Legacy pre-1.0 C API


Data Structures

struct  yuv_buffer
 A YUV buffer for passing uncompressed frames to and from the codec. More...
struct  theora_info
 Theora bitstream info. More...
struct  theora_state
 Codec internal state and context. More...
struct  theora_comment
 Comment header metadata. More...

Files

file  theora.h
 The libtheora pre-1.0 legacy C API.

Defines

#define OC_FAULT   -1
 General failure.
#define OC_EINVAL   -10
 Library encountered invalid internal data.
#define OC_DISABLED   -11
 Requested action is disabled.
#define OC_BADHEADER   -20
 Header packet was corrupt/invalid.
#define OC_NOTFORMAT   -21
 Packet is not a theora packet.
#define OC_VERSION   -22
 Bitstream version is not handled.
#define OC_IMPL   -23
 Feature or action not implemented.
#define OC_BADPACKET   -24
 Packet is corrupt.
#define OC_NEWPACKET   -25
 Packet is an (ignorable) unhandled extension.
#define OC_DUPFRAME   1
 Packet is a dropped frame.

Enumerations

enum  theora_colorspace { OC_CS_UNSPECIFIED, OC_CS_ITU_REC_470M, OC_CS_ITU_REC_470BG, OC_CS_NSPACES }
 A Colorspace. More...
enum  theora_pixelformat { OC_PF_420, OC_PF_RSVD, OC_PF_422, OC_PF_444 }
 A Chroma subsampling. More...

Functions

const char * theora_version_string (void)
 Retrieve a human-readable string to identify the encoder vendor and version.
ogg_uint32_t theora_version_number (void)
 Retrieve a 32-bit version number.
int theora_encode_init (theora_state *th, theora_info *ti)
 Initialize the theora encoder.
int theora_encode_YUVin (theora_state *t, yuv_buffer *yuv)
 Submit a YUV buffer to the theora encoder.
int theora_encode_packetout (theora_state *t, int last_p, ogg_packet *op)
 Request the next packet of encoded video.
int theora_encode_header (theora_state *t, ogg_packet *op)
 Request a packet containing the initial header.
int theora_encode_comment (theora_comment *tc, ogg_packet *op)
 Request a comment header packet from provided metadata.
int theora_encode_tables (theora_state *t, ogg_packet *op)
 Request a packet containing the codebook tables for the stream.
int theora_decode_header (theora_info *ci, theora_comment *cc, ogg_packet *op)
 Decode an Ogg packet, with the expectation that the packet contains an initial header, comment data or codebook tables.
int theora_decode_init (theora_state *th, theora_info *c)
 Initialize a theora_state handle for decoding.
int theora_decode_packetin (theora_state *th, ogg_packet *op)
 Input a packet containing encoded data into the theora decoder.
int theora_decode_YUVout (theora_state *th, yuv_buffer *yuv)
 Output the next available frame of decoded YUV data.
int theora_packet_isheader (ogg_packet *op)
 Report whether a theora packet is a header or not This function does no verification beyond checking the header flag bit so it should not be used for bitstream identification; use theora_decode_header() for that.
int theora_packet_iskeyframe (ogg_packet *op)
 Report whether a theora packet is a keyframe or not.
int theora_granule_shift (theora_info *ti)
 Report the granulepos shift radix.
ogg_int64_t theora_granule_frame (theora_state *th, ogg_int64_t granulepos)
 Convert a granulepos to an absolute frame index, starting at 0.
double theora_granule_time (theora_state *th, ogg_int64_t granulepos)
 Convert a granulepos to absolute time in seconds.
void theora_info_init (theora_info *c)
 Initialize a theora_info structure.
void theora_info_clear (theora_info *c)
 Clear a theora_info structure.
void theora_clear (theora_state *t)
 Free all internal data associated with a theora_state handle.
void theora_comment_init (theora_comment *tc)
 Initialize an allocated theora_comment structure.
void theora_comment_add (theora_comment *tc, char *comment)
 Add a comment to an initialized theora_comment structure.
void theora_comment_add_tag (theora_comment *tc, char *tag, char *value)
 Add a comment to an initialized theora_comment structure.
char * theora_comment_query (theora_comment *tc, char *tag, int count)
 Look up a comment value by tag.
int theora_comment_query_count (theora_comment *tc, char *tag)
 Look up the number of instances of a tag.
void theora_comment_clear (theora_comment *tc)
 Clear an allocated theora_comment struct so that it can be freed.
int theora_control (theora_state *th, int req, void *buf, size_t buf_sz)
 Encoder control function.

