137 lines
4.3 KiB
Plaintext
137 lines
4.3 KiB
Plaintext
Name
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ANDROID_presentation_time
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Name Strings
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EGL_ANDROID_presentation_time
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Contributors
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Jamie Gennis
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Andy McFadden
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Jesse Hall
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Contact
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Jamie Gennis, Google Inc. (jgennis 'at' google.com)
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Status
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Draft
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Version
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Version 3, June 26, 2013
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Number
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EGL Extension #XXX
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Dependencies
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Requires EGL 1.1
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This extension is written against the wording of the EGL 1.4 Specification
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Overview
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Often when rendering a sequence of images, there is some time at which each
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image is intended to be presented to the viewer. This extension allows
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this desired presentation time to be specified for each frame rendered to
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an EGLSurface, allowing the native window system to use it.
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New Types
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/*
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* EGLnsecsANDROID is a signed integer type for representing a time in
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* nanoseconds.
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*/
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#include <khrplatform.h>
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typedef khronos_stime_nanoseconds_t EGLnsecsANDROID;
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New Procedures and Functions
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EGLboolean eglPresentationTimeANDROID(
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EGLDisplay dpy,
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EGLSurface sur,
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EGLnsecsANDROID time);
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New Tokens
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None.
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Changes to Chapter 3 of the EGL 1.2 Specification (EGL Functions and Errors)
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Add a new subsection before Section 3.9.4, page 53 (Posting Errors)
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"3.9.4 Presentation Time
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The function
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EGLboolean eglPresentationTimeANDROID(EGLDisplay dpy, EGLSurface
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surface, EGLnsecsANDROID time);
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specifies the time at which the current color buffer of surface should be
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presented to the viewer. The time parameter should be a time in
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nanoseconds, but the exact meaning of the time depends on the native
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window system's use of the presentation time. In situations where
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an absolute time is needed such as displaying the color buffer on a
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display device, the time should correspond to the system monotonic up-time
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clock. For situations in which an absolute time is not needed such as
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using the color buffer for video encoding, the presentation time of the
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first frame may be arbitrarily chosen and those of subsequent frames
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chosen relative to that of the first frame.
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The presentation time may be set multiple times, with each call to
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eglPresentationTimeANDROID overriding prior calls. Setting the
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presentation time alone does not cause the color buffer to be made
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visible, but if the color buffer is subsequently posted to a native window
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or copied to a native pixmap then the presentation time of the surface at
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that time may be passed along for the native window system to use.
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If the surface presentation time is successfully set, EGL_TRUE is
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returned. Otherwise EGL_FALSE is returned and an appropriate error is
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set. If <dpy> is not the name of a valid, initialized EGLDisplay, an
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EGL_BAD_DISPLAY error is generated. If <surface> is not a valid EGLSurface
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then an EGL_BAD_SURFACE error is generated.
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Issues
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1. How is the presentation time used?
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RESOLVED: The uses of the presentation time are intentionally not specified
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in this extension. Some possible uses include Audio/Video synchronization,
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video frame timestamps for video encoding, display latency metrics, and
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display latency control.
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2. How can the current value of the clock that should be used for the
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presentation time when an absolute time is needed be queried on Android?
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RESOLVED: The current clock value can be queried from the Java
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System.nanoTime() method, or from the native clock_gettime function by
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passing CLOCK_MONOTONIC as the clock identifier.
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3. Should the presentation time be state which is used by eglSwapBuffers,
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or should it be a new parameter to an extended variant of eglSwapBuffers?
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RESOLVED: The presentation time should be new state which is used by
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the existing eglSwapBuffers call. Adding new state composes better with
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other (hypothetical) extensions that also modify the behavior of
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eglSwapBuffers.
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Revision History
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#3 (Jesse Hall, June 26, 2013)
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- Enumerated errors generated by eglPresentationTimeANDROID.
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- Added Issue #3 with resolution.
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#2 (Jamie Gennis, April 1, 2013)
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- Clarified how uses that either do or do not need an absolute time should
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be handled.
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- Specified the eglPresentationTimeANDROID return value.
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#1 (Jamie Gennis, January 8, 2013)
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- Initial draft.
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