Image Processing Reference
In-Depth Information
It is likely that early service launches in Europe will go with a 720p format but not
pass up the opportunity of upgrading that to 1080p should the industry go in that direc-
tion with screen sizes and DVD formats in the future.
Aspect Ratio
Aspect ratio is a term used to describe the proportional difference between width and
height. The values are often normalized so that the number on the right is 1. Sometimes
they are normalized so that only integer values are used.
Common TV Aspect Ratio Formats
If you look at the back of DVD or video packaging you will see aspect ratios quoted.
Normal aspect ratio is taken to be 4:3 and widescreen TV is 16:9. Some broadcasters com-
promise and deliver the content as 14:9. This allows for a small portion to be cropped on
4:3 aspect ratio TV sets, and a small black bar is present on either size of the picture on a
16:9 TV set. Figure 5-14 illustrates the various common aspect ratios in use.
Some common TV aspect ratios are summarized in Table 5-4.
Normal Aspect Ratio
Normal aspect ratio material is presented in a raster where the width to height ratio is
4:3. On a wide-screen monitor this is distorted to fill the screen, so a border is placed on
each side to retain the original aspect ratio. Center cutout on a wide-screen TV sacrifices
horizontal resolution. Letterboxing does the same when playing wide-screen content on
a normal TV except that it wastes lines. In both cases, cropping is necessary to remove
the wasted space if you are compressing for the Web. That will help to achieve the
best possible compression ratio. There is an inevitable trade-off between quality and
4:3 (12:9)
Figure 5-14 Aspect ratios.
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