Bitmaps page 4
RGB colours are also called RGB channels. The reason for this is that there is an optional fourth channel a pixel can have, namely its alpha channel.
The alpha channel ('A') is used to represent transparency.
Not all digital images have transparency information. In cases where
they don't, (a plain RGB image), the images are 100% "solid".
If you were to layer such an image over another image, it would
obscure the one underneath as it is completely opaque. When an image
has transparency information in it, it is often referred to as an
RGBA image and this type of image is very useful in compositing
work as the edges of images which overlap are nicely antialiased.
In a 24 bit image, there are 8 bits per channel/colour,
which is the same as saying that each channel/colour has 256 possible
shades. The same will apply to the alpha channel. It too will be
8 bits hence have 256 shades from 0 = transparent, to 255 = opaque.
The shades in between are degrees of transparency.
Whilst you will likely encounter 64 bits per pixel
images or even higher, you will almost never encounter images with
alpha channels that have less than 32 bits per pixel with
the exception of GIF bitmaps.
Premultiplied versus "Straight" Alpha
As a rule, using alpha channel is by far the easiest
way of layering images one over another. The only downside is that
there are two ways in which alpha channels can be written to a file:
premultiplied and "straight".
Ignoring the technical explanation, all you need to know is that if you import an image to layer over another and you get very odd fringing effects where you were expecting a seamless blend, you are using the wrong sort of alpha information. Fortunately, most "high end" programs will quickly convert the image to the right sort. Almost always, you will be trying to use a premultiplied image where a straight one is needed. If it is the other way round, instead of bad fringing, you will get more subtle but unexpected changes in brightness.
Wherever possible, use "straight" alpha where a choice is offered.