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12 Using Flash Video Encoder
About encoding video with non-square pixels
Most static computer graphics use square pixels, which have a width-to-height ratio of 1:1.
When working with digital video, pixels often have a different width-to height ratio and are
referred to as rectangular pixels. The reason for this behavior is to allow analog video (e.g.
broadcast television) and digital video (e.g. DVD video) to coexist. When encoding video
formats with non-square pixels (also known as
anamorphic video ), you need to resample the
video image to the correct Display Aspect Ratio (DAR).
For example, standard NTSC digital video (DV), has a frame size of 720 x 480 pixels, and it’s
usually displayed at an aspect ratio of 4:3. This means that each pixel is rectangular, with a
pixel aspect ratio (PAR) of 10:11 (a tall, narrow pixel). Both MPEG 1 and 2 video are also
produced in various sizes (720 x 480 or 480 x 480 are common) even though they are
typically displayed with either a 4:3 or 16:9 (wide screen) aspect ratio.
To calculate the image frame size to use when encoding video using non-square pixels, you
must first decide which dimension—width or height—is the preferred master dimension, and
then compute the other dimension as follows:
If height is the master dimension, calculate the width using the following formula:
For example, if your video uses an aspect ratio of 4:3 the equation would appear as:
If width is the master dimension, calculate the height using the following formula:
For example, if your video uses an aspect ratio of 4:3 the equation would appear as:
As an example, if you are encoding a video whose frame size is 720 x 480 pixels, and you want
to encode it with a 4:3 aspect ratio, you must determine the width at which to encode the
video frame in pixels:
The result is a video image height of 640 pixels.
Thus, you need to encode the 720 x 480 image to 640 x 480, a standard 4:3 aspect ratio.
width height
aspect ratio width
aspect ratio height
-------------------------------------------- ×
=
width height
3
4
---
×
=
height width
aspect ratio height
aspect ratio width
--------------------------------------------
×
=
height width
4
3
--- ×
=
640 480
4
3
--- ×
=
000_FLVEncoder.book Page 12 Friday, August 12, 2005 3:50 PM