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    Video slides Video slides Presentation Transcript

    • Video
      • Video consists of image frames captured from real motion and shown in succession
      • Animation is similar except that the frames are synthesized
      • The frame rate is the number of frames shown per second (fps)
      • One minute of uncompressed video requires significant storage: 1.6 GB to 1.85 GB
      • Compression can be accomplished on the recording device or using a video capture card
      • The IEEE 1394 (firewire) interface standard is often used for video capture
    • Analog Video
      • An image on a TV is drawn row by row from left to right, starting at the top left of the screen
      • After the image has been drawn, it moves back up and begins again
      • The refresh rate of a TV is the number of times it draws a new image per second
      • The refresh rate is measured in Hertz (basically, per second)
    • Analog Video
      • There are two main TV formats:
        • NTSC - National Television System Committee
        • PAL – Phase Alternating Line
      • NTSC is used in America and Japan, while PAL is used in Europe and Australia
      • An NTSC TV has a refresh rate of 60 Hz, while a PAL TV has a refresh rate of 50 Hz
      • Standard movie frame rate: 24 fps
    • Video Codecs
      • An algorithm that shrinks the size of a movie to allow the movie to be played on a computer or from a network
      • codec is short for co mpressor / dec ompressor
      • Use lossy compression
      • Some video codecs are “software only” while others require special hardware
    • Video Codecs
      • Most codecs are block-oriented
      • Divide the frame into blocks
      • Sometimes you can see the block of pixels when a video plays
      • Codecs differ in how the blocks are encoded
    • Video Architectures
      • Handles and synchronizes video files
      • An architecture creates file formats with its codecs
      • Examples of Architectures:
        • QuickTime
        • RealSystem G3
        • Microsoft Windows Media
        • MPEG
      • The architecture you choose determines which codecs you can use
      • Not all codecs are available for every architecture
    • Compression Techniques
      • Codecs compress video using two techniques:
        • Temporal Compression
        • Spatial Compression
      • Temporal compression eliminates information that is not necessary for visual continuity over time
      • Spacial compression eliminates information that is not necessary for continuity over area
      • Other compression can be accomplished by reducing colors, frame rate, and audio quality
    • Temporal Compression
      • Temporal compression looks for differences between consecutive frames
      • A keyframe is a frame that is used as the basis for differences
      • Keyframes contain the entire image
      • A delta frame (or difference frame) contains only the areas that are different from one frame to the next
      • keyframe, delta, delta, …, keyframe, delta, delta, ...
      • Several keyframes are chosen throughout the movie whenever significant changes occur
    • Spacial Compression
      • Generally focuses on individual frames
      • If an area is all one color, the area is defined using geometry and the color stored once
      • Combines bitmap and raster graphics concepts
      • Temporal and spacial compression work in concert
    • Asymmetric vs. Symmetric
      • A codec can be asymmetric :
        • Encoding process is slower than the decoding process
        • Better suited for movies that are prerecorded and played back
      • Or symmetric :
        • Encoding process takes about the same time as the decoding process
        • Better suited for live broadcasts or video teleconferencing
    • Video Codecs
      • Some popular video codecs:
        • Sorenson Video
        • Cinepak
        • MPEG:
          • MPEG-1
          • MPEG-2
          • MPEG-4
        • Real Video:
          • RealVideo G2
          • RealVideo 8
        • H.261 / H.263
    • Sorenson Video Codec
      • Developed by Sorenson Vision, Inc (www.s-vision.com)
        • High compression
        • High-quality/low data rates
        • High-end processors NOT needed
      • Recommended for:
        • Web video (fast viewing rates)
        • CD-ROM video (fit more movies on a CD-ROM)
      • It is asymmetric
      • A hardware specific version is also available
    • Sorenson Video Codec
      • Sorenson is supported by the QuickTime 3 and 4 architectures
      • It uses:
        • variable encoding
        • vector quantization
        • motion compensation
    • Variable Encoding
      • Adjusts the bit depth according to how much action the scene contains
      • Two-pass encoding technique
      • Movie is analyzed to determine “easy” and “difficult” sections
      • Movie is compressed intelligently:
        • Attempts to give each frame the optimum number of bytes
    • Vector Quantization
      • Image divided into segments (vectors)
      • The mean value of each segment is subtracted from the individual pixels in the segment (residual vector pattern)
      • Each residual segment pattern is compared to a set of previously determined patterns (stored in a codebook )
      • The codebook entry pattern closest to the residual vector pattern is determined
      • The binary address of the codebook entry is sent to receiver to be decoded
    • Vector Quantization
      • The decoder only needs the address of the residual pattern and the mean value of the individual segments
      • If codebook pattern entries are determined ahead of time according to test imagery, the quality of the movie will be good
      • The number of codebook entries can be increased substantially without too much effect on the bandwidth needed
    • Motion Compensation
      • From frame-to-frame, there is usually some translation of blocks of pixels due to motion
      • Successive frames in a motion sequence are searched to determine the motion compensation vectors
      • Motion compensation vectors indicate the direction and distance of translation of block of pixels that has moved
      • The whole frame does not have to be translated - the vectors contain the information on how to reconstruct the new frame from the previous frame
    • Sorenson Tools
      • Allows for a media key so that the video can be locked with a password
      • Tries to determine when new scenes begin and marks them as key frames (smart screen change detection)
      • Allows the user to insert a custom watermark on all frames
      • Allows the user to specify intervals at which frames should be dropped to increase playback performance
    • Cinepak
      • Codec maintained by Compression Techniques, Inc.
      • Medium-quality CD-ROM video
      • Does not require a lot of processing power
      • Is asymmetric
      • Uses vector quantization
      • Cinepak video codec is supported by the following architectures:
        • Video for Windows
        • QuickTime
    • MPEG
      • Developed by Motion Picture Expert’s group
      • MPEG is both a file format and a codec (a file format that employs compression algorithms)
      • Usually requires special hardware for encoding
      • Usually requires substantial CPU power/special hardware for playback
      • Mainly used for high-end desktop video
      • MPEG is an open standard
    • MPEG
      • MPEG is a family of codecs containing:
        • MPEG-1
        • MPEG-2
        • MPEG-4 (approved in 1999)
      • MPEG supported by the following architectures:
        • QuickTime 3
        • DirectShow 6
        • Video CD
        • DVD Video
        • Windows Media
    • RealVideo
      • RealVideo a family of codecs containing:
        • RealVideo 8
        • RealVideo G2
      • Usually used for streaming video
      • Both RealVideo codecs are scalable
        • compensates if the connection bit rate is slower than the bit rate used to encode the video
        • Allows for smoother playback on a wide range of machines
      • Asymmetric
      • Requires a high-end computer for optimal encoding/decoding
      • RealPlayer 8 supports variable encoding