Image Processing Reference
In-Depth Information
A raster whose size is 720
240 should be scaled down to 320
240 for a 4:3 aspect
ratio or 432
240 for 16:9.
If you must scale a raster down or interpolate between lines or pixels, use bi-cubic algo-
rithms or sine curves. The nearest neighbor introduces some blocky artifacts that don't look
good on a still image and on a moving image they are gross and very distracting. The impli-
cations of these interpolation methods with regard to quality are discussed in a little while.
Honor the pixel aspect-ratio rules—they are important to remember:
Use non-square pixels on TV.
Use square pixels on computers.
Some input sources are so close to the desired output size that applying a sensible
crop is better than trying to scale the image.
Input taken from 486 line sources should be simply cropped to 480 lines. The obvi-
ous default crop to apply would seem to be removing 3 lines from the top and bottom of
the image. This will move the video up or down an odd number of lines, and this could
switch pixels from the odd to the even field, which might lead to field-dominance issues
(that is, which field is broadcast first) later on in the broadcast chain.
The best approach is to crop using an even number of lines at the top and bottom, as
in the following examples:
Remove 0 lines at the top and 6 at the bottom.
Or crop 2 and 4.
Or 4 and 2.
Or crop 6 and 0.
The even number ensures that field-dominance problems will not show up later.
Because you may be using various different tools to perform the stages of prepro-
cessing, it is worth knowing that on the Mac OS platform, the Compressor application that
is shipped as part of the Final Cut and DVD Studio tool kit is superior to QuickTime when
scaling because the processing algorithms are better. But if you have only the basic
QuickTime tools available, that will have to do.
A simple rule of thumb is that for 320
240 or higher, you should minimize cropping
and use adaptive de-interlace processing or inverse telecine to keep as many lines intact
from your original source as possible.
Scaling Up
Scaling the size of the video to a larger format is likely to introduce artifacts that appear as
jaggies in any non-vertical edges. Horizontal edges may also show a tendency to bounce a
little. Using high-quality interpolation algorithms in the scaling system will alleviate this.
There are several occasions when this scaling technique would be used when
rebroadcasting archived material, for example, when transforming NTSC 525-line stan-
dard-density (SD) video into European 625, which is shown in Figure 34-5.
Search WWH ::

Custom Search