theora_control() codes

#define TH_DECCTL_GET_PPLEVEL_MAX   (1)
 Get the maximum post-processing level.
#define TH_DECCTL_SET_PPLEVEL   (3)
 Set the post-processing level.
#define TH_ENCCTL_SET_KEYFRAME_FREQUENCY_FORCE   (4)
 Sets the maximum distance between key frames.
#define TH_DECCTL_SET_GRANPOS   (5)
 Set the granule position.
#define TH_DECCTL_SET_TELEMETRY_MBMODE   (9)
#define TH_DECCTL_SET_TELEMETRY_MV   (11)
#define TH_DECCTL_SET_TELEMETRY_QI   (13)
#define TH_DECCTL_SET_TELEMETRY_BITS   (15)
#define TH_ENCCTL_SET_QUANT_PARAMS   (2)
 Sets the quantization parameters to use.
#define TH_ENCCTL_SET_VP3_COMPATIBLE   (10)
 Disables any encoder features that would prevent lossless transcoding back to VP3.
#define TH_ENCCTL_GET_SPLEVEL_MAX   (12)
 Gets the maximum speed level.
#define TH_ENCCTL_SET_SPLEVEL   (14)
 Sets the speed level.
#define TH_ENCCTL_SET_DUP_COUNT   (18)
 Sets the number of duplicates of the next frame to produce.

Define Documentation

#define OC_BADHEADER   -20

Header packet was corrupt/invalid.

#define OC_BADPACKET   -24

Packet is corrupt.

#define OC_DISABLED   -11

Requested action is disabled.

#define OC_DUPFRAME   1

Packet is a dropped frame.

#define OC_EINVAL   -10

Library encountered invalid internal data.

#define OC_FAULT   -1

General failure.

#define OC_IMPL   -23

Feature or action not implemented.

#define OC_NEWPACKET   -25

Packet is an (ignorable) unhandled extension.

#define OC_NOTFORMAT   -21

Packet is not a theora packet.

#define OC_VERSION   -22

Bitstream version is not handled.

#define TH_DECCTL_GET_PPLEVEL_MAX   (1)

Get the maximum post-processing level.

These are the available request codes for theora_control() when called with a decoder instance. By convention, these are odd, to distinguish them from the encoder control codes. Keep any experimental or vendor-specific values above 0x8000. The decoder supports a post-processing filter that can improve the appearance of the decoded images. This returns the highest level setting for this post-processor, corresponding to maximum improvement and computational expense.

#define TH_DECCTL_SET_GRANPOS   (5)

Set the granule position.

Call this after a seek, to update the internal granulepos in the decoder, to insure that subsequent frames are marked properly. If you track timestamps yourself and do not use the granule postion returned by the decoder, then you do not need to use this control.

#define TH_DECCTL_SET_PPLEVEL   (3)

Set the post-processing level.

Sets the level of post-processing to use when decoding the compressed stream. This must be a value between zero (off) and the maximum returned by TH_DECCTL_GET_PPLEVEL_MAX.

#define TH_DECCTL_SET_TELEMETRY_BITS   (15)

#define TH_DECCTL_SET_TELEMETRY_MBMODE   (9)

#define TH_DECCTL_SET_TELEMETRY_MV   (11)

#define TH_DECCTL_SET_TELEMETRY_QI   (13)

#define TH_ENCCTL_GET_SPLEVEL_MAX   (12)

Gets the maximum speed level.

Higher speed levels favor quicker encoding over better quality per bit. Depending on the encoding mode, and the internal algorithms used, quality may actually improve, but in this case bitrate will also likely increase. In any case, overall rate/distortion performance will probably decrease. The maximum value, and the meaning of each value, may change depending on the current encoding mode (VBR vs. CQI, etc.).

Parameters:
[out] buf int: The maximum encoding speed level.
Return values:
TH_FAULT theora_state or buf is NULL.
TH_EINVAL buf_sz is not sizeof(int).
TH_IMPL Not supported by this implementation in the current encoding mode.

#define TH_ENCCTL_SET_DUP_COUNT   (18)

Sets the number of duplicates of the next frame to produce.

Although libtheora can encode duplicate frames very cheaply, it costs some amount of CPU to detect them, and a run of duplicates cannot span a keyframe boundary. This control code tells the encoder to produce the specified number of extra duplicates of the next frame. This allows the encoder to make smarter keyframe placement decisions and rate control decisions, as well as reduces CPU usage, when compared to just submitting the same frame for encoding multiple times. This setting only applies to the next frame submitted for encoding. You MUST call th_encode_packetout() repeatedly until it returns 0, or the extra duplicate frames will be lost.

Parameters:
[in] _buf int: The number of duplicates to produce. Unless this is positive, no duplicates will be produced.
Return values:
TH_EFAULT _enc_ctx or _buf is NULL.
TH_EINVAL _buf_sz is not sizeof(int), or the number of duplicates is greater than or equal to the maximum keyframe interval. In the latter case, NO duplicate frames will be produced. You must ensure that the maximum keyframe interval is set larger than the maximum number of duplicates you will ever wish to insert prior to encoding.
TH_IMPL Not supported by this implementation in the current encoding mode.

#define TH_ENCCTL_SET_KEYFRAME_FREQUENCY_FORCE   (4)

Sets the maximum distance between key frames.

This can be changed during an encode, but will be bounded by 1<<th_info::keyframe_granule_shift. If it is set before encoding begins, th_info::keyframe_granule_shift will be enlarged appropriately.

Parameters:
[in] buf ogg_uint32_t: The maximum distance between key frames.
[out] buf ogg_uint32_t: The actual maximum distance set.
Return values:
TH_FAULT theora_state or buf is NULL.
TH_EINVAL buf_sz is not sizeof(ogg_uint32_t).
TH_IMPL Not supported by this implementation.

#define TH_ENCCTL_SET_QUANT_PARAMS   (2)

Sets the quantization parameters to use.

These are the available request codes for theora_control() when called with an encoder instance. By convention, these are even, to distinguish them from the decoder control codes. Keep any experimental or vendor-specific values above 0x8000. The parameters are copied, not stored by reference, so they can be freed after this call. NULL may be specified to revert to the default parameters. For the current encoder, scale[ci!=0][qi] must be no greater than scale[ci!=0][qi-1] and base[qti][pli][qi][ci] must be no greater than base[qti][pli][qi-1][ci]. These two conditions ensure that the actual quantizer for a given qti, pli, and ci does not increase as qi increases.

Parameters:
[in] buf th_quant_info
Return values:
TH_FAULT theora_state is NULL.
TH_EINVAL Encoding has already begun, the quantization parameters do not meet one of the above stated conditions, buf is NULL and buf_sz is not zero, or buf is non-NULL and buf_sz is not sizeof(th_quant_info).
TH_IMPL Not supported by this implementation.

#define TH_ENCCTL_SET_SPLEVEL   (14)

Sets the speed level.

By default a speed value of 1 is used.

Parameters:
[in] buf int: The new encoding speed level. 0 is slowest, larger values use less CPU.
Return values:
TH_FAULT theora_state or buf is NULL.
TH_EINVAL buf_sz is not sizeof(int), or the encoding speed level is out of bounds. The maximum encoding speed level may be implementation- and encoding mode-specific, and can be obtained via TH_ENCCTL_GET_SPLEVEL_MAX.
TH_IMPL Not supported by this implementation in the current encoding mode.

#define TH_ENCCTL_SET_VP3_COMPATIBLE   (10)

Disables any encoder features that would prevent lossless transcoding back to VP3.

This primarily means disabling block-level QI values and not using 4MV mode when any of the luma blocks in a macro block are not coded. It also includes using the VP3 quantization tables and Huffman codes; if you set them explicitly after calling this function, the resulting stream will not be VP3-compatible. If you enable VP3-compatibility when encoding 4:2:2 or 4:4:4 source material, or when using a picture region smaller than the full frame (e.g. a non-multiple-of-16 width or height), then non-VP3 bitstream features will still be disabled, but the stream will still not be VP3-compatible, as VP3 was not capable of encoding such formats. If you call this after encoding has already begun, then the quantization tables and codebooks cannot be changed, but the frame-level features will be enabled or disabled as requested.

