Name
KHR_surfaceless_context
Name Strings
EGL_KHR_surfaceless_context
Contributors
Acorn Pooley
Jon Leech
Kristian Hoegsberg
Steven Holte
Contact
Acorn Pooley: apooley at nvidia dot com
Notice
Copyright (c) 2010-2013 The Khronos Group Inc. Copyright terms at
http://www.khronos.org/registry/speccopyright.html
Status
Complete.
Approved by the EGL Working Group on June 6, 2012.
Approved by the Khronos Board of Promoters on July 27, 2012.
Version
Version 4, 2012/05/03
Number
EGL Extension #40
Dependencies
EGL 1.0 is required.
The functionality of this extension is not supported by client OpenGL ES
contexts unless the GL_OES_surfaceless_context extension is supported by
those contexts.
Written against wording of EGL 1.4 specification.
Overview
These extensions allows an application to make a context current by
passing EGL_NO_SURFACE for the write and read surface in the
call to eglMakeCurrent. The motivation is that applications that only
want to render to client API targets (such as OpenGL framebuffer
objects) should not need to create a throