Video on Your Computer, Pixel Aspect Ratio

Unlike video, which uses lines of resolution to create images on a television screen, computer monitors create images using thousands of small squares called pixels. When you work with video on a computer, applications such as Final Cut Pro and After Effects display your video as non-square pixels. As you can tell from the name, these pixels are shaped differently than the pixels in other computer images. If you’re creating images in a program such as Adobe Photoshop, and you plan to use them in a video, you need understand pixel aspect ratio to ensure your images don’t distort.

Pixel aspect ratio (Figure 2-4) describes the width of a pixel in relation to its height. Square pixels, generated by most computer applications, are equal in height and width —that’s what makes them square. Non-square pixels display at different proportions: they appear taller than they are wide. The difference becomes important when you start to combine video and still images. If you create a title or a graphic and then import it into your video, the image may distort slightly due to the difference in pixel aspect ratios. For example, if you create a circle in Photoshop and then import the circle into Final Cut Pro or After Effects, it might display as an oval. Likewise, if you export a still image of a person from a frame of video, the image can distort on a computer screen making the person appear shorter and fatter.

Figure 2-4. In the figure at left, a circle ...

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