Image Processing Reference
In-Depth Information
When cropping, keep the following points in mind:
Cropping asymmetrically is permitted to remove noise.
Low-resolution output is cropped to the safe area.
Crop off any letter boxing and center-cutout blanked areas.
Crop to the final aspect ratio if it is not going back to TV.
If you are exceeding your desired bit rate, it is preferable to shrink the frame size in
order to keep the frame rate up.
Cropping for Aspect-Ratio Correction
For output to the web, you may be processing some wide-screen footage with black bars
at the top and bottom of the frame. There is nothing to be gained by encoding these areas
of the frame, and they will only consume valuable bit rate that is better used on genuine
picture information. Figure 34-14 shows how to crop these.
You should leave in the letterboxing effect when you are encoding wide-screen
footage with an aspect ratio wider than 16:9 for eventual deployment on a DVD since a
complete raster must be encoded. The overall cropping outline should be set to 16:9 for
a DVD but may be anamorphically scaled to 4:3 for encoding.
Ragged Edges
Another reason to crop is that the edges may be ragged due to timing issues when the
footage was ingested at the digitizer. This might show up as noise at the extreme left and
right edges and some tearing at the bottom of the picture. Often there is also a half line at
16:9 on 4:3
Crop this
4:3 on 16:9
Crop this
Figure 34-14 Cropping to correct aspect ratios.
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