Video Video consists of image frames captured from real motion and shown in succession Animation is similar except that the frames are synthesized The.
Download ReportTranscript Video Video consists of image frames captured from real motion and shown in succession Animation is similar except that the frames are synthesized The.
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 1 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) 2 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 3 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 compressor / decompressor Use lossy compression Some video codecs are “software only” while others require special hardware 4 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 5 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 6 Compression Techniques Codecs compress video using two techniques: • Temporal Compression • Spatial Compression Temporal compression eliminates information that is not Spacial compression eliminates information that is not necessary for visual continuity over time necessary for continuity over area Other compression can be accomplished by reducing colors, frame rate, and audio quality 7 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 8 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 9 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 10 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 11 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 12 Sorenson Video Codec Sorenson is supported by the QuickTime 3 and 4 architectures It uses: • variable encoding • vector quantization • motion compensation 13 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 14 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 15 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 16 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 17 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 18 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 19 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 20 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 21 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 22