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Aspect Ratio (non-widescreen content)

Aspect ratio is important - it defines what shape you video will be. if you display things at the wrong aspect ratio your video will be distorted.
Video comes in many different formats - for the purposes of this document we'll stick to looking at PAL video - we will look video that originates from DV cameras and video that is entirely created on computer.
Digital video's aspect ratio is controlled by two basic factors, the pixel dimensions of the frame and the aspect ratio of the pixels.
  • Standard non-widescreen video has an aspect ratio of 4:3
  • PAL Video is 768x576 square pixels
  • PAL DV Video is 720x576 D1 Pixels
  • D1 Pixels are rectangular they have an aspect ratio of 1.066:1
  • Square Pixels have an aspect ratio of 1:1
Square Pixels (1:1) D1 Pixels (1.066:1) Overlay on one another its easier to see
Square pixels are square D1 pixels are rectangular Overlayed on each other its easy to see the different shapes

When we capture video from a DV camera via firewire the video is saved at a resolution of 720x576 D1 pixels. Because computers use square pixel displays the image appears squashed, to display the video at the correct 4:3 aspect ratio it is nesasery to stretch the image to be 768 pixels across.

Original 720x576 DV source file Corrected 768x576 image
Note how the image is squashed - the square is rectangular and the circle is an oval Scaled to the correct aspect ratio

We can see that PAL D1 has an effective square pixel dimension of 768x576 by multiplying its D1 pixel dimensions (720x576) by the D1 pixel aspect ratio (1.066:1)
720 * 1.066 = 767.52 (rounded to nearest pixel = 768)
576 * 1 = 576

Lets assume you have some DV clips captured from dv camera and you want to make some VJing clips - for the best speed and quality in most VJing applications it is better to do any scaling when you make your clips rather than use computer power to scale clips live so we'll take a DV source clip and convert it to 768x576 in a good codec for VJing - this creates an avi that is the correct shape with no squeeze. if you want to make a smaller clip for use on an older computer i'd recommend a 2:1 reduction from the full square pixel size (i.e. 384x288 pixels for footage from PAL DV)


© Tom Bassford 2005-2006