Graphics Programs Reference
In-Depth Information
Chapter 6. Alpha Sports
This chapter starts by providing an understanding of the significance of an alpha
channel and some issues related to it. After realizing the alpha channel, this chapter
deals with different keying and masking techniques. Following is the list of topics
covered in this chapter:
• Significance of the alpha channel
• Blender's alpha modes
• Layering concept and formula
• Using the alpha channel when layering the foreground over the background
with the Mix and Alpha Over nodes
• Solving fringing issues while using the alpha channel with the Mix and Alpha
Over nodes
• Generating matte masks using the ID Mask node
• Different ways to invert matte information
• Edge filtering
• Value and luminance
• Concept of keying and what to inspect in the footage to be keyed
• Understanding Blender's keying nodes
What is an Alpha channel?
The alpha channel first introduced by
Alvy Ray Smith
in the 1970s, can store values
between 0-1, signifying whether a specific pixel is transparent, opaque, or semitrans-
parent. Though this channel is boring to look at, this information is very essential when
merging footages. Alpha information can be stored either independent of RGB, re-
ferred to as
straight alpha
, or by multiplying it with RGB, referred to as
premultiplied
alpha
. A value of zero in the alpha channels signifies that the RGB pixel should be
completely transparent, and opaque when the value is one. The following screenshot
portrays an alpha channel:
