Image Processing Reference
Figure 36-17 Example of the Gibbs effect on text.
1. Give up some compression efficiency.
2. Replace the footage with stationary text pages.
3. Try different colors and aliased edges on the text.
4. Replace the text with a clean version against a more neutral, stationary background.
Encoding video with burned-in captions is not ideal. There are nasty artifacts (such as the
Gibbs effect) that cause horrible fringing patterns around text when the DCT transform is
applied. In many cases, the size of the video is such that any text you burn in will be
If there are already burned-in captions that didn't survive the compression process
before, then use the cropping mechanisms to remove them.
If you cannot avoid burning captions into the video, try to make them sensibly large
so they can be read on the screen at the resolution at which the image will be playing back.
If they have to be added as part of your processing, make sure it is the last thing you do
before compression begins. Then any scaling, noise filtering, or other operations will not
have spoiled the clean edges of the text.
The ideal scenario is to encode an extra text track and let the player subtitle the video
in a separate area of the screen. This can be done with a Real Text track in an .rt file, a
QuickTime text track in a .mov file, or a Windows Media .smi or .sami file.