![]() ![]() Decoding is much simpler and can be done by today’s desktop CPUs or with low cost decoder chips. The scheme is asymmetric the MPEG encoder is very complex and places a very heavy computational load for motion estimation. Consequently, compression ratios above 100:1 are common. Since minimal information is sent between every four or five frames, a significant reduction in bits required to describe the image results. MPEG involves fully encoding only key frames through the JPEG algorithm (described above) and estimating the motion changes between these key frames. This group works on standards for the coding of moving pictures and associated audio. MPEG is the “Moving Picture Experts Group”, working under the joint direction of the International Standards Organization (ISO) and the International Electro-Technical Commission (IEC). Note that JPEG does not address the question of audio tracks and audio/video synchronization. However, full-motion JPEG will be employed in professional video processing, since there are no missing frames in the bit stream, and frame-by-frame editing can be precise. Motion JPEG is possible if the compression/decompression algorithm is executed fast enough (on a fast chip or chip set) to keep up with the video data stream, but at typical compression ratios of about 16:1, there is not a considerable saving (compared to the 100:1 ratios of MPEG-1) and not good enough compression to play from a CD-ROM (which is constrained to transfer data at 150 kbps or 300 kbps, depending on JPEG, while designed for still images, is often applied to moving images, or video. Since the computational complexity is virtually identical in either direction, JPEG is considered a symmetrical compression method. This results in additional compression of about 3x.ĭecompression reverses this procedure, beginning with the Huffman tree decoding and inverse DCT, transforming the image back to the pixel domain. The remaining components are then entropy coded by the Huffman tree method which assigns short codes to frequent symbolsĪnd longer codes to infrequent symbols. This quantization step is where most of the actual compression takes place. Near-zero components are truncated to zero and need not be sent on, either. This produces two-dimensional spatial-frequency components, many of which will be zero and discarded. The frequency domain by a discrete cosine transform (DCT), a DSP algorithm similar to the fast Fourier transform (FFT). The image information is then transformed from the pixel (spatial) domain to In the pixel format, there is usually a large amount of low-spatial-frequency information and relatively small amounts of high-frequency information. Because the human eye is less sensitive to high-frequency color information, JPEG calls for the coding of chrominance (color) information at a reduced resolution compared to the luminance (brightness) information. The image frame consists of three 2-D patterns of pixels, one for luminance and two for chrominance. JPEG compression involves several processing stages, starting with an image from a camera or other video source. ![]()
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