9.5.2

690 • CHAPTER 13
RENDERING • RENDER SETTINGS • 691
Straight Alpha
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.
You can use this option if straight alphas are supported by your compositing program to avoid the dark
seam associated with pre-multiplied alphas. Note that straight alphas are suitable for compositing
only; they are unusable as conventional pictures.
From left to right: the rendered image, the
alpha channel, the result.
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 TIF format.
The alpha channel can be integrated into a movie only if alpha channels are supported by the chosen
codec.
Separate Alpha
Alpha channels are usually integrated into TARGA, TIFF or PICT pictures — that is, they are saved as
part of the image le. However, if you want to save the alpha channel as a separate le, enable this
option. In addition to your color picture (e.g. ‘room.tif’), you will also have a le containing the alpha
channel (e.g. ‘A_room.tif’). Alpha channels are always saved in the TIFF format.
24 Bit Dithering
Dithering is a process that adds a random pattern to colors to prevent color banding. Although
dithering enhances the image quality, it also increases le size. For web graphics in particular, you
may want to disable dithering to reduce image le size.