Image Processing Reference
In-Depth Information
If you don't remove the dot crawl, it will increase the bit rate on your compressed-
video output. You will be wasting a lot of that bit rate preserving the crawling pixels. Their
motion will be perfectly rendered when the compressed video is played back.
ATI Radeon video cards offer an option to switch between stationary and moving
dot crawl. Ideally, you would like to click a check box to turn it off completely, but that is
not possible. This is your only option on a composite output signal carrying this artifact.
Figure 36-15 Dot crawl.
1. Reduce the color saturation.
2. Apply grain removal.
3. Modify the parameters of your video capture or source device if they provide sta-
tionary/moving dot crawl selection.
Temporal Noise Removal
If your processing system is designed to cope with film artifacts, it may have a grain-
removal filter. Random noise is similar enough to film grain that the filter does a good job
of removing it.
This does a sort of averaging between frames, but be careful because it may lead
to blurring if it is overused. It is sometimes called temporal noise because it is due to
changes at the same pixel from one frame to another. Hence it is on the time axis rather
than X or Y.
An example from a cartoon movie is shown in Figure 36-16.
The picture shows a zoomed-in rectangle cropped out of two adjacent frames of a
1939 animated movie, which has been converted via telecine and compressed using
MPEG-2. The background changes subtly from frame to frame due to grain and fading of
the emulsion. The compression didn't introduce the background noise, but it did intro-
duce some macroblock artifacts. The differences between one frame and the next were cal-
culated using Photoshop and exaggerated by equalization of the resulting image.
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