MLT Video Control Protocol (MVCP) Reference Documentation

Copyright (C) 2004-2009 Ushodaya Enterprised Limited
Copyright (C) 2010-2011 Dan R. Dennedy
Author: Dan Dennedy <dan@dennedy.org>
Last Revision: 2011-04-17


General Format
--------------
   MVCP is an ASCII-based request/response TCP protocol much like FTP and
   inspired by the SGI MVCP [1] (Multiport Video Computer Protocol). Each
   command is three to eight characters long followed by zero or more
   arguments. Every item (command or argument) in the request is delimited
   by a space and terminated with a new line. Arguments that contain spaces
   must be surrounded by double quotation marks. The new line must contain
   a line feed optionally preceeded by a carriage return. There are no
   request header lines or body.

   [1] http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi?coll=0650&db=bks&fname=/SGI_EndUser/MSB3xx_UG/ch09.html


Response Codes
--------------
   Responses consist of a numeric result code followed by a space folowed
   by a brief textual description of the result. No quoting is applied to
   descriptions regardless if it contains spaces. The result codes are
   grouped by the hundreds into general categories of responses. Anything
   in the 200-299 range is considered a success and anything 300 and above
   is an error or exception. Most responses do not contain a body except
   some of the success results that report information and sometimes the
   500 Server Error returns specific information. 

   A 200 result code contains no body.
   A 201 result code contains one or more lines in the body, and an empty
   line terminates the response.
   A 202 result code contains only a single response line in the body.

   Errors in the 400 range indicate a normally handled error where the
   command could not perform its action due to protocol syntax errors or
   problems with validation of one or more of the arguments. This usually
   indicates that the client is responsible for performing an illegal
   request.
   
   Errors in the 500 range indicate a server error or exception.
   
   The following is a list of response codes and their descriptions:
   200 OK
   201 OK
   202 OK
   400 Unknown command
   401 Operation timed out
   402 Argument missing
   403 Unit not found
   404 Failed to locate or open clip
   405 Argument value out of range
   500 Server Error


Establishing a Connection
-------------------------
   One can connect to the melted server using telnet or a custom client,
   preferrably one developed using the libmvcp client API. The default port
   is 5250. Connections can be broken at will or use the BYE command to
   request the server to terminate the connection.


General Command Information
---------------------------

   All commands are case insensitive. Arguments may or may not be case
   sensitive. There are two categories of commands: global and unit. Global
   commands operate at the server level. Unit commands address a specific
   unit. melted is a multi-unit system. Units are named as U? where ?
   is the unit number, for example, U0. As units are added to the server,
   the unit number increases; the first unit is U0.
   
   The command HELP lists all commands known to the server with a brief
   description of their purpose and arguments. Most commands take zero or
   one argument outside of the unit name. Sometimes an argument is
   optional, and an optional argument always follows required arguments.
   All units command required a unit name argument.
   
   {} = required argument
   [] = optional argument
   () = one of a set of pre-defined values
   

Global Commands
---------------

HELP
   List the commands and their brief description.

BYE
   Close the connection.
   
SHUTDOWN
   Shutdown the server and all client connections.

SET {key=value}
   Set a global server configuration property.
   Currently, the only planned key is "root" to set the base directory
   path for the CLS and LOAD commands. The default root value is /.

GET {key}
   Get the current value of a configuration property.
   The value is returned by itself in the body of the response.

CLS {path}
   List the clips and subdirectories at {path} on the server.
   Only subdirectories, non-hidden regular files, symbolic links, and NFS
   shares are supported.
   The response body contains one line per item.
   The name of the subdirectory/file is always surrounded by double
   quotation marks in case it contains spaces.
   Subdirectories are listed before files and have a trailing / in their
   name.
   File entries have a size value in bytes in the second column position.

RUN {file}
   Process the commands in a file located on the server.
   Commands are executed one after the other with no delay until the end
   of file is reached or a command returns a response code not in the 200
   range.
   The response body contains each command sent along with its arguments,
   followed by each command's response status code and response body.

STATUS
   Responds with the output of USTA for each unit and accepts no further
   input. Each time the state of the unit changes, a new row is returned by
   the server containing the state of the unit. 


Unit Management

   The following global commands manage the playout units within the server.
   Currently there is a maximum of four units, and units can not be
   removed. Each unit may be in an online or offline state. Offline units
   can not be used, and any unit commands issued against an offline unit
   results in a 403 response. 
   
NLS
   * NOT IMPLEMENTED IN MELTED YET *


UADD mlt-consumer[:argument]
   Add a unit based upon the mlt-consumer id and optional constructor
   argument.
   If the consumer is not found, then it still added but in an
   offline manner. Later, by adding the device to the bus, the unit will
   automatically become online.
   The response body contains the name of the new unit: U0, U1, U2, or U3.
   Channel is an optional setting. 

ULS
   List the units.
   The response body contains a space-delimited row for each unit in the
   server containing the following columns:
   - unit name (one of U0, U1, U2, or U3)
   - mlt-consumer[:argument] from uadd
   - 1394 node GUID (defunt - always 0 with melted for now)
   - online flag (1 = online, 0 = offline)

SHUTDOWN
   Shutdown the server.


Unit Commands
-------------

   The first argument of any unit command is the unit name (U0 - U3). A
   unit must be loaded with a file before it can play anything. A "clip"
   refers to the presence of a file loaded into the unit. A clip can
   contain an in and out point to set the playback region. The default in
   point is 0, and the default out point is the number of frames in the
   file minus one. Therefore, all frame positions are zero-based.

USET {unit} {key=value}
   Set a unit's configuration property.
   Key is one of the following: eof, points.
   
   Property "eof" determines what the playback engine does when it reaches
   the end of a clip. The eof property takes one of the following values:
   stop, loop, continue or pause. The default is pause.
   
   Property "points" determines whether the playback engine restricts the
   playback region to the in and out points. It takes one of the following
   values: use, ignore.
   
UGET {unit} {key}
   Get a unit's configuration property.
   Key is one of the following: eof, points.
   The response body contains only the key's value. See USET for information 
   about each property.

LIST {unit}
   List the clips associated to the unit.
   The response body consists of two sections - the first section is a single row
   containing the generation number of the playlist associated to the unit (an
   integer starting from 0 which is incremented on each action which changes the
   playlist). The second sections contais a space-delimited row for each clip in the
   units playlistcontaining the following columns:
   - clip index (starts from 0)
   - file name
   - in point
   - out point
   - real length of the files
   - calculated length of file
   When USET points=use is specified (default), the calculated size is (out-in)+1. 
   When points are ignored, the real length of the file is returned.

LOAD {unit} {filename} [in out]
   Load a clip into the unit.
   Optionally set the in and out points to the specified absolute frame numbers.
   Sets the current position to the first frame in the clip.
   Preface the filename with '!' to tell the disk reader thread to remove only
   duplicate frames from the tail of its buffer queue (from a previously loaded
   and playing clip). Otherwise, melted flushes all of its buffers upon LOAD
   to make the effect of LOAD instantaneous. The LOAD !, USET eof=pause, and
   extended USTA information can be used for client-side playlists (see the 
   demo programs).

APND {unit} {filename} [in out]
   Append a clip onto the unit's playlist.
   Optionally set the in and out points to the specified absolute frame numbers.
   
INSERT {unit} {filename} [ [+|-]clip [ in out ] ]
   Insert a clip into the units playlist at the specified clip index or relative
   to the currently playing clip index.

REMOVE {unit} [ [+|-]clip ]
   Removes a clip from the specified clip index or position relative to the 
   currently playing clip index.
   
CLEAN {unit}
   Removes all but the playing clip.
   
WIPE {unit}
   Removes all clips before the playing clip.
   
MOVE {unit} [+|-]clip [ [+|-]clip ]
   Move a clip in the playlist to position specified or position relative to the
   currently playing clip.

PLAY {unit} [speed]
   Commence unit playback from the current position.
   The default speed is 100% if not specified.
   Speed is represented as a percentage value multiplied by 10. Therefore
   the default playback speed is 1000 (1X or 100%), 2X is 2000.
   Negative speed values play in reverse.

STOP {unit}
   Terminate the unit playback resulting in no video being sent.

PAUSE {unit}
   Pause the unit playback causing the current frame position to he held
   indefinitely.

REW {unit}
   Rewind the unit.
   If the unit it playing, then REW sets the playback speed to -2000
   (200%).
   If the unit is stopped, then the frame position is reset to the first
   frame. First frame depends upon the "points" unit configuration property
   and whether an in point has been established for the clip using the SIN
   command.
   Set the currently loaded clip's in point.
   Frame is zero-based and absolute. It is not dependent upon the clip's
   current in point.
   A frame-number of -1, resets the in point to 0.

FF {unit}
   Fast forward the unit.
   If the unit it playing, then FF sets the playback speed to 2000 (200%
   in reverse).
   If the unit is stopped, then the frame position is reset to the first
   frame. First frame depends upon the "points" unit configuration property
   and whether an in point has been established for the clip using the SIN
   command.

STEP {unit} {number-of-frames}
   Adjust the current frame position by the number of frames specified.
   Number-of-frames can accept positive or negative values.

GOTO {unit} {frame-number} [ [+|-]clip ]
   Set the current frame position to frame-number.
   Frame-number is zero-based and absolute within the clip, which means it is 
   relative to the file beginning and not the clip in point.
   It does not alter the playback status of the unit. 

SIN {unit} {frame-number} [ [+|-]clip ]
   Set the currently loaded clip's in point.
   The in point is the logical starting frame of the clip.
   Frame is zero-based and absolute. It is not dependent upon the clip's
   current in point.
   A frame-number of -1, resets the in point to 0.

SOUT {unit} {frame-number} [ [+|-]clip ]
   Set the currently loaded clip's out point.
   The out point is the logical last frame of the clip.
   Frame is zero-based and absolute. It is not dependent upon the clip's
   current out point.
   A frame-number of -1, resets the out point to the number of frames in
   the file minus 1.

USTA {unit}
   Get the unit status report.
   The response body contains the following fields delimited by spaces:
   - unit number: U0, U1, U2, or U3 without the "U" prefix
   - mode: (offline|not_loaded|playing|stopped|paused|disconnected|unknown)
     "unknown" means the unit has not been added
     "disconnected" means the server has closed the connection to the client.
   - current clip name: filename
   - current position: in absolute frame number units
   - speed: playback rate in (percent * 10)
   - fps: frames-per-second of loaded clip
   - current in-point: starting frame number
   - current out-point: ending frame number
   - length of the clip
   - buffer tail clip name: filename
   - buffer tail position: in absolute frame number units
   - buffer tail in-point: starting frame number
   - buffer tail out-point: ending frame number
   - buffer tail length: length of clip in buffer tail
   - seekable flag: indicates if the current clip is seekable (relates to head)
   - playlist generation number
   - current clip index (relates to head)
    
   The status contains information based not only on the current frame being
   output (current above) but also based upon the most recent frame read by
   the disk reader thread and added to the tail of the input buffer queue
   (buffer tail above).

XFER {unit} {target-unit}
   Transfer the unit's clip to the target unit.
   The clip inherently includes the in- and out-point information.
   The target unit's "points" configuration property is set to "use."

PUSH {unit}
{size}
{XML}
   Append an in-band MLT XML document to the unit.
   Do note that the size and XML arguments are on new lines.
   Size is the size of the XML payload in bytes.
   Returns 404 if the XML is malformed or if the XML producer fails parsing.

Topic revision: r3 - 2011-04-17 - DanDennedy
 
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