9.5.2

690 • CHAPTER 13
RENDERING • RENDER SETTINGS • 691
Alpha Channel
The entire alpha channel is masked if you use a Sky, Floor, Foreground or
Background object in your scene. Do not use any of these objects if you need
the alpha channel.
If you enable this option, a pre-multiplied alpha channel will be calculated during rendering. The alpha
channel is a grayscale image of the same resolution as your color picture. Pixels in the alpha channel
are either black or white. A white pixel in the alpha channel indicates the presence of an object at that
position in the image while a black pixel indicates no object.
You can use the alpha channel for compositing in video programs. For example, suppose you have
scanned a photograph of an aireld and you want to render an aircraft and place it on the runway.
Render the aircraft with an alpha channel in CINEMA 4D, then use that alpha channel in your
compositing program to cut out the non-aircraft parts of the render so that the aireld shows though.
The edges of the alpha channel picture are antialiased to ensure a soft transition in the composited
picture.
Pre-multiplied alphas have one particular shortcoming, illustrated below.
From left to right: the rendered image, the
alpha channel, the result.
In the illustration, the alpha channel causes a dark seam. This is because both the picture and the alpha
channel were rendered with antialiasing. By denition, the color picture and the alpha channel must
be multiplied and so the black is calculated twice. You can avoid this dark seam by using the Straight
Alpha option instead. Note that straight alphas are suitable for compositing only; they are unusable
as conventional pictures.
Alpha channels are integrated automatically for TARGA, TIFF, PICT, PSD and QuickTime Movie formats.
If you have enabled Separate Alpha or if you have chosen a different picture format, the alpha channel
is saved separately to the color picture. These les are indicated by an ‘A_’ before the lename, e.g.
‘A_room.tif’. Separate alphas are saved in the TIFF format.
The alpha channel can be integrated into a movie only if alpha channels are supported by the chosen
codec.