sight A has to look through more glass, while line of sight B looks through a very thin piece of glass. Oblique effects let you simulate the change in appearance of such oblique points of view.

Figure 1.14 Left, sphere object with low opacity at edges; right, same object with high opacity at edges

If you use oblique effects, you need to specify the falloff rate for diffusion, opacity, specular strength, reflection, and refraction from the center of the object to the edge of the object in the Effects section of the Attributes Editor.

Copyright © 1996, Nichimen Graphics Corporation. All rights reserved.