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.
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 object’s 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.
