H261(3)
NAME
H261 − H.261 decompressor for compressed image sequences
DESCRIPTION
CCITT Recommendation H.261, Video Codec for Audiovisual Services at p x 64 kbit/s, is an international standard for videophone and videoconferencing. It describes the moving picture component of audiovisual services at the rates of p x 64 kbit/s, where p is in the range 1 to 30.
The XIL H261 codec implements the H.261 standard without the transmission coder/decoder; i.e., the XIL bitstream does not contain any Error Correction Framing bits.
The current release of the XIL library does not contain an implementation of an H.261 compressor. Calls to xil_compress(3) will produce an error unless a third party H.261 compressor has been installed.
Image Types
The H261 decompressor produces 3-band, XIL_BYTE images in the XIL library’s "ycc601" color space (The XIL image color space will not be examined or set by the H261 codec, but the codec assumes its input image has the proper color space). The width and height of the images must be either Common Intermediate Format (CIF), which is 352 wide by 288 high, or Quarter CIF (QCIF), which is 176 wide by 144 high.
Creating a CIS
To create an H.261 compressed image sequence (CIS), specify "H261" for the compressorname argument in xil_cis_create(3).
Getting and Setting H261 Attributes
Use xil_cis_get_attribute(3) and xil_cis_set_attribute(3) to get and set H261 CIS attributes. These attributes are as described in the following sections. Refer to the example section for additional information.
H261 Compression Attributes
The following paragraphs describe the H.261 CIS attributes available with the XIL library. All structures and enumerations are defined via xil.h. Note that if you are setting an attribute and that attribute is a structure, you must pass the address of that structure. If you are getting an attribute, you must always pass the address of the attribute. If you are getting a structure attribute, you must pass a pointer to a pointer to the structure and XIL will set the pointer to the structure. You must free the memory for this structure (using free(3C)) when it is no longer needed.
COMPRESSOR_BITS_PER_IMAGE
Description Encode images with this number of bits per image. This is normally bits_per_second/frames_per_second.
Access get and set
Type Integer
Values value must be greater than or equal to 0
Default 5069 (0.2 bits/pixel at QCIF resolution)
COMPRESSOR_IMAGE_SKIP
Description Number of images that the application is skipping between encoded frames. Controls the Temporal Reference counter in the bitstream. (Temporal Reference is incremented by 1 + COMPRESSOR_IMAGE_SKIP)
Access get and set
Type Integer
Values 0-31
Default 0
COMPRESSOR_MV_SEARCH_RANGE
Description Set motion vector search range. Value 15 is the maximum H.261 search range. Value 0 means that the search range is limited to the spatially corresponding block in the previous picture. This attribute is only a suggestion and may be ignored by the compressor. It may be used to speed up compression at the expense of compression quality.
Access get and set
Type typedef struct {
int x; /∗ horizontal search limit ∗/
int y; /∗ vertical search limit ∗/
} XilH261MVSearchRange;
Values x: Can have a value in the range of 0-15 y: Can have a value in the range of 0-15
Default 15 for both x and y
COMPRESSOR_LOOP_FILTER
Description Allow encoder to use loop filtering. This attribute is only a suggestion and may be ignored by the compressor. It may be used to minimize the bitstream size (at the expense of image quality) by reducing inter-frame differences.
Access get and set
Type Xil_boolean
Values TRUE: Loop filtering turned on FALSE: Loop filtering turned off
Default TRUE
COMPRESSOR_ENCODE_INTRA
Description Cause encoder to encode pictures in INTRA mode with coding parameters to avoid buffer overflow. (This attribute can be used by the application in response to a Fast Update signal sent via H.221).
Access get and set
Type Xil_boolean
Values TRUE: Intra-only coding turned on FALSE: Intra-only coding turned off
Default FALSE
COMPRESSOR_FREEZE_RELEASE
Description Set the Freeze Picture Release bit in each picture in the bitstream, starting with the next compressed picture.
Access get and set
Type Xil_boolean
Values TRUE: Set the Freeze Picture Release bit in the bitstream. FALSE: Do not set the Freeze Picture Release bit in the bitstream.
Default FALSE
COMPRESSOR_SPLIT_SCREEN
Description Set the Split Screen Indicator bit in each picture in the bitstream, starting with the next compressed picture.
Access get and set
Type Xil_boolean
Values TRUE: Set the Split Screen Indicator bit in the bitstream. FALSE: Do not set the Split Screen Indicator bit in the bitstream.
Default FALSE
COMPRESSOR_DOC_CAMERA
Description Set the Document Camera Indicator bit in each picture in the bitstream, starting with the next compressed picture.
