Here is a real-world example: NTSC-M analog video contains 480 scan lines in the active image area. Note that image width and image height in this formula refer to the image in memory, not the displayed image. The following formula relates PAR, display aspect ratio (DAR), and image size in pixels:ĭAR = ( image width in pixels / image height in pixels) × PAR If the image PAR and the display PAR do not match, the image must be scaled in one dimension, either vertically or horizontally, in order to display correctly. Computer monitors generally use square pixels. The physical shape of the display device and the physical pixel resolution (across and down) determine the PAR of the display device. Pixel aspect ratio also applies to the display device. Pixels that are not square are called non-square pixels. Specifically, the shape of each pixel will be 3×4. The shape of each resulting pixel will be taller than it is wide. Suppose that the original image is square (that is, the picture aspect ratio is 1:1) and suppose the sampling grid contains 12 elements, arranged in a 4×3 grid. 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.