Image Processing Reference
In-Depth Information
companies are free to use less precision or compress more harshly at the entropy-coding
and quantization stage if they wish.
Some widely available DVD titles exhibit particularly bad artifacts, but it would be
unfair to single them out because most DVDs exhibit some artifacts when you inspect
them closely. It may also be dependent on the player you are watching them on because
some players only implement the very minimum functionality required to decode and
display a basic image. They don't necessarily exploit everything that the encoder put
there.
For examples that exhibit artifacts, check out scenes that are shot underwater with a
searchlight beam in the shot. These are guaranteed to demonstrate contouring artifacts if
the encoding process did not deal with them properly. Scenes with predominantly sky and
sunset or a glaring sun also exhibit the same problem. Early DVDs with rapid scene move-
ment such as car races demonstrate some very unpleasant motion artifacts. A lot of DVDs
have been compressed with far too much edge enhancement applied during the telecine
process.
11.6
Concealment Motion Vectors Tool
This is a useful tool to provide some redundancy in the coding. It works very much like
the disk block reallocation technique in a hard disk. If a macroblock is lost, then the con-
cealment motion vector in an adjacent macroblock may provide an alternative that is less
optimal but still better than a total dropout in the reconstructed image.
11.7
Pulldown Support
Pulldown was explained in Chapter 4 when film formats were discussed. The MPEG-2
standard provides the tools to remove the pulldown frames to restore the 24-frame time-
line. This is then compressed more effectively than the video might have been, but the
information is added to the compressed data to instruct the decoder to recreate the 3:2
pulldown as necessary in order to play the content out again at 30 fps when it is viewed
in a locale with 60-Hz displays.
11.8
Pan and Scan Support
A useful enhancement to MPEG-2 allows content to be displayed sensibly when the source
aspect ratio does not match the playback aspect ratio.
A wide-screen image can be panned to center a 4:3 aspect ratio window where the
most interesting part of the action is taking place. Figure 11-3 shows how wide-screen con-
tent gets cropped for a normal aspect ratio.
In the opposite situation, a 4:3 aspect ratio source image is scanned so that a wide-screen
viewing rectangle is moved up and down to display part of the image (see Figure 11-4).
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