DirectX Video Acceleration Video Subtypes

The following subtypes are used by decoders that are using DirectX Video Acceleration (DXVA). AI44 and IA44 are surfaces with a bits-per-pixel value of 8. There are 4 bits of I and 4 bits of A. I represents an index into a 16-entry YUV palette. A represents 4 bits of transparency information (also known as per-pixel-alpha). Therefore, AI44 and IA44 surfaces allow for 16 different colors at 16 different transparency values, or 256 different pixel representations. With AI44 the alpha is stored in the hi-nibble, in IA44 the alpha is stored in the lo-nibble. Both formats are very useful for DVD sub-picture images and Line21 and Teletext images. Microsoft prefers the AI44 version because it is slightly easier to generate text using this format.

See Also

• Video Subtypes