ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

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The
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77
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Quarter Pixel
In a digitized video each frame is composed of a 2-
dimensional array of
pixels , the small colored
squares that when viewed collectively from a
distance make up the picture.
Recall from
Forward-Macroblocks and motion that
during the motion search DivX attempts to locate a
matching image for every block in the current frame
from a similarly sized area in the reference frame. One way to think of the motion
search is to imagine the encoder taking each square of video in the current frame
and overlaying it in various positions in the reference frame until it finds a good
match, continually maneuvering it and evaluating the difference.
The simplest way to perform this operation would be to compare each block in
the current frame against the surrounding area in the reference frame, moving
the block one pixel in any direction at a time. The relative vector offsets from the
origin of the current block during this search might be
(0, 0) , (0, 1) , (0, 2) , (1, 0)
and so on.
This scheme would be called whole-pixel because the search was performed
against the reference frame at 1-pixel resolution.
In fact, DivX actually searches at half-pixel resolution by default, achieved by use
of filtering. Applying a filter to the block allows the encoder to create a virtual
block that represents how the block would appear maneuvered half a pixel in any
direction against reference frame. The motion search can then return a vector
accurate to half-pixel resolution. The relative vector offsets from the origin of the
current block during a half-pixel resolution search might be
(0, 0) , (0, 0.5) , (0, 1) ,
(0, 1.5) , (0, 2) , (0.5, 0) , (0.5, 0.5) , (0.5, 1) and so on.
Quarter-pixel, as its name suggests, performs the motion search accurate to
quarter-pixel resolution. Relative vector offsets from the origin of the current
block during a quarter-pixel resolution search might be
(0, 0) , (0, 0.25) , (0, 0.5) ,
(0, 0.75) , (0, 1) and so on.
What is quarter-pixel?