Image Processing Reference
Figure 14-8 Rate control based on quantization scale factor.
Quantization Scale-Factor tools
The MPEG-1 standard provides a tool called the “quantization scale factor.” This is used
in a feedback loop that measures the buffering being consumed by the encoder. The feed-
back throttles the quantization level as necessary to stop the buffer from overflowing. This
part of the encoding technology relates to the performance of encoders when you select
variable bit-rate (VBR) or constant bit-rate (CBR) algorithms.
Being able to adjust the quantization scale factor allows us to more easily generate
video that is delivered at a constant bit rate. The workings of this and the flow of feedback
information are shown in Figure 14-8.
The feedback is quite sophisticated because it will affect the way that macroblocks are
encoded. Knowing something about the complexity of the macroblocks will help you decide
where to scavenge more bit rate with a quantization scale factor that increases the compres-
sion ratio. Performing some metrics on the original source image data provides a measure of
the amount of detail. There are a variety of ways to do this. The best performance is achieved
by carefully examining the source material before starting to encode the macroblocks.
This section of the encoder includes the video buffer verifier (VBV) and some com-
pression tools (Popwire Compression Master, for example) allow you to control the behav-
ior. Often it will not be called the VBV control. It masquerades as a minimum and
maximum target bit-rate range control in the variable bit-rate settings. This is where traf-
fic shaping and bit starving are controlled to even out the peaks in the bit rate.
If you have the analyzing tools, Figure 14-9 is a typical display showing buffer occu-
Bit Starving to Empty Buffers Just Prior to Complex Scene Switches
You will need to make sure that there is sufficient bit rate to cope when the video is com-
plex. For example, the compression ratio will be less efficient when you are dealing with
Footage of fire
Footage of explosions