Access get and set
Type Xil_boolean
Values TRUE: Set the Document Camera Indicator bit in the bitstream. FALSE: Do not set the Document Camera Indicator bit in the bitstream.
Default FALSE
H261 Decompression Attributes
IGNORE_HISTORY
Description If TRUE, perform forward seeks without updating the decoding history and allow backward seeking (decompression after these seeks may yield incomplete results). If FALSE, maintain proper decoding history during forward seeks and disallow backward seeking. xil_cis_get_random_access(3) will return TRUE if IGNORE_HISTORY is TRUE, and will return FALSE if IGNORE_HISTORY is FALSE.
Access get and set
Type Xil_boolean
Values TRUE: Allow backward seeks and perform fast forward seeks. FALSE: Perform correct seeking.
Default FALSE
DECOMPRESSOR_FREEZE_RELEASE
Description Return value of the Freeze Picture Release bit from the picture header of the most recently decompressed picture. Value is available immediately after executing an xil_decompress(3) call and may be "gotten" and tested without compromising the execution of a decompression molecule.
Access get
Type Xil_boolean
Values TRUE: Freeze Picture Release bit is set. FALSE: Freeze Picture Release bit is not set.
Default Value is undefined if no pictures have been decompressed.
DECOMPRESSOR_SPLIT_SCREEN
Description Return value of the Split Screen Indicator bit from the picture header of the most recently decompressed picture. Value is available immediately after executing an xil_decompress(3) call and may be "gotten" and tested without compromising the execution of a decompression molecule.
Access get
Type Xil_boolean
Values TRUE: Split Screen Indicator bit is set. FALSE: Split Screen Indicator bit is not set.
Default Value is undefined if no pictures have been decompressed.
DECOMPRESSOR_DOC_CAMERA
Description Return value of the Document Camera Indicator bit from the picture header of the most recently decompressed picture. Value is available immediately after executing an xil_decompress(3) call and may be "gotten" and tested without compromising the execution of a decompression molecule.
Access get
Type Xil_boolean
Values TRUE: Document Camera Indicator bit is set. FALSE: Document Camera Indicator bit is not set.
Default Value is undefined if no pictures have been decompressed.
DECOMPRESSOR_SOURCE_FORMAT
Description Return value of the Source Format bit from the picture header of the most recently decompressed picture. Value is available immediately after executing an xil_decompress(3) call and may be "gotten" and tested without compromising the execution of a decompression molecule.
Access get
Type typedef enum {
QCIF, CIF
} XilH261SourceFormat;
Values CIF: Source Format (picture size) is Common Intermediate Format (CIF) QCIF: Source Format (picture size) is Quarter Common Intermediate Format (QCIF)
Default Value is undefined if no pictures have been decompressed.
DECOMPRESSOR_TEMPORAL_REFERENCE
Description Return value of the Temporal Reference from the picture header of the most recently decompressed picture. Temporal Reference is formed by incrementing its value in the previously transmitted picture header by one plus the number of non-transmitted pictures (at 29.97 Hz) since the last transmitted one. Arithmetic is performed modulo 32. Value is available immediately after executing an xil_decompress(3) call and may be "gotten" and tested without compromising the execution of a decompression molecule.
Access get
Type Integer
Values value can be an integer from 0 to 31.
Default Value is undefined if no pictures have been decompressed.
EXAMPLES
The following example opens and closes an H.261 CIS using the XIL library:
XilSystemState State;
XilCis cis;
State = xil_open();
cis = xil_cis_create(State, "H261");
-- calls to H261-specific compression routines --
xil_cis_destroy(cis);
xil_close(State);
The following example sets an H.261 CIS attribute called COMPRESSOR_LOOP_FILTER to TRUE. Note that because this attribute is not a structure, it is not necessary to pass the address of COMPRESSOR_LOOP_FILTER when setting it.
XilCis cis;
xil_cis_set_attribute(cis,"COMPRESSOR_LOOP_FILTER", (void ∗) TRUE);
The following example returns the value of an H.261 CIS attribute called DECOMPRESSOR_DOC_CAMERA. Note that when getting an attribute it is always necessary to pass the address.
Xil_boolean on;
XilCis cis;
xil_cis_get_attribute(cis, "DECOMPRESSOR_DOC_CAMERA", (void ∗∗) &on);
NOTES
The xil_cis_set_attribute () and xil_cis_get_attribute () calls are used to modify the default behavior of a specific compressor. Generic attributes of compressors are set by individual function calls.
SEE ALSO
xil_cis_create(3), xil_cis_get_attribute(3), xil_cis_get_bits_ptr(3), xil_compress(3), xil_decompress(3).
SunOS 5.6 — Last change: 3 August 1993