ADS Technologies USBAV-708 INSTANT VIDEOMPX, Руководство пользователя

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The
Guide
67
Cro
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a
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Re
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Wh
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iz
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This extra bit spend means throughout the video less bits are available to encode
the actual picture and thus video quality suffers. By cropping borders no bits are
wasted outside the actual picture area and thus overall quality is improved.
Resizing is used in a variety of situations, the simplest case being where there is
a desire to reduce the video dimensions. At lower bitrates reducing video
dimensions may assist in preserving quality by reducing the number of blocks
required to make up the picture - with fewer blocks each block should receive a
greater number of bits on average.
It should be noted that there is a moving balance to be drawn between
dimensions and quality at any particular bitrate. A small reduction in dimensions
will generally mean fewer blocks and thus a greater bit spend per block giving
higher overall quality, but if the video dimensions are reduced too far then within
each block the texture and motion may
become so complex that intra-blocks require
more bits to store and the motion search
becomes less effective leading to a greater
proportion of intra-blocks, intra-frames and
lower overall quality. At lower resolutions fine
details and sharp edges may also become
degraded.
Resizing is frequently used to scale video on its
vertical axis to correct the
aspect ratio when
encoding from DVD. The aspect ratio describes the
relationship between the horizontal and vertical
dimensions of the video with respect to their
proportionality. If this ratio is not correct video will
appear to be either
compressed or stretched
vertically when viewed,
leading to characters that
appear either short and fat or
tall and thin.
Higher aspect ratio
Original aspect ratio
Lower aspect ratio
Video dimensions
Perceptual quality
Optimal dimensions