barcode generator project source code in vb.net Birds Over the Phone: Understanding Video Compression in Software

Encode QR in Software Birds Over the Phone: Understanding Video Compression

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Birds Over the Phone: Understanding Video Compression
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After compact discs appeared in 1982, digital audio became a commodity It took many years before the same transformation could begin to work its magic on video The step up from digital audio to digital video is a doozy, for in any segment of television there is about 250 times as much information as in the same-length segment of CD audio Despite its larger capacity, however, DVD is not even close to 250 times more spacious than CD-ROM The trick is to reduce the amount of video information without significantly reducing the quality of the picture The solution is digital compression In a sense, you employ compression in daily conversations Picture yourself talking on the phone to a friend You are describing the antics of a particularly striking bird outside your window You might begin by depicting the scene and then mentioning the size, shape, and color of the bird But when you begin to describe the bird s actions, you naturally do not repeat your description of the background scene or the bird You take it for granted that your friend remembers this information, so you only describe the action the part that changes If you had to continually refresh your friend s memory of every detail, you would have very high phone bills The problem with TV is that it has no memory the picture has to be refreshed continually, literally It is as if the TV were saying, There s a patch of grass and a small tree with a 4-inch green and black bird with a yellow beak sitting on a branch Now there s a patch of grass and a small tree with a 4-inch green and black bird with a yellow beak hanging upside down on a branch Now there s a patch of grass and a small tree with a 4-inch green and black bird with a yellow beak hanging upside down on a branch trying to eat some fruit, and so on, only in much more detail, redescribing the entire scene 30 times a second In addition, a TV individually describes each piece of the picture even when they are all the same It would be as if you had to say, The bird has a black breast and a green head and a green back and green wing feathers and green tail feathers and (again, in much more meticulous detail) rather than simply saying, The bird has a black breast, and the rest is green This kind of conversational compression is second nature to us, but for computers to do the same thing requires complex algorithms Coding only the changes in a scene is called conditional replenishment The simplest form of digital video compression takes advantage of spatial redundancy areas of a single picture that are the same Computer pictures are made up of a grid of dots, each one a specified color But many of
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Figure 31 Run-length compression example
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the dots are the same color Therefore, rather than storing, say, a hundred red dots, you store one red dot and a count of 100 This reduces the amount of information from 100 pieces to 3 pieces (a marker indicating a run of similar colored dots, the color, and the count) or even 2 pieces (if all information is stored as pairs of color and count) (Figure 31) This is called run-length compression It is a form of lossless compression, meaning that the original picture can be reconstructed perfectly with no missing detail Run-length compression is great for simple pictures and computer data but does not reduce a large, detailed picture enough for most purposes DVD-Video uses run-length compression for subpictures, which contain captions and simple graphic overlays The legibility of subtitles is critical, so it is important that no detail be lost DVD limits subpictures to four colors at a time, so there are lots of repeating runs of colors, making them perfect candidates for run-length compression Compressed subpicture data makes up less than one-half of 1 percent of a typical DVD-Video program In order to reduce picture information even more, lossy compression is required In this case, information is removed permanently The trick is to remove detail that will not be noticed Many such compression techniques, known as psychovisual encoding systems, take advantage of a number of aspects of the human visual system 1 The eye is more sensitive to changes in brightness than in color 2 The eye is unable to perceive brightness levels above or below certain thresholds 3 The eye cannot distinguish minor changes in brightness or color This perception is not linear In other words, certain ranges of brightness or color are more important visually than others For example, variegated shades of green such as leaves and plants in a forest are more easily discriminated than various shades of dark blue such as in the depths of a swimming pool 4 Gentle gradations of brightness or color (such as a sunset blending gradually into a blue sky) are more important to the eye and more readily perceived than abrupt changes (such as pinstriped suits or confetti)
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