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Lighting In Layers - By Jeremy Birn
Modeled and Rendered by Jeremy Birn, using Softimage and Mental Ray.
The color and shading of the backdrop is very important to the perceived look of the ant in the final rendering. Even without the ant appearing in this rendering, you can already get a sense that it is going to be red and translucent.
The key and fill shadows are cast by area lights, for naturally soft shadows. Unlike some soft shadow algorithms that are possible from point/spot lights, area light soft shadows are focused and sharp at the point where the foot touches the ground. They grow softer and more diffuse, and eventually fade away, with greater distances between the shadow caster and the receiving surface.
By having rendered each layer separately, parameters such as the key/fill ratio, and color temperature of the lights, can be adjusted and re-adjusted without rerendering. Most of the savings in render time come from reducing the number of re-renders in the revision and approval process.
To add warmth and shading to the shadows on the backdrop, a row of reddish point lights were positioned inside the ant. These lights served both to simulate indirect light that could have bounced off of the reddish ant into the environment, and further added to the sense that the ant itself was made from a translucent material.
Note that the main rendering of the ant isn't shiny yet. Highlights will be added in another layer. Having the diffuse layer in isolation is handy in a number of ways. If the ant stepped into a shadow, the highlight layer could be held out of the shadow area, and a darker version of the diffuse layer could be used without highlights for the shadowed ant. Color correction is also made easier, because it is possible to adjust the tint or saturation of the ant body without affecting its highlights.
This pass was rendered with no lights in the scene, and no ambience. This way, there was no ambient, diffuse, or specular illumination on any of the surfaces, which are phong shaded. The only thing making the phong shaded surfaces appear brighter than pure black is the reflections of 3 reflect-only objects (flat cards that are not visible to primary rays). The 3 cards are texture mapped with an image of a window, and given a constant (sometimes called lightsource) shading model. If you look at the smoother plate of the tail, you can see the different panes of the window textured onto the reflected cards.
Other advantages of having an isolated highlight pass include the option to adjust the shininess, and the sharpness of the highlights, during the composite. Also, in most renderers, a single bump map on a surface will perturb the surface normals for the purpose of shading the diffuse area, and will also distort the reflections and highlights. By rendering layers separately, you can use different bump maps on the different layers, creating effects that would otherwise require customized or layered shaders. Finally, visual effects such as glows on surfaces are created easily in post by superimposing a blurred version of the highlight pass.
Software Note: Since this page was first posted, many visitors have e-mailed to ask whether this technique was something that used some special features of a particular renderer, or whether there was as much reason to work this way in Lightwave, Max, or other programs as in Renderman or Mental Ray. Rendering and compositing multiple passes and layers has been a staple of visual effects work since long before 3D rendering was even used, and this is the way high-end professional productions are created, without regard for which software is being used. In fact, one of the ways that some lower-end renderers can be "pushed" to achieve more controllable, photorealistic results that move beyond the shading characteristics of a renderer's standard output, is precisely by isolating and manipulating multiple passes.
Also see the related page Multi-Pass Rendering, an overview of 3D pass types and compositing tips from Jeremy Birn.
Jeremy Birn's book Digital Lighting & Rendering (now in a 2nd edition) covers this type of rendering in much more detail.
See the Main Index for more rendering tutorials.