NV_triple_buffer
NV_quadruple_buffer

EGL_NV_triple_buffer
EGL_NV_quadruple_buffer

Daniel Kartch, NVIDIA Corporation
Tom McReynolds, NVIDIA Corporation
Santanu Thangaraj, NVIDIA Corporation

Daniel Kartch, NVIDIA Corporation (dkartch 'at' nvidia.com)

Complete.

Version 1 - February 28, 2019

133

EGL display extension

Requires EGL 1.0

This extension is written against the wording of the EGL 1.3
Specification.

NV_triple_buffer and NV_quadruple_buffer allow applications to
request additional back buffers, in order to produce greater and
less variable frame rates.

This document describes two related extensions, one dependent on
the other. Implementations may choose to support only
NV_triple_buffer and not NV_quadruple_buffer, but not vice versa.

None

None

Added by NV_triple_buffer:

  Accepted as a value for EGL_RENDER_BUFFER in the <attrib_list>
  parameter of eglCreateWindowSurface:

      EGL_TRIPLE_BUFFER_NV                    0x3230

Added by NV_quadruple_buffer:

  Accepted as a value for EGL_RENDER_BUFFER in the <attrib_list>
  parameter of eglCreateWindowSurface:

      EGL_QUADRUPLE_BUFFER_NV                 0x3231

Insert after third sentence of second paragraph of Section 2.2.2
(Rendering Models):

    Windows may have more than one back buffer, allowing rendering
    of a new frame to proceed while the copy requested by 
    eglSwapBuffers is still pending.

Replace the third sentence of the EGL_RENDER_BUFFER description in
Section 3.5.1 (Creating On-Screen Rendering Surfaces):

    If its value is EGL_BACK_BUFFER, EGL_TRIPLE_BUFFER_NV, or
    EGL_QUADRUPLE_BUFFER_NV, then client APIs should render into
    the current back buffer. The implementation should provide
    at least one, two, or three back buffers, respectively, which
    will be used in rotation each frame.

Change first sentence of third bullet point of eglQueryContext
description in Section 3.7.4 (Context Queries):

    If the context is bound to a window surface, then either
    EGL_SINGLE_BUFFER, EGL_BACK_BUFFER, EGL_TRIPLE_BUFFER_NV, or
    EGL_QUADRUPLE_BUFFER_NV may be returned.

Replace first sentence of eglSwapBuffers description in
Section 3.9.1
(Posting to a Window):

    If surface is a back-buffered window surface, then the current
    color buffer is copied to the native window associated with
    that surface. If there is more than one back buffer, then the
    next color buffer in rotation becomes current, and rendering
    of the next frame may proceed before the copy takes place,
    provided any previous swaps from the new current buffer have
    completed.

1. Why do we need triple-buffering?

   RESOLVED: With only a single back buffer and a non-zero swap
   interval, eglSwapBuffers must block rendering to the back-
   buffer until the copy has completed. This can leave the CPU
   and/or GPU idle, wasting valuable compute time, and possibly
   cause the next frame to be delivered later than otherwise could
   have been. Additional buffers allow rendering to continue even
   when a frame is awaiting display, maximizing our use of
   computational resources.

2. Why quadruple-buffering? Isn't triple-buffering enough to
   produce frames as fast as the processor(s) and swap interval
   allow?

   RESOLVED: When there is only a single rendering stream
   operating on a system, triple-buffering is sufficient. However,
   if other threads are contending for resources, variable 
   latencies may be introduced. This is especially problematic
   with video, where any deviation in frame rate from the recorded
   media can produce visible artifacts. Additional buffers smooth
   out these latencies, allowing a steady frame rate.

3. Then why not arbitrary n-buffering?

   RESOLVED: The TRIPLE/QUADRUPLE buffer specification fits nicely
   into the RENDER_BUFFER attribute already in use for
   eglCreateWindowSurface. Arbitrary buffer counts would require a
   new attribute. Additionally, case studies indicated no
   significant benefit to using more than three back buffers,
   especially when factoring in the added memory cost.

#2 (February 28, 2019) Santanu Thangaraj
   - Marked issues 1,2 and 3 as resolved.
   - Included extension type section.
   - Corrected line length violations.

#1 (August 12, 2008) Daniel Kartch
   - Initial Draft