ADS Technologies USBAV-708 INSTANT VIDEOMPX, Руководство пользователя

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The
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Motion vectors accurate to quarter-pixel resolution allow each motion-
compensated frame to be reconstructed more accurately than those accurate to
half-pixel resolution. The result tends to be that video appears sharper and of
slightly higher quality.
Consider that the eye is most sensitive to motion where objects in a scene are
moving very slowly. It would be difficult, for example, to accurately see how many
pixels a rocket had traveled across the screen after firing, but if instead we were
watching a snail crawling in front of the camera then accuracy would be far more
important.
The effect of perspective (where distant objects appear smaller than near
objects) biases the improvements made by quarter-pixel motion search with
respect to an objects depth in the scene. With respect to the image at the camera
objects will appear to move slowly in the distance
where perspective causes space to appear
compressed, and fast near to the camera where
perspective has the least significant effect.
In this scene the
effect of
perspective is apparent—although the car does
not physically shrink as it becomes more distant it
occupies less area in the image. We could say
that distant objects have lower spatial resolution
in the image. Quarter pixel counter-acts this.
Imagine a character walking across the video behind the car while it is positioned
near to the camera. The motion accuracy would not be critical because the
resolution of near objects is already high. In contrast, imagine the same
character walking across the video behind the while it is distant from the camera.
The character would occupy far fewer pixels and motion accuracy would be more
important for fluid and accurate motion.
Therefore, although quarter-pixel reduces the compensation error throughout the
entire video its most significant impact is on the fluidity in motion of objects
distant from the camera.
Why use quarter-pixel?