The QCIF frame is the ideal size to
The QCIF frame is the ideal size to place a video window within a web page. There is still room left for the menus and textual content of interactive applications. The larger frame sizes are more useful where video is the primary content and will occupy most or all of the display. Temporal scaling This is the process of dropping frames below the normal video rate of 30 frames per second (fps). For certain subject material the frame rate can be reduced to one-half or even more without serious degradation in picture quality. The deciding factor is the amount of motion in the picture. A talking head can be viewed at rates as low as one-fifth of television rates of course a fast-moving subject like a sportscast would not be satisfactory at these low speeds. This sounds like a big problem for streaming video, but much of the demand for low bit rate content is for applications like distance learning. Here the typical program uses a talking head, just the kind of material that can stand the highest temporal compression. RealVideo will encode at a variable frame rate. The user sets the maximum rate, and then the codec automatically adjusts the frame rate based on the clip size, the target delivery bit rate, and emphasis set for smoothness or visual clarity. One scene may be 7 fps, for example, while another is 10. A maximum set to 15 fps means the frame rate could vary anywhere between 15 and 0.25 fps. Color resolution The camera and display are both RGB devices. The usual coding is to take an 8-bit sample each of the red, green, and blue values at each sample point or pixel. This gives a total of 24 bits, but it is often padded for ease of processing to give a 32-bit word. A 24-bit pixel can display 16 million colors. To reduce the bit rate of the streaming video, the bit depth of a sample can be reduced, albeit at a reduction in the number of colors displayed. Video encoding 165 Table 8.5 AVI Formats AVI format Bit depth Padding Bytes per pixel R G B RGB32 8 8 8 8 4 RGB24 8 8 8 3 RGB16 5 6 5 2 RGB15 5 5 5 1 2 RGB8 Indexed palette 1
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