GL_APPLE_fence

Name

    APPLE_fence

Name Strings

    GL_APPLE_fence

Contact

    Geoff Stahl, Apple (gstahl 'at' apple.com)

Status

    Shipping as of August 24, 2002 (Mac OS X v10.2)

Version

    $Date: 2002/08/23 00:31:45 $ $Revision: 1.3 $
	

Number

    272

Dependencies

    None

Overview

    This extension is provided a finer granularity of synchronizing GL command
    completion than offered by standard OpenGL, which currently offers only two
    mechanisms for synchronization: Flush and Finish. Since Flush merely assures
    the user that the commands complete in a finite (though undetermined) amount
    of time, it is, thus, of only modest utility.  Finish, on the other hand,
    stalls CPU execution until all pending GL commands have completed forcing
    completely synchronous operation, which most often not the desired result.
    This extension offers a middle ground - the ability to "finish" a subset of
    the command stream, and the ability to determine whether a given command has
    completed or not.

    This extension introduces the concept of a "fence" to the OpenGL command
    stream with SetFenceAPPLE.  Once the fence is inserted into the command
    stream, it can be tested for its completion with TestFenceAPPLE. Moreover,
    the application may also request a partial Finish up to a particular "fence"
    using the FinishFenceAPPLE command -- that is, all commands prior to the
    fence will be forced to complete until control is returned to the calling
    process.  These new mechanisms allow for synchronization between the host
    CPU and the GPU, which may be accessing the same resources (typically
    memory).

    Fences are created and deleted, as are other objects in OpenGL, specifically
    with GenFencesAPPLE and DeleteFencesAPPLE.  The former returns a list of
    unused fence names and the later deletes the provided list of fence names.

    In addition to being able to test or finish a fence this extension allows
    testing for other types of completion, including texture objects, vertex
    array objects, and draw pixels. This allows the client to use
    TestObjectAPPLE or FinishObjectAPPLE with FENCE_APPLE, TEXTURE,
    VERTEX_ARRAY, or DRAW_PIXELS_APPLE with the same type of results as
    TestFenceAPPLE and FinishFenceAPPLE.  Specifically, using the FENCE_APPLE
    type is equivalent to calling TestFenceAPPLE or FinishFenceAPPLE with the
    particular fence name.  Using TEXTURE as the object type tests or waits for
    completion of a specific texture, meaning when there are no pending
    rendering commands which use that texture object. Using the VERTEX_ARRAY
    type will test or wait for drawing commands using that particular vertex
    array object name.  Finally, DRAW_PIXELS_APPLE will wait or test for
    completion of all pending DrawPixels commands.  These tests and finishes
    operate with the same limitations and results as test and finish fence.

    One use of this extension is in conjunction with APPLE_vertex_array_range to
    determine when graphics hardware has completed accessing vertex data from a
    vertex array range.  Once a fence has been tested TRUE or finished, all
    vertex indices issued before the fence must have completed being accessed.
    This ensures that the vertex data memory corresponding to the issued vertex
    indices can be safely modified (assuming no other outstanding vertex indices
    are issued subsequent to the fence).
    

Issues

	How is TestObjectAPPLE or FinishObjectAPPLE used with DRAW_PIXELS_APPLE?
		
		Resolution:  Currently there is no support DrawPixels with storage using
		the APPLE_client_storage extension and thus this option has no utility,
		due to implementation specifics, which always copy the DrawPixels
		buffer, thus allowing the client to immediately modify the data used by
		DrawPixels.  Once the APPLE_client_storage extension is supported,
		DrawPixels modification synchronization will be required after drawing
		with a buffer, which resides in client space.

    Do we need an IsFenceAPPLE command?

        RESOLUTION:  Yes.  IsFenceAPPLE makes APPLE_fence's API orthogonal to
        other OpenGL object interfaces, and can be used as any other Is...
        command would be.

    Are the fences sharable between multiple contexts?

        RESOLUTION:  No.

    What is the relative performance of the calls?

        Execution of a SetFenceAPPLE is not free.  In the initial
        implementation, a Flush is generated.  This will likely change for
        future implementations and should not be depended on.  A Finish will not
        be generated in any case.

    Is the TestFenceAPPLE call really necessary?  How often would this be used
    compared to the FinishFenceAPPLE call (which also flushes to ensure this
    happens in finite time)?

        TestFenceAPPLE allows clients to provide logic to handle
        synchronization issues rather than forcing completely synchronous
        operation at the point of testing or finishing.

    Should we allow these commands to be compiled within display list?
    Which ones?  How about within Begin/End pairs?

        RESOLUTION:  DeleteFencesAPPLE, GenFencesAPPLE, TestFenceAPPLE,
        TestObjectAPPLE, and IsFenceAPPLE are executed immediately while
        FinishFenceAPPLE, FinishObjectAPPLE and SetFenceAPPLE are compiled. 
        None of these commands are allowed within Begin/End pairs.

New Procedures and Functions

	void GenFencesAPPLE(sizei n, uint *fences);

	void DeleteFencesAPPLE(sizei n, const uint *fences);

	void SetFenceAPPLE(uint fence);

	boolean IsFenceAPPLE(uint fence);

	boolean TestFenceAPPLE(uint fence);

	void FinishFenceAPPLE(uint fence);

	boolean TestObjectAPPLE(enum object, uint name);

	void FinishObjectAPPLE(enum object, int name);

New Tokens

    Accepted by the <object> parameter of TestObjectAPPLE and FinishObjectAPPLE:

        DRAW_PIXELS_APPLE                    0x8A0A
        FENCE_APPLE                          0x8A0B

Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions)

    Add to the end of Section 5.4 "Display Lists"

    "DeleteFencesAPPLE, GenFencesAPPLE, TestFenceAPPLE, IsFenceAPPLE, and
    TestObjectAPPLE are not complied into display lists but are executed
    immediately."

    After the discussion of Flush and Finish (Section 5.5) add a
    description of the fence operations:

    "5.X  Fences

    The command 

       void SetFenceAPPLE(uint fence);

    sets a fence within the GL command stream and assigns the fence a status of
    FALSE. No other state is affected by execution of the fence command.  A
    fence's state can be queried by calling the command

      boolean TestFenceAPPLE(uint fence);

    The command

      void FinishFenceAPPLE(uint fence);

    forces all GL commands prior to the fence to complete. FinishFenceAPPLE does
    not return until all effects from these commands on GL client and server
    state and the frame buffer are fully realized.

    The fence must first be created before it can be used.  The command

      void GenFencesAPPLE(sizei n, uint *fences);

    returns n previously unused fence names in fences.  These names are marked
    as used for the purposes of GenFencesAPPLE only and acquire boolean state
    only when they have been set.

    Fences are deleted by calling

      void DeleteFencesAPPLE(sizei n, const uint *fences);

    fences contains n names of fences to be deleted.  After a fence is deleted,
    it has no state, and its name is again unused.  Unused names in fences are
    silently ignored.

    If the fence passed to TestFenceAPPLE or FinishFenceAPPLE is not the name of
    a fence, the error INVALID_OPERATION is generated.  In this case,
    TestFenceAPPLE will return TRUE, for the sake of consistency. Note, fences
    that have note been set prior to calling TestFenceAPPLE or FinishFenceAPPLE
    act as if the state is TRUE and the fence command has already been
    completed.  In other words TestFenceAPPLE returns TRUE and FinishFenceAPPLE
    will not block on fences that have not been set.

    State must be maintained to indicate which fence integers are currently used
    or set.  In the initial state, no indices are in use. When a fence integer
    is set, status of the fence is also maintained.  The status is a boolean.

    Once the status of a fence has been finished (via FinishFenceAPPLE) or
    tested and the returned status is TRUE (via TestFenceAPPLE), the status
    remains TRUE until the next SetFenceAPPLE of the fence.
    
    The command
    
      boolean TestObjectAPPLE(enum object, uint name);
    
    and the command

      void FinishObjectAPPLE(enum object, int name);
    
    work in a similar fashion to TestFenceAPPLE and FinishFenceAPPLE but on
    other types of "objects".  Both of these commands take an object, which can
    be FENCE_APPLE, TEXTURE, VERTEX_ARRAY, or DRAW_PIXELS_APPLE and an object
    name. These commands are useful for synchronizing the update of buffers for
    textures, draw pixels, or vertex arrays, especially when using extensions
    such as Apple's vertex array range or client storage.

    If the object parameter for TestObjectAPPLE or FinishObjectAPPLE is
    FENCE_APPLE then these commands work in exactly the same manner as
    TestFenceAPPLE and FinishFenceAPPLE, respectively.  If the object parameter
    is TEXTURE then these routines test or finish the use of a texture object,
    thus FinishObjectAPPLE will block and TestFenceAPPLE will return FALSE while
    there are pending rendering commands utilizing the texture object in
    question. If the object parameter is VERTEX_ARRAY, FinishObjectAPPLE will
    block and TestFenceAPPLE will return FALSE while there are pending rendering
    commands utilizing the vertex array object in question.  Note, in both these
    cases object name 0 will work as expected, thus testing or finishing the
    default texture or vertex array object. If the object parameter is
    DRAW_PIXELS_APPLE, FinishObjectAPPLE will block and TestFenceAPPLE will
    return FALSE while there are pending DrawPixels commands.  For all other
    cases, assuming a valid object type and name are used, FinishObjectAPPLE
    will return immediately and TestFenceAPPLE will return TRUE.

    INVALID_OPERATION error is generated if FinishObjectAPPLE or TestFenceAPPLE
    is called with either an invalid object type enumeration or a name, which is
    not the name of a valid object of the type specified in the object
    parameter.

Additions to Chapter 6 of the OpenGL 1.2.1 Specification (State and State Requests)

    Insert new section after Section 6.1.10 "Minmax Query"

    "6.1.11 Fence Query

    The command

      boolean IsFenceAPPLE(uint fence);

    returns TRUE if texture is the name of a fence.  If fence is not the name of
    a fence, or if an error condition occurs, IsFenceAPPLE returns FALSE.  A
    name returned by GenFencesAPPLE, but not yet set via SetFenceAPPLE, is not
    the name of a fence.

Additions to the GLX Specification

    None

GLX Protocol

    None

Errors

    INVALID_VALUE is generated if GenFencesAPPLE parameter <n> is negative.

    INVALID_VALUE is generated if DeleteFencesAPPLE parameter <n> is negative.

    INVALID_OPERATION is generated if the fence used in TestFenceAPPLE or
    FinishFenceAPPLE is not the name of a fence. 
    
    INVALID_OPERATION is generated if the object name used in TestObjectAPPLE or
    FinishObjectAPPLE is not the name of an object of the type requested in the
    object parameter.

    INVALID_OPERATION is generated if any of the commands defined in
    this extension is executed between the execution of Begin and the
    corresponding execution of End.
    
    INVALID_VALUE is generated if DeleteFencesAPPLE or GenFencesAPPLE are
    called where n is negative.

New State

	None

New Implementation Dependent State

    None

Implementation Details

    This section describes implementation-defined limits:

        SetFenceAPPLE calls are not free.  They should be used prudently, and a
        "good number" of commands should be sent between calls to SetFenceAPPLE.
        Testing or finishing a fence may cause a Flush if commands up to the
        fence being tested have not been submitted to the hardware.

Revision History

	None

Implementation Support

   List of OpenGL implementations supporting the GL_APPLE_fence extension

Original File

   Original text file for the GL_APPLE_fence extension