Parameters:
[in] buf int: a non-zero value to enable VP3 compatibility, or 0 to disable it (the default).
[out] buf int: 1 if all bitstream features required for VP3-compatibility could be set, and 0 otherwise. The latter will be returned if the pixel format is not 4:2:0, the picture region is smaller than the full frame, or if encoding has begun, preventing the quantization tables and codebooks from being set.
Return values:
TH_FAULT theora_state or buf is NULL.
TH_EINVAL buf_sz is not sizeof(int).
TH_IMPL Not supported by this implementation.

Enumeration Type Documentation

enum theora_colorspace

A Colorspace.

Enumerator:
OC_CS_UNSPECIFIED  The colorspace is unknown or unspecified.
OC_CS_ITU_REC_470M  This is the best option for 'NTSC' content.
OC_CS_ITU_REC_470BG  This is the best option for 'PAL' content.
OC_CS_NSPACES  This marks the end of the defined colorspaces.

enum theora_pixelformat

A Chroma subsampling.

These enumerate the available chroma subsampling options supported by the theora format. See Section 4.4 of the specification for exact definitions.

Enumerator:
OC_PF_420  Chroma subsampling by 2 in each direction (4:2:0).
OC_PF_RSVD  Reserved value.
OC_PF_422  Horizonatal chroma subsampling by 2 (4:2:2).
OC_PF_444  No chroma subsampling at all (4:4:4).

Function Documentation

void theora_clear ( theora_state *  t  ) 

Free all internal data associated with a theora_state handle.

Parameters:
t A theora_state handle.

void theora_comment_add ( theora_comment *  tc,
char *  comment 
)

Add a comment to an initialized theora_comment structure.

Parameters:
tc A previously initialized theora comment structure
comment A null-terminated string encoding the comment in the form "TAG=the value"
Neither theora_comment_add() nor theora_comment_add_tag() support comments containing null values, although the bitstream format supports this. To add such comments you will need to manipulate the theora_comment structure directly.

void theora_comment_add_tag ( theora_comment *  tc,
char *  tag,
char *  value 
)

Add a comment to an initialized theora_comment structure.

Parameters:
tc A previously initialized theora comment structure
tag A null-terminated string containing the tag associated with the comment.
value The corresponding value as a null-terminated string
Neither theora_comment_add() nor theora_comment_add_tag() support comments containing null values, although the bitstream format supports this. To add such comments you will need to manipulate the theora_comment structure directly.

void theora_comment_clear ( theora_comment *  tc  ) 

Clear an allocated theora_comment struct so that it can be freed.

Parameters:
tc An allocated theora_comment structure.

void theora_comment_init ( theora_comment *  tc  ) 

Initialize an allocated theora_comment structure.

Parameters:
tc An allocated theora_comment structure

char* theora_comment_query ( theora_comment *  tc,
char *  tag,
int  count 
)

Look up a comment value by tag.

Parameters:
tc Tn initialized theora_comment structure
tag The tag to look up
count The instance of the tag. The same tag can appear multiple times, each with a distinct and ordered value, so an index is required to retrieve them all.
Returns:
A pointer to the queried tag's value
Return values:
NULL No matching tag is found
Note:
Use theora_comment_query_count() to get the legal range for the count parameter.

int theora_comment_query_count ( theora_comment *  tc,
char *  tag 
)

Look up the number of instances of a tag.

Parameters:
tc An initialized theora_comment structure
tag The tag to look up
Returns:
The number on instances of a particular tag.
Call this first when querying for a specific tag and then interate over the number of instances with separate calls to theora_comment_query() to retrieve all instances in order.

int theora_control ( theora_state *  th,
int  req,
void *  buf,
size_t  buf_sz 
)

Encoder control function.

This is used to provide advanced control the encoding process.

Parameters:
th A theora_state handle.
req The control code to process. See the list of available control codes for details.
buf The parameters for this control code.
buf_sz The size of the parameter buffer.

int theora_decode_header ( theora_info *  ci,
theora_comment *  cc,
ogg_packet *  op 
)

Decode an Ogg packet, with the expectation that the packet contains an initial header, comment data or codebook tables.

