Picture-Level Frame/Field Coding
PicAFF, or Picture-adaptive Frame-Field Encoding, is an advanced video encoding technique used in MPEG-4 AVC. Maintaining the quality of interlaced video can be a challenge in video encoding because of the larger spaces between horizontal lines in the same field. While MPEG-1 doesn't support interlaced video at all and MPEG-2 requires entire streams (files) to be either interlaced or progressive, AVC allows individual frames, or even macroblocks to be encoded as interlaced or progressive, depending on whether there are significant differences between the top and bottom fields.
In addition to quality improvements, progressive frames require fewer bits making them more compressible for the same quality as interlaced frames. Just as high quality deinterlacers first look to see if there's motion from one field to another within the same frame, when using PicAFF encoding an AVC encoder looks for motion to determine whether both fields shold be encoded as a single progressive frame or separate interlaced fields. While not as efficient as the alternative, MBAFF, in terms of file size, it results in faster encoding and is supported by more AVC decoding software.