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DIGITAL CCTV A Security Professional’s Guide phần 3

Khi Ethernet và TCP / IP tiêu chuẩn được phát triển, ứng dụng dựa trên mạng trở nên khả thi. Tiếp theo, tiến bộ trong truyền thông không dây cung cấp di động chưa từng có để ứng dụng giám sát. Tất cả những tiến bộ trong công nghệ là có thể bởi vì các kỹ thuật số video. | In the Beginning 51 The cones are made up of three types one is sensitive to red-orange light the second to green light and the third to blue-violet light. When a single cone is stimulated the brain perceives the corresponding color. If our green cones are stimulated we see green if our red-orange cones are stimulated we see red. If both our green and red-orange cones are simultaneously stimulated we see yellow. The human eye cannot tell the difference between spectral yellow and some combination of red and green. Because of this physiological response the eye can be fooled into seeing the full range of visible colors through a proportionate adjustment of just three colors red green and blue. Colors are represented by bits and the more bits that are available the more precise the color definition is portrayed. Digital video uses a non-linear variation of RGB called YCbCr. Cb represents luminance and Cr represents chrominance. Subtractive Color Subtractive color is the basis for printing. It is called subtractive because white its base color reflects all spectral wavelengths and any color added to white absorbs or subtracts different wavelengths. The longer wavelengths of the visible spectrum which are normally perceived as red are absorbed by cyan. Magenta Figure 3-7 Primary Colors in Subtractive Color System 52 Digital CCTV absorbs the middle wavelengths green and yellow absorbs the shorter wavelengths of the visible spectrum blue-violet . Mixing cyan magenta and yellow together subtracts all wavelengths of visible light and as a result we see black. Printing inks comprised of the colors cyan magenta and yellow combine to absorb some of the colors from white light and reflect others. Figure 3-7 illustrates how cyan magenta and yellow when printed as three overlapping circles work to produce black as well as red green and blue. In practice the black produced by combining cyan magenta and yellow is often not black enough to provide a large contrast range so printers