Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 93

and quality in compositing, 3D renders are usually rendered in passes . Each pass repres-
ents a part of what makes up the final image. The amount of light that falls on the object,
for example, is rendered separately (represented as a color image where bright pixels are
strong light and dark pixels low light) from its color. Reflections, shadows, and specular
highlightsareallexamplesofhowanimagemightbesplitintodifferentpasses.Thecom-
positor'srolethenistotakealltheseelementsinandrebuildwhatthe3Dprogramusually
does—creating the beauty pass , the composite of all these passes together.
Once the layers are composited, it is very easy for the compositor to change the look of
the beauty pass, as there is easy access for anything that makes up the look of the render.
For example, because the light is separate from the color, it's easy to color correct it so it
is brighter—meaning more light—or to change the reflection so it is a little blurry, mak-
ing the object look like its material is a little worn, for example. Rendering just a single
final image from the 3D software, on the other hand, means that changing the look (such
as adding blur) will be more difficult.
Working with Channels
Down deep, digital images are really an array of numbers representing brightness levels.
There is a brightness value for each location in the X and Y resolution of the image. A
single location in X and Y is called a pixel . An array of a single value for each pixel
is called a channel . A color image generally consists of the four standard channels: red,
green, blue, and alpha. Nuke allows you to create or import additional channels as masks,
lighting passes, and other types of image data. A Nuke script can include up to 1023
uniquely named channels per compositing script.
All channels in a script must exist as part of a channel set (also called a layer ). You're
probably familiar with the default channel set—RGBA—which includes the channels
with pixel values for red, green, blue, and also the alpha channel for transparency.
All channels in a composite must belong to a channel set. Channel names always include
the channel set name as a prefix, like this: setName.channelName. So the red channel is
actually called rgba.red.
Note
OpenEXR has the .exr extension and is simply called EXR for short.
Most image file types can hold only one channel set—RGBA. The PNG file format holds
only RGBA. The JPEG file format holds only RGB. However, TIFF and PSD can hold
morechannels.AllthelayersyoucreateinPhotoshopareactuallyotherchannelsets.One