H.261 / H.263 Video CODEC Library For Philips Trimedia

Transcription

1 Revision 1.0 H.261 / H.263 Video CODEC Library For Philips Trimedia June 1999 Overview This data sheet describes 4i2i Communications Ltd s H.261 and H.263 video encoder/decoder (CODEC). The CODEC is available as a set of library functions for the Philips Trimedia DSP platform. Features Software library written in C, optimized for Trimedia platform Real-time video compression and decompression in software Ideal for a wide range of digital video applications including videoconferencing desktop video surveillance remote inspection Reliable and robust encoding and decoding Takes full advantage of Trimedia Custom Opcodes and architecture Selectable H.261 or H.263 operation Compatible with ITU-T H.261 and H.263 International Standards Flexible API for easy integration into your video application Suitable for low bit rates (under 10kbps) Excellent video quality at higher bit rates Supplied with extensive documentation and support Contents 1. CODEC STRUCTURE 2. API DESCRIPTION 2.1 API FUNCTIONS 2.2 DATA INPUT AND OUTPUT 2.3 CODEC OPTIONS 2.4 INFORMATION FROM CALLING APPLICATION TO CODEC 2.5 SIGNALLING INFORMATION FROM CODEC 3. SOFTWARE DESCRIPTION 3.1 CODEC MODULES 3.2 CODEC FILES 4. COMPATIBILITY AND RELIABILITY 4.1 H.261 COMPATIBILITY 4.2 H.263 COMPATIBILITY 4.3 RELIABILITY 5. REFERENCES 6. FURTHER INFORMATION, Doig Scott Building, Craibstone Estae, Aberdeen, AB21 9YA, Scotland Page 1 of 8

2 1. CODEC STRUCTURE The CODEC consists of a series of interconnected functional modules. Figure 1 shows how the modules are interconnected for Encoder operation. CODEC configured for Encode API CodecGO YUV input FrameStore frame data manipulation ENCODE IOcodec bit level stream manipulation Video Stream output VideoFrame current VideoFrame previous Rate maintains buffered Rate Control Picture maintains frame sequencing and picture header information Gob maintains Gob header information Macroblock controls Encode procedures for current Macroblock unit VLCtables look-up tables for variable length coding MPredict motion prediction routines (+ filter H.261) Dct Dct/Quantize/Zigzag forward and inverse routines Figure 1 Each modular unit contains the code required to perform a specific CODEC task. Modules with utility functions have one C file, while modules with distinct Encode and Decode tasks have two C files, one containing Encode functions and the other containing Decode functions. This facilitates separate Encoder and Decoder software and keeps code size to the minimum. The library is designed to be linked with, and called by, a calling application., Doig Scott Building, Craibstone Estate, Aberdeen, AB21 9YA, Scotland Page 2 of 8

3 2. API DESCRIPTION Configuration and initialisation parameters are held in the structures CodecConfigure and CodecInit. The calling application should declare these structures and include the appropriate source and header files for Encoder or Decoder. The API functions are called in the following order: 1. CodecConfigEncode - sets the configuration structure members to the default Encoder settings OR CodecConfigDecode - sets the configuration structure members to the default Decoder settings 2. CodecConstruct - Initialises each CODEC module 3. CodecGo Processes video data for one frame and generates encoded stream (Encoder) or decoded video frame (Decoder). CALLING APPLICATION MUST INCLUDE FOLLOWING CODE #include config.h struct CodecConfig configure; struct CodecInit base; API function for default configuration API CodecConstruct START Calling application maintains configuration options EXIT FROM LOOP TO STOP The first call to the API function CodecGo starts video data processing. Each subsequent call continues to process the video data on a frame by frame basis. An exit from the CodecGo calling loop stops current operation. API CodecGo CODEC process frame unit CODEC feedback information Figure 2 Codec API diagram 2.1 API Functions The API consists of the following functions: void CodecConfigEncode(struct CodecConfigure* configure) This function is used when the CODEC is to function as an Encoder. It sets the CodecConfigure members to their default ENCODE settings. It must be called before starting the Encoder and before CODEC initialisation. void CodecConfigDecode(struct CodecConfigure* configure) This function is used when the CODEC is to function as a Decoder. It sets the CodecConfigure members to their default DECODE settings. It must be called before starting the Decoder and before CODEC initialisation. void CodecConstruct(struct CodecConfigure* configure, struct CodecInit* base) This function must be called before starting the CODEC and after the configuration API function. This function performs CODEC initialisation. void CodecGo(struct CodecConfigure* configure, struct CodecInit* base) This function must be called each time a frame unit is ready to process (Encode or Decode). Any changes to configuration required between frame units are the responsibility of the calling application. An internal, Doig Scott Building, Craibstone Estate, Aberdeen, AB21 9YA, Scotland Page 3 of 8

