Here is an example that uses small numbers to keep the math simple. The shape of the sampling grid determines the shape of the pixels in the digitized image. When a digital image is captured, the image is sampled both vertically and horizontally, resulting in a rectangular array of quantized samples, called pixels or pels. Pixel aspect ratio (PAR) measures the shape of a pixel. The term 'pan' in pan-and-scan refers to the panning effect that is caused by moving the pan-and-scan area. The area that is cropped can move from frame to frame, as the area of interest shifts. The resulting image fills the entire display, without requiring black letterbox areas, but portions of the original image are cropped out of the picture. Pan-and-scan is a technique whereby a widescreen image is cropped to a 4×3 rectangular area, for display on a 4:3 display device. However, the term letterbox is also used in a general sense, to mean scaling a video image to fit any given display area. The reverse case, scaling a 4:3 image to fit a widescreen display, is sometimes called pillarboxing. The resulting rectanglular areas at the top and bottom of the image are typically filled with black, although other colors can be used. The process of scaling a widescreen image to fit a 4:3 display is called letterboxing, shown in the next diagram. Stretching the image to fit the display area is almost always wrong, because it does not preserve the correct picture aspect ratio.
I have only explained resolutions for NTSC. This is a short video I made to describe pixel aspect ratios, explain why they're important in digital content creation, and how to work with them. Scale the image to fit the display area, while maintaining the original picture aspect ratio.Stretch the image along one axis to fit the display area.In that case, there are three ways the image can be made to fit within the display area: In computer video, the video might be shown inside a window that has an arbitrary size. For example, a 4:3 video might be shown on a widescreen (16×9) television. Sometimes the video image does not have the same shape as the display area. Picture aspect ratio is also called display aspect ratio (DAR). Cinema film often uses a 1:85:1 or 1:66:1 aspect ratio. The 16:9 aspect ratio is commonly called widescreen. Most video standards use either 4:3 or 16:9 picture aspect ratio. Picture aspect ratio is notated X:Y, where X:Y is the ratio of picture width to picture height. Picture aspect ratio defines the shape of the displayed video image.
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