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Render Passes, Layers, and 3D Compositing

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Adapted from
Digital Lighting & Rendering
Digital Lighting & Rendering

By Jeremy Birn

  Multi-pass Rendering

While some people use the terms interchangeably, Rendering in layers is the process of rendering different objects in your scene separately, so that a different image is rendered for each layer of objects. Rendering in passes is the process of rendering different attributes of your scene separately, such as the different pass types below.

Pass Types

Multi-Pass Examples:

Download the 1 meg file SpaceSample.psd, and examine the 10 layer composite in Photoshop's Layers Window.

Or see the case-study rendering an ant for a pass-by-pass breakdown of a real project.

 

A beauty pass (sometimes called diffuse pass or color pass) is the main, full-color rendering of your subject, including diffuse illumination, color, and color maps. A beauty pass usually will not include reflections, highlights, and shadows, which are usually separate passes.

Highlight passes (sometimes called specular passes) isolate the specular highlights from your objects. You can render highlight passes by turning off any ambient light and making the objectís diffuse shading and color mapping to pure black. The result will be a rendering of all the specular highlights in the scene, over a black background, without any other types of shading.

A reflection pass includes reflections of other objects or the surrounding environment, and can either replace or complement the highlight pass. To isolate reflections, usually all you need to do is turn off ambient, diffuse, and specular shading from a surface, so that only reflections appear.

A shadow pass is a rendering that shows the locations of shadows in a scene. A shadow pass often appears as a white shadow region against a black background, a black shadow against a white background, or a rendering with the shadow shape embedded in the alpha channel. Cast shadows are where an object casts a shadow onto another 3D object or darkens an area of a live-action plate. Separate shadow passes can depict attached shadows, where an object casts shadows onto itself, such as the shadow a character's nose casts onto his own face.

A lighting pass is an optional part of multi-pass rendering, that adds flexibility and control to the compositing process. Instead of rendering a beauty pass all at once, you could instead render multiple lighting passes, with each individual lighting pass showing the influence of one light (or one group of lights) on an element. Other lights are hidden or deleted when rendering the lighting pass.

Effects passes may sometimes be rendered, depending on the needs of your project. An effects pass is a separate rendering of a visual effect or a mask for a visual effect. An effects pass might be an optical effect, such as a light glow or lens flare, or a particle effect, such as a cloud of smoke or plume of jet exhaust.

A depth map (also called Z-Depth or a depth pass) is a pass that stores depth information at each point in your scene. Some productions use depth maps rendered in a special depth map file format. Other productions use simulated depth maps which are rendered as standard image files just like any other pass, but with a depth-fading effect over objects with constant white shading.

Passes can be rendered one at a time by rendering differently modified versions of your 3D scene, or some software can set them up automatically or render more than one pass type at once.

Sample Uses For Passes

Here are ten example uses of why scenes are split into different passes in professional work (instead of trying to render the whole scene in one pass as most hobbyists do.)

Memory can be saved by not putting every object into a rendering at once, making complex renderings possible on ordinary PCs.

Changes can be made with little or no re-rendering. Often, just one element (such as a character) needs to be re-rendered, instead of a whole environment. Other adjustments, such as adjusting the darkness of a shadow or the color of a glow, can be made without any re-rendering, just by adjusting your composite.

Integration with live-action plates depends on separate passes. For example, if your character needs to cast a shadow onto the ground in a photograph, or appear reflected in a real pool of water.

Still Images can sometimes be used for some elements, especially if the camera isn't moving. For example, one still frame of a room might be composited behind every frame of a separately rendered animated character.

Recycling is often possible, where a separate element can be reused in different positions or times within a shot.

Reflections can be softly blurred in the composite, which eliminates the need for anti-aliasing or high-quality rendering for the reflection pass.

Particles can be used to create different effects in compositing. For example, if you wanted a natural refraction or heat-ripple effect from your particles, you could render them as a separate effects pass, then use the rendered particles as a mask for a glass distortion filter or warping of the background plate behind them.

Bump mapping can be applied selectively to reflections, beauty pass, or highlight pass, instead of having the same bump map on each element of the shader.

Glows can be created and manipulated in post by adding a blurred copy of your specular pass on top of the composite.

Depth of Field effects can be created without requiring any extra rendering time, by blurring separately rendered background layers, or by using a depth pass as a mask for a blur filter. You can also create atmospheric perspective (fading to blue or gray with distance) with a color correction masked by the depth pass.

About Chapter 11
If you have the book Digital Lighting & Rendering Second Edition, see Chapter 11, "Rendering Passes and Compositing," for a much more up to date (including global illumination and occlusion passes, etc.) and complete discussion of rendering in passes and compositing 3D with live-action footage. This web page quotes fewer than 1000 words from a 7800 word chapter.

Copyright © 2000-2006 by Jeremy Birn.