Specifications

Image Architecture
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Alpha (Opacity) Channel
In addition to the color channels, an image can have one alpha channel,
also known as an opacity channel, which is mainly used for image
compositing operations (see “Image Compositing Based on Opacity”in
Chapter 5). The alpha channel must be the last channel in the image.
The interpretation of operations on the alpha channel is usually different
from that for color channels. For example, adding a constant to the RGB
channels in an RGBA image would brighten the image, while adding a
constant to the A (alpha) channel would make the image more opaque.
For this reason, by default most functions ignore the alpha channel if one is
specified. The exceptions are the compositing functions, which use this
channel as the image’s opacity value, and geometric transform functions,
which treat it as any other channel.
To apply any other function to the alpha channel, in the
IplImage structure
temporarily set the alphaChannel field to 0 before calling the function.
Scanline Alignment
Image row data (scanline) can be aligned on doubleword (32-bit) or
quadword (64-bit) boundaries. Each row is padded with zeros if required.
For maximum performance with MMX technology, it is important to have
the image data aligned on quadword boundaries.
Image Dimensions
There is no practical limit of the image size. A long integer is used for the
height and width of the image. This allows you to create images of such
sizes that are much beyond the hardware and OS constraints of today’s PCs
or workstations. For large image support, see also “Image Tiling.”