Name

    EXT_swap_control_tear

Name Strings

    GLX_EXT_swap_control_tear

Contributors

    Brian Nguyen, NVIDIA, brnguyen at nvidia.com

Contact

    Brian Nguyen, NVIDIA, brnguyen at nvidia.com

Status

    Complete.

Version

    Version 1 (November 4, 2011)

Number

    414

Dependencies

    This specification is written against the GLX 1.4 specification, but
    only GLX 1.1 or greater with the ARB_get_proc_address extension is
    required, for glXQueryExtensionString and glXGetProcAddressARB
    respectively.

    GLX_EXT_swap_control is required.

    Based on WGL_EXT_swap_control_tear version 1.

    This extension affects GLX_NV_swap_group.

Overview

    This extension extends the existing GLX_EXT_swap_control extension by
    allowing a negative <interval> parameter to glXSwapIntervalEXT. 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.

IP Status

    There are no known IP issues.

Issues

    This extension is incompatible with GLX_NV_swap_group. If a drawable is
    currently part of a swap group, late unsynchronized swaps will implicitly be
    disabled for that drawable.

New Procedures and Functions

    None

New Tokens

    GLX_LATE_SWAPS_TEAR_EXT	 0x20F3

Additions to Chapter 2 of the OpenGL 1.2.1 Specification (OpenGL
Operation)

    None.

Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization)

    None.

Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment
Operations and the Frame Buffer)

    None.

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

    None.

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

    None.

Additions to the GLX 1.4 Specification

    In Section 3.3.10 of the GLX specification (Double
    Buffering), after the following paragraph:

    "If <interval> is set to a value of 0, buffer swaps are not
    synchronized 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."

    In Section 3.3.10 of the GLX specification (Double
    Buffering), modify the paragraph beginning:

    "The current swap interval and implementation-dependent max swap
    interval for a particular drawable..."

    To read:

    "The current swap interval and implementation-dependent max swap
    interval for a particular drawable can be obtained by calling
    glXQueryDrawable with the attributes GLX_SWAP_INTERVAL_EXT and
    GLX_MAX_SWAP_INTERVAL_EXT respectively. The application can also
    determine whether a particular drawable allows late unsynchronized
    swaps to occur by calling glXQueryDrawable with the attribute
    GLX_LATE_SWAPS_TEAR_EXT. If late swaps are enabled, <value> will
    be set to 1; otherwise, it will be set to 0. The value returned by
    glXQueryDrawable is undefined if the drawable is not a GLXWindow
    and these attributes are given."

Errors

    Remove the following error from GLX_EXT_swap_control:

    "glXSwapIntervalEXT generates BadValue if parameter <interval> is
    less than zero."

Usage Examples

    Example 1: Set swap interval for the current drawable

    Display *dpy = glXGetCurrentDisplay();
    GLXDrawable drawable = glXGetCurrentDrawable();
    const int interval = -1;

    glXSwapIntervalEXT(dpy, drawable, interval);

Version History

    1. 04 Nov 2011 - BrianN
	-Initial version