4 function detects changes in operation and re-configures the CODEC before starting the next Encode/Decode task. This function returns the appropriate signalling information. 2.2 Data Input and Output Video buffers Video buffers Y Encoder Stream Buffer Decoder Y U V U V Figure 3 Encoder: When operating in Encoder mode, the CODEC requires to be passed a set of pointers to a Y buffer, a U buffer and a V buffer 1 and to a stream buffer (see Section 2.4). When CodecGo is called, the CODEC reads the data in the Y, U and V buffers (corresponding to one input video frame), encodes this data and writes the encoded data to the stream buffer. Upon returning from CodecGo, the stream buffer contains H.261 or H.263 coded data for one video frame. Decoder: When operating in Decoder mode, the CODEC requires to be passed a set of pointers to a Y buffer, a U buffer and a V buffer and to a stream buffer (see Section 2.4). When CodecGo is called, the CODEC reads data from the stream buffer corresponding to one coded frame, decodes this data and writes the decoded frame to the Y, U and V buffers. After returning from CodecGo, the Y, U and V buffers contain a single decoded video frame. Note: the decoder can handle a stream buffer containing a whole or partial coded frame. If the stream buffer contains a partial frame, the decoder reads and decodes the contents of the stream buffer and then (optionally) requests a new buffer from the calling application. 1 H.261 and H.263 handle video frames in Y:U:V format, i.e. each video frame consists of three components : Y (luminance), U (colour difference) and V (colour difference)., Doig Scott Building, Craibstone Estate, Aberdeen, AB21 9YA, Scotland Page 4 of 8

5 2.3 CODEC Options Changes in these options are read by the CODEC each time the API function CodecGo is called. Variable Encoder Decoder Description ModeAndResolution Set mode (H.261 or H.263) and frame resolution FrameRate Target frame rate in frames per second (between 1 and 30) BitRate Target bit rate in bits per second BufferSizeRequest Set buffer size externally or calculate within Encoder H261BCH H.261 BCH framing enabled or disabled H261MotionComp H.261 motion compensation enabled or disabled H261LoopFilter H.261 loop filter option enabled or disabled FreezePictureRequest Freeze current frame, search for Freeze Picture Release FastUpdateRequest Encode an INTRA frame and set Freeze Picture Release H263UnrestrictedMV 1 Unrestricted motion vectors optional mode H263SyntaxBasedAC 1 Syntax-based Arithmetic Coding optional mode H263AdvancedPred 1 Advanced Prediction optional mode H263PBFrames 1 PB Frames optional mode H263GOBSync Insert GOB synchronization headers H263HalfPixelME Enable half-pixel motion estimation 1 Please enquire for the current status of the optional modes H263UnrestrictedMV, H263SyntaxBasedAC, H263AdvancedPred and H263PBFrames. These are not provided within our standard package. They can be purchased at additional cost. 2.4 Information from Calling Application to CODEC The following information should be supplied to the CODEC. Variable Encoder Decoder Description LoopControl Set execution as Encoder or Decoder Ydata, Udata, Vdata Pointers to Y, U, V frame buffers Ystride, UVStride Indicates extra bytes at the end of each row of pixels in a frame, for example due to 64-byte alignment of data VideoStream Pointer to coded video stream buffer location StreamBufferSize Size of video stream buffer in bytes NewSequence Indicate start of new sequence (Encoder uses INTRA mode), Doig Scott Building, Craibstone Estate, Aberdeen, AB21 9YA, Scotland Page 5 of 8

