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Aspect ratio essentials

The pixel aspect ratio is the width (x) of the pixel compared to its height (y). A square pixel has a ratio of 1:1, but a non-square (rectangular) pixel does not have the same height and width. This concept is similar to the frame aspect ratio, which is the total width of an image compared to its height. In general, television pixels are rectangular and computer pixels are square. Therefore, images that look correct on a computer monitor display in a distorted manner on a television screen. This is especially evident with spherical images. In Microsoft Expression Encoder, you can set options to display your non-square pixel media correctly, and to output non-square pixel content.

NoteNote

The frame aspect ratio of an image does not necessarily dictate the pixel aspect ratio. A widescreen image with a frame aspect ratio of 16:9 can be composed of square or non-square pixels. For example, digital video with the frame aspect ratio of 720x486 is typically considered widescreen video and is created using rectangular pixels. However, an image with the same frame aspect ratio may actually have been created in an image editing program, and, therefore, was created in a square pixel environment.

Pixel aspect ratios

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If you encode a video source with non-square pixels as if the pixels are square, the output will be distorted, as shown in the following diagram.

A circle created using rectangular pixels (left), and the same circle displayed on a computer monitor (right)

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If you know the frame aspect ratio (Ix:Iy) and exact height and width of your image source, you can use the following formula to determine the x and y values of the pixel aspect ratio:

PixelAspectRatioX / PixelAspectRatioY = (Ix * height) / (Iy * width)

For example, if your image is 720 pixels by 360 pixels, and the frame aspect ratio is widescreen (16:9), you would use the following formulas:

PixelAspectRatioX=8 and PixelAspectRatioY=9
PixelAspectRatioX / PixelAspectRatioY = (16*360) / (9*720) = 8/9
NoteNote

Microsoft Silverlight does not currently support non-square pixel display.

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