extensions/NV/WGL_NV_delay_before_swap.txt

Name

    NV_delay_before_swap

Name Strings

    WGL_NV_delay_before_swap

Contributors

    Jeannot Breton, NVIDIA
    Eric Werness, NVIDIA
    Andy Ritger, NVIDIA
    James Jones, NVIDIA
    Cass Everitt, NVIDIA
    John Carmack, id Software

Contact

    Jeannot Breton, NVIDIA Corporation (jbreton 'at' nvidia.com)

Status

    Shipping.

Version

    Last Modified Date:         02/04/2013
    Revision:                   2

Number

    OpenGL Extension #436

Dependencies

    OpenGL 2.1 is required.

Overview

    For most interactive applications, the standard rendering loop responding
    to input events on a frame granularity is sufficient.  Some more demanding
    applications may want to exchange performance for the ability to sample
    input closer to the final frame swap and adjust rendering accordingly. 
    This extension adds functionality to allow the application to wait until a
    specified time before a swapbuffers command would be able to execute.
    
    
New Procedures and Functions

    BOOL wglDelayBeforeSwapNV(HDC hDC, GLfloat seconds)

    
New Tokens

    None

Additions to the WGL Specification

        BOOL wglDelayBeforeSwapNV(HDC hDC, GLfloat seconds)

    wglDelayBeforeSwapNV blocks the CPU until <seconds> seconds before a
    synchronized swap would occur on the window associated with <hDC>.  It also
    returns a boolean value equal to TRUE when the implementation had to wait
    for the synchronized swap and FALSE otherwise.
    
    The parameter <hDC> must be a valid device context associated with a
    graphic adapter.  If <hDC> is not valid, GetLastError will return 
    WGL_INVALID_HDC, wglDelayBeforeSwapNV will return FALSE and will not wait
    for the swap.  If <hDC> is not associated with a graphic adapter,
    GetLastError will return ERROR_DC_NOT_FOUND, wglDelayBeforeSwapNV will
    return FALSE and will not wait for the swap.

    The parameter <seconds> accepts positive floating point values not larger
    than the length in seconds of the swap period on the associated window.
    When buffer swaps are synchronized, the swap period is composed of one or
    multiple video frame periods.  A video frame period is the time required by
    the monitor to display a full frame of video data.  A swap interval set to
    a value of 2 means that the color buffers will be swapped at most every
    other video frame.  If <seconds> is smaller than 0, GetLastError will
    return ERROR_INVALID_DATA, wglDelayBeforeSwapNV will return FALSE and will
    not wait for the end of the swap period.  If <seconds> is greater than a 
    swap period, wglDelayBeforeSwapNV will return immediately without 
    generating any error and the return value will be FALSE. 

    The application should use a <seconds> delay large enough to have time to
    complete its work before the end of the swap period.  If <seconds> is close
    to 0.0, the application may miss the end of the swap period and it will
    have to wait an additional swap period before it can swap.
    
    If wglDelayBeforeSwapNV detects that there is less than <seconds> seconds
    before the end of the swap period, it will return immediately and the 
    return value will be FALSE. The implementation will not wait an additional
    video frame period to have an exact delay of <seconds> seconds.

    If buffer swaps are unsynchronized, wglDelayBeforeSwapNV will return
    immediately and the return value will be FALSE.  It could happen for
    multiple reasons, for example if the swap interval is equal to 0, if the
    window is in a mode switch or if no monitors are active.


Usage Examples

    Here is a simple example that shows how an application can use
    WGL_NV_delay_before_swap to lower input latency when rendering its frames.

    void DrawFrame()
    {
        // Render the slowest part of the frame
        DrawScene();
        // Make sure there's no remaining work on the GPU
        glFinish();
        // Wait for the end of the swap period
        wglDelayBeforeSwapNV(hDC, 0.0015);
        
        // Sample inputs and adjust the image before the SwapBuffer
        
        SwapBuffer(hDC); 
    }
    
Issues

    (1) What happens if wglDelayBeforeSwapNV is called and the <seconds> delay
        is larger than the time left until the end of the swap period?
       
        RESOLVED.  The function returns immediately.  We also added a return
        value to wglDelayBeforeSwapNV to help the application detects this
        situation.  When wglDelayBeforeSwapNV returns FALSE, but didn't
        generate any error, it means that it didn't have to wait because it got
        called less than <seconds> seconds before the end of the swap period.

    (2) Should we add a function that return the amount of time until the end
        of the swap period?
        
        RESOLVED.  It would be nice to know exactly when the current swap
        period is going to end, but in some configurations it's not possible
        to return a value that we can guarantee will always be accurate.

    (3) How does wglDelayBeforeSwapNV interact with WGL_EXT_swap_control_tear?

    	RESOLVED. wglDelayBeforeSwapNV always attempts to stall until the
    	specified time before the SwapBuffers command could complete. With
    	swap_control_tear, the swap will wait until a fixed swap period if
    	possible, but perform an unsynchronized swap otherwise. If the
    	swapbuffers would wait, then wglDelayBeforeSwapNV will wait similarly if
    	required, but if the swap period is already past and the swapbuffers
    	would execute unsynchronized, then wglDelayBeforeSwapNV would return
    	immediately.

    (4) Why does this extension delay before execution of SwapBuffers rather
        than on a potential swap period?

	RESOLVED. Given that the expected use case is to wait until before the
	SwapBuffers would execute to sample input, having any cases where the
	behavior of the delay mismatches the behavior of the swap (such as
	swap_control_tear or swap_interval!=1) can cause significant issues in
	when the input is sampled.

Revision History

    Rev.    Date    Author    Changes
    ----  --------  --------  -----------------------------------------
     2    02/04/13  jbreton    Add a return value to wglDelayBeforeSwapNV.
                               Add an example and issues (1) and (2)
     
     1    01/29/13  jbreton    Internal revisions.