6 2.5 Signalling Information from CODEC The following information is returned by the CODEC. Variable Encoder Decoder Description FramesSkipped Count of skipped (non-transmitted) frames (if any) InputWaiting New Y, U, V frame buffers required OutputWaiting New stream buffer required DecodedBytes Count of bytes extracted from stream buffer for current frame EncodedBytes Count of bytes written to stream buffer for current frame EncodedFrameVoid Stream buffer was not large enough for current coded frame, data is not complete 3. SOFTWARE DESCRIPTION 3.1 CODEC Modules Each module consists or a group of functions required for its own internal processing., Doig Scott Building, Craibstone Estate, Aberdeen, AB21 9YA, Scotland Page 6 of 8

7 MODULE SOURCE NAME DESCRIPTION OPERATIONAL UNIT FrameStore fstore get next frame and control frame access frame VideoFrame [2] vframe current and previous frames frame RateControl rcontrol buffered Rate Control frame Picture picture frame level synchronisation frame Gob gob gob level synchronisation group of blocks Macroblock mblock controls macroblock encode/decode processing macroblock Dct dct groups DCT, Quantize and ZigZag utility routines block Mpredict mpredict motion estimation and compensation macroblock IO io read/write video bit stream operations byte VLCtables tables search routines to find VLC from look-up tables VLC 3.2 CODEC Files FILE config.c config.h codec.h utility.h tables.h encoder.h decoder.h encoder.c decoder.c genfstore.c genvframe.c genrcontrol.c encpicture.c decpicture.c encgob.c decgob.c encmblock.c decmblock.c gendct.c encmpredict.c decmpredict.c genio.c enctables.c dectables.c DESCRIPTION CODEC API functions Contains declarations for CodecConfigure and CodecInit structures CODEC module declarations Prototypes for utility functions H.261 and H.263 VLC look-up tables Prototypes for Encoder functions Prototypes for Decoder functions H.261/H.263 encode frame routines H.261/H.263 decode frame routines Source code for FrameStore module - Encoder and Decoder Source code for VideoFrame module - Encoder and Decoder Source code for RateControl module - Encoder and Decoder Source code for Picture module - Encoder Source code for Picture module - Decoder Source code for Gob module - Encoder Source code for Gob module - Decoder Source code for Macroblock module - Encoder Source code for Macroblock module - Decoder Source code for Dct module - Encoder and Decoder Source code for Mpredict module - Encoder Source code for Mpredict module - Decoder IO bit level functions - Encoder and Decoder VLCtables module - search routines for Encoder VLCtables module - search routines for Decoder 4. COMPATIBILITY AND RELIABILITY 4.1 H.261 Compatibility The CODEC (operating in H.261 mode) has been successfully tested for compatibility with: test sequences from Inria, Doig Scott Building, Craibstone Estate, Aberdeen, AB21 9YA, Scotland Page 7 of 8

8 Osprey 1000 H.261 compression board Array Microsystems video compression board 4.2 H.263 Compatibility The CODEC (operating in H.263 mode) interworks successfully with the H.263 Test Model (TMN-5). 4.3 Reliability The CODEC is designed for continuous, robust operation. The Encoder and Decoder have run continuously for several days without any failures. The Decoder uses 4i2i s proprietary error-recovery techniques to continue decoding, regardless of transmission errors and impairments. This means that the CODEC library will not fail or crash under normal conditions. 5. REFERENCES [1] ITU-T Recommendation H.261, Video CODEC For Audiovisual Services At p x 64 kbits, March 1993 [2] ITU-T Recommendation H.263, Video Coding For Low Bitrate Communication, May FURTHER INFORMATION To find out more about the technical features of this library or to request information on pricing and licensing, please contact. Address Doig Scott Building Craibstone Estate Bucksburn Aberdeen AB21 9YA UK Telephone Fax info@4i2i.com Web Doig Scott Building, Craibstone Estate, Aberdeen, AB21 9YA, Scotland Page 8 of 8