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Diffstat (limited to 'docs/EXT_buffer_age.spec')
| -rw-r--r-- | docs/EXT_buffer_age.spec | 172 |
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diff --git a/docs/EXT_buffer_age.spec b/docs/EXT_buffer_age.spec new file mode 100644 index 0000000000..8da2538717 --- /dev/null +++ b/docs/EXT_buffer_age.spec @@ -0,0 +1,172 @@ +EGL_EXT_buffer_age
+
+ EXT_buffer_age
+
+Name Strings
+
+ EGL_EXT_buffer_age
+
+Notice
+
+ Copyright 2011,2012 Intel Cooperation. All rights reserved.
+
+Contributors
+
+ Robert Bragg
+ Neil Roberts
+ Tapani Pälli
+ Kristian Høgsberg
+
+Contacts
+
+ Robert Bragg, Intel (robert.bragg 'at' intel.com)
+
+Status
+
+ Draft
+
+Version
+
+ 5 - Feb 12, 2012
+
+Number
+
+ TBD
+
+Dependencies
+
+ Requires EGL 1.4
+
+ This extension is written against the wording of the EGL 1.4
+ Specification.
+
+ This extension refers to the EGL_EXT_start_frame extension
+
+Overview
+
+ The aim of this extension is to expose enough information to
+ applications about how the driver manages the set of front and
+ back buffers associated with a given surface to allow applications
+ to re-use the contents of old frames and minimize how much must be
+ redrawn for the next frame.
+
+ There are lots of different ways for a driver to manage these
+ buffers, from double buffering, different styles of triple
+ buffering and even n-buffering or simply single buffer rendering.
+ We also need to consider that power management events or memory
+ pressure events might also result in some of the buffers not
+ currently in-use being freed.
+
+ This extension lets you query the age of the back buffer contents
+ for an EGL surface as the number of frames elapsed since the
+ contents were originally defined. The back buffer can either be
+ reported as invalid (has an age of 0) or it may be reported to
+ contain the contents from n frames prior to this new frame.
+
+ For many use-cases this extension can provide an efficient
+ alternative to using the EGL_BUFFER_PRESERVED swap behaviour. The
+ EGL_BUFFER_PRESERVED swap behaviour adds a direct dependency for
+ any frame n on frame n - 1 which can affect the pipelining of
+ multiple frames but also implies a costly copy-back of data to
+ initialize the back-buffer at the start of each frame.
+
+ For example if you consider a double buffered application drawing
+ a small spinning icon, but everything else in the scene is static.
+ If we know that 2 buffers are continuously being recycled each
+ time eglSwapBuffers is called then even though 100s of frames may
+ need to be drawn to animate the icon it can be seen that the two
+ buffers are remaining unchanged except within the bounds of the
+ icon. In this scenario ideally the application would simply
+ perform an incremental update of the old buffer instead of
+ redundantly redrawing all the static parts of the scene. The
+ problem up until now though has been that EGL doesn't report how
+ buffers may be recycled so it wasn't safe for applications to try
+ and reuse their contents. Now applications can keep track of all
+ the regions that have changed over the last n frames and by
+ knowing the age of the buffer they know how to efficiently repair
+ buffers that are re-cycled instead of redrawing the entire scene.
+
+New Procedures and Functions
+
+ None
+
+New Tokens
+
+ EGL_BUFFER_AGE_EXT TBD
+
+Additions to Section 3.5 of the EGL 1.4 Specification (Rendering Surfaces)
+
+ Add the following to the table of "Queryable surface attributes
+ and types":
+
+ +----------------------+---------+-----------------------------+
+ | Attribute | Type | Description |
+ +----------------------+---------+-----------------------------+
+ | EGL_BUFFER_AGE_EXT | Integer | Age of back-buffer contents |
+ +----------------------+---------+-----------------------------+
+ Table aaa: Queryable surface attributes and types.
+
+
+ Add the following text to the subsection titled "Surface
+ Attributes" in the description for eglQuerySurface
+
+ Querying EGL_BUFFER_AGE_EXT returns the age of the
+ color contents of the current back-buffer as the number of
+ frames elapsed since it was defined. For example with a single
+ buffered surface the age would usually be 1. With a double
+ buffered surface the age would usually be 2. An age of 0 means
+ that the contents have only just been initialized and the
+ contents are undefined.
+ Since this attribute depends on the progression of frames
+ and for it to really be useful it should be queried at the
+ start of a frame before rendering anything, applications
+ should usually call eglStartFrameEGL() to explicitly start a
+ new frame before querying the back-buffer age.
+ If the EGL driver decides to free un-used back-buffers
+ when the system is under memory pressure or in response to
+ power-management events then EGL will return an age of 0 when
+ it allocates a new buffer at the start of a new frame.
+ Applications can use this age to safely rely on the
+ contents of old back-buffers to potentially reduce the amount
+ of redrawing they do each frame.
+ If the EGL_BUFFER_PRESERVED swap behaviour is in use then
+ it can be assumed that the age will always be 1. It is
+ recommended where possible though that the
+ EGL_BUFFER_PRESERVED swap behaviour not be used since it can
+ have severe performance consequences. Keeping track of the
+ buffer age instead and the regions that have changed over the
+ last 2 or 3 frames can often replace the need for using
+ EGL_BUFFER_PRESERVED without adding a dependency for each
+ frame on the previous frame which is what usually make
+ EGL_BUFFER_PRESERVED in-efficient.
+
+Dependencies on OpenGL ES
+
+ None
+
+Dependencies on OpenVG
+
+ None
+
+Issues
+
+ What are the semantics if EGL_BUFFER_PRESERVED is in use
+
+ RESOLVED: The age will always be 1 in this case. More
+ clarification about this was added along with the recommendation
+ to use the buffer age to reuse buffers instead of
+ EGL_BUFFER_PRESERVED when possible to avoid the in-efficiencies of
+ introducing a dependency for each frame on the previous frame.
+
+Revision History
+
+ Version 1, 25/07/2011
+ - First draft
+ Version 2, 03/08/2011
+ - Clarified semantics for using EGL_BUFFER_PRESERVED
+ Version 3, 01/09/2011
+ - Fixed a prototype inconsistency
+ Version 3, 03/11/2011
+ - Split out the buffer age parts from EGL_INTEL_start_frame
+ Version 4, 12/02/2012
+ - Rename to EGL_EXT_buffer_age
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