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from black to white would be a change of 10000, or 100%.
This comparison is performed for all 330 blocks in the frames. The total num-
ber of blocks that have changed is counted and used with the Percentage
Area setting, as described in the following section.
PERCENTAGE AREA
The final step in motion detection is to compare the number of blocks that
have changed (as determined in the previous step) to the threshold com-
puted from the Percentage Area setting. If this threshold is reached, motion
is detected.
Percentage Area is the percentage of the 330 blocks in the image that need
to change for motion to be detected, multiplied by 100. For example, a Per-
centage Area setting of 250 (2.5 percent) means that more than eight of the
blocks (.025×330) would have to change for motion to be detected. The low-
est setting you can use is about 30 (or 0.3 percent)—which requires a single
block in the image to change from the reference frame for motion to be de-
tected. Setting the Percentage Area smaller than 30 would require zero blocks
to change for motion to be detected—which means motion would be de-
tected all the time.
MASKING BLOCKS
Any of the 330 blocks in a frame can be masked. Whenever a block is masked,
it is completely ignored during the block-by-block comparison. A masked
block is never counted as one of the blocks that have changed even if it
changes completely from black to white. If you mask everything in the pic-
ture, motion will never be detected no matter what happens.
Even if you mask one or more blocks, the Percentage Area setting uses all
330 blocks for its computations. For example, suppose you mask 230 blocks
and set Percentage Area to 2000. In this case, 20 percent of all 330 blocks—
or 66 blocks—must change for motion to be detected. However, this requires
66 of the 100
unmasked blocks
to change for motion to be detected.
PAL DIFFERENCES
The motion detection process is exactly the same for PAL, except that the
sizes are slightly different. In PAL, the original image is 768×576 and the
smaller black-and-white image is 22×18 (compared to 22×15 for NTSC).
Summary of Contents for Dvx 1000
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