Parameters:
ci A theora_info structure to fill. This must have been previously initialized with theora_info_init(). If op contains an initial header, theora_decode_header() will fill ci with the parsed header values. If op contains codebook tables, theora_decode_header() will parse these and attach an internal representation to ci->codec_setup.
cc A theora_comment structure to fill. If op contains comment data, theora_decode_header() will fill cc with the parsed comments.
op An ogg_packet structure which you expect contains an initial header, comment data or codebook tables.
Return values:
OC_BADHEADER op is NULL; OR the first byte of op->packet has the signature of an initial packet, but op is not a b_o_s packet; OR this packet has the signature of an initial header packet, but an initial header packet has already been seen; OR this packet has the signature of a comment packet, but the initial header has not yet been seen; OR this packet has the signature of a comment packet, but contains invalid data; OR this packet has the signature of codebook tables, but the initial header or comments have not yet been seen; OR this packet has the signature of codebook tables, but contains invalid data; OR the stream being decoded has a compatible version but this packet does not have the signature of a theora initial header, comments, or codebook packet
OC_VERSION The packet data of op is an initial header with a version which is incompatible with this version of libtheora.
OC_NEWPACKET the stream being decoded has an incompatible (future) version and contains an unknown signature.
0 Success
Note:
The normal usage is that theora_decode_header() be called on the first three packets of a theora logical bitstream in succession.

int theora_decode_init ( theora_state *  th,
theora_info *  c 
)

Initialize a theora_state handle for decoding.

Parameters:
th The theora_state handle to initialize.
c A theora_info struct filled with the desired decoding parameters. This is of course usually obtained from a previous call to theora_decode_header().
Return values:
0 Success

int theora_decode_packetin ( theora_state *  th,
ogg_packet *  op 
)

Input a packet containing encoded data into the theora decoder.

Parameters:
th A theora_state handle previously initialized for decoding.
op An ogg_packet containing encoded theora data.
Return values:
0 Success
OC_BADPACKET op does not contain encoded video data

int theora_decode_YUVout ( theora_state *  th,
yuv_buffer *  yuv 
)

Output the next available frame of decoded YUV data.

Parameters:
th A theora_state handle previously initialized for decoding.
yuv A yuv_buffer in which libtheora should place the decoded data. Note that the buffer struct itself is allocated by the user, but that the luma and chroma pointers will be filled in by the library. Also note that these luma and chroma regions should be considered read-only by the user.
Return values:
0 Success

int theora_encode_comment ( theora_comment *  tc,
ogg_packet *  op 
)

Request a comment header packet from provided metadata.

A pointer to the comment data is placed in a user-provided ogg_packet structure.

Parameters:
tc A theora_comment structure filled with the desired metadata
op An ogg_packet structure to fill. libtheora will set all elements of this structure, including a pointer to the encoded comment data. The memory for the comment data is owned by libtheora.
Return values:
0 Success

int theora_encode_header ( theora_state *  t,
ogg_packet *  op 
)

Request a packet containing the initial header.

A pointer to the header data is placed in a user-provided ogg_packet structure.

Parameters:
t A theora_state handle previously initialized for encoding.
op An ogg_packet structure to fill. libtheora will set all elements of this structure, including a pointer to the header data. The memory for the header data is owned by libtheora.
Return values:
0 Success

int theora_encode_init ( theora_state *  th,
theora_info *  ti 
)

Initialize the theora encoder.

Parameters:
th The theora_state handle to initialize for encoding.
ti A theora_info struct filled with the desired encoding parameters.
Return values:
0 Success

int theora_encode_packetout ( theora_state *  t,
int  last_p,
ogg_packet *  op 
)

Request the next packet of encoded video.

The encoded data is placed in a user-provided ogg_packet structure.

Parameters:
t A theora_state handle previously initialized for encoding.
last_p whether this is the last packet the encoder should produce.
op An ogg_packet structure to fill. libtheora will set all elements of this structure, including a pointer to encoded data. The memory for the encoded data is owned by libtheora.
Return values:
0 No internal storage exists OR no packet is ready
-1 The encoding process has completed
1 Success

int theora_encode_tables ( theora_state *  t,
ogg_packet *  op 
)

Request a packet containing the codebook tables for the stream.

A pointer to the codebook data is placed in a user-provided ogg_packet structure.

Parameters:
t A theora_state handle previously initialized for encoding.
op An ogg_packet structure to fill. libtheora will set all elements of this structure, including a pointer to the codebook data. The memory for the header data is owned by libtheora.
Return values:
0 Success

int theora_encode_YUVin ( theora_state *  t,
yuv_buffer *  yuv 
)

Submit a YUV buffer to the theora encoder.

Parameters:
t A theora_state handle previously initialized for encoding.
yuv A buffer of YUV data to encode. Note that both the yuv_buffer struct and the luma/chroma buffers within should be allocated by the user.
Return values:
OC_EINVAL Encoder is not ready, or is finished.
-1 The size of the given frame differs from those previously input
0 Success

ogg_int64_t theora_granule_frame ( theora_state *  th,
ogg_int64_t  granulepos 
)

Convert a granulepos to an absolute frame index, starting at 0.

The granulepos is interpreted in the context of a given theora_state handle.

Note that while the granulepos encodes the frame count (i.e. starting from 1) this call returns the frame index, starting from zero. Thus One can calculate the presentation time by multiplying the index by the rate.

Parameters:
th A previously initialized theora_state handle (encode or decode)
granulepos The granulepos to convert.
Returns:
The frame index corresponding to granulepos.
Return values:
-1 The given granulepos is undefined (i.e. negative)
Thus function was added in the 1.0alpha4 release.

int theora_granule_shift ( theora_info *  ti  ) 

Report the granulepos shift radix.

When embedded in Ogg, Theora uses a two-part granulepos, splitting the 64-bit field into two pieces. The more-significant section represents the frame count at the last keyframe, and the less-significant section represents the count of frames since the last keyframe. In this way the overall field is still non-decreasing with time, but usefully encodes a pointer to the last keyframe, which is necessary for correctly restarting decode after a seek.

This function reports the number of bits used to represent the distance to the last keyframe, and thus how the granulepos field must be shifted or masked to obtain the two parts.

Since libtheora returns compressed data in an ogg_packet structure, this may be generally useful even if the Theora packets are not being used in an Ogg container.

Parameters:
ti A previously initialized theora_info struct
Returns:
The bit shift dividing the two granulepos fields
This function was added in the 1.0alpha5 release.

double theora_granule_time ( theora_state *  th,
ogg_int64_t  granulepos 
)

Convert a granulepos to absolute time in seconds.

The granulepos is interpreted in the context of a given theora_state handle, and gives the end time of a frame's presentation as used in Ogg mux ordering.

Parameters:
th A previously initialized theora_state handle (encode or decode)
granulepos The granulepos to convert.
Returns:
The absolute time in seconds corresponding to granulepos. This is the "end time" for the frame, or the latest time it should be displayed. It is not the presentation time.
Return values:
-1. The given granulepos is undefined (i.e. negative), or
-1. The function has been disabled because floating point support is not available.

void theora_info_clear ( theora_info *  c  ) 

Clear a theora_info structure.

All values within the given theora_info structure are cleared, and associated internal codec setup data is freed.

Parameters:
c A theora_info struct to initialize.

void theora_info_init ( theora_info *  c  ) 

Initialize a theora_info structure.

All values within the given theora_info structure are initialized, and space is allocated within libtheora for internal codec setup data.

Parameters:
c A theora_info struct to initialize.

int theora_packet_isheader ( ogg_packet *  op  ) 

Report whether a theora packet is a header or not This function does no verification beyond checking the header flag bit so it should not be used for bitstream identification; use theora_decode_header() for that.

Parameters:
op An ogg_packet containing encoded theora data.
Return values:
1 The packet is a header packet
0 The packet is not a header packet (and so contains frame data)
Thus function was added in the 1.0alpha4 release.

int theora_packet_iskeyframe ( ogg_packet *  op  ) 

Report whether a theora packet is a keyframe or not.

Parameters:
op An ogg_packet containing encoded theora data.
Return values:
1 The packet contains a keyframe image
0 The packet is contains an interframe delta
-1 The packet is not an image data packet at all
Thus function was added in the 1.0alpha4 release.

ogg_uint32_t theora_version_number ( void   ) 

Retrieve a 32-bit version number.

This number is composed of a 16-bit major version, 8-bit minor version and 8 bit sub-version, composed as follows: 

   (VERSION_MAJOR<<16) + (VERSION_MINOR<<8) + (VERSION_SUB)
Returns:
The version number.

const char* theora_version_string ( void   ) 

Retrieve a human-readable string to identify the encoder vendor and version.

Returns:
A version string.

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