Name

    EXT_swap_control_tear

Name Strings

    WGL_EXT_swap_control_tear
    
Contact

    Piers Daniell, NVIDIA (pdaniell 'at' nvidia.com)
    
Contributors

    Antonio Tejada, NVIDIA
    Jeff Bolz, NVIDIA
    Pat Brown, NVIDIA
    Udo Lugauer, NVIDIA

Status

    Complete. Shipping with NVIDIA Forceware 285.62 and AMD Catalyst 11.10
    drivers. 

Version

    Last Modified Date:         03/16/2011
    Author revision:            1

Number

    415

Dependencies

    WGL_EXT_extensions_string is required.
    
    WGL_EXT_swap_control is required.

Overview

    This extension extends the existing WGL_EXT_swap_control extension
    by allowing a negative <interval> parameter to wglSwapIntervalEXT.
    The negative <interval> allows late swaps to occur without
    synchronization to the video frame. This reduces the visual stutter
    on late frames and reduces the stall on subsequent frames.

New Procedures and Functions

    None

New Tokens

    None

Additions to the WGL Specification

    In the section that starts:

    "wglSwapIntervalEXT specifies the minimum number of video frame periods
    per buffer swap for the window associated with the current context."

    After the following paragraph:

    "If <interval> is set to a value of 0, buffer swaps are not synchron-
    ized to a video frame.  The <interval> value is silently clamped to
    the maximum implementation-dependent value supported before being
    stored."

    Add the following paragraph:

    "If <interval> is negative, the minimum number of video frames between
    buffer swaps is the absolute value of <interval>. In this case, if 
    abs(<interval>) video frames have already passed from the previous swap 
    when the swap is ready to be performed, the swap will occur without
    synchronization to a video frame. When an unsynchronized swap happens,
    the missed frame will count towards the minimum number of video frames
    for the next swap."

Dependencies on WGL_EXT_extensions_string

    Because there is no way to extend wgl, these calls are defined in
    the ICD and can be called by obtaining the address with
    wglGetProcAddress.  Because this extension is a WGL extension, it
    is not included in the GL_EXTENSIONS string.  Its existence can be
    determined with the WGL_EXT_extensions_string extension.

Errors

    In the section that starts:

    "If the function succeeds, the return value is TRUE. If the function
    fails, the return value is FALSE.  To get extended error information,
    call GetLastError."
    
    Remove the following error:

       ERROR_INVALID_DATA      The <interval> parameter is negative.

New State

    None

New Implementation Dependent State

    None

Issues

    1) This extension doesn't provide a way to know when a video frame was
       missed and an unsynchronized swap was performed.
    
       UNRESOLVED: This extensions isn't meant to solve the issue of repeated
       tearing if the application falls behind the vsync. This extension just
       eliminates the extra stall that will occur if the application misses
       the target vsync. The application can use a CPU timer or the GL timer
       queries to determine if it is falling behind continually and can
       adjust it's workload appropriatly.

Revision History

    Rev.    Date    Author    Changes
    ----  --------  --------  -----------------------------------------
      1   03/16/11  pdaniell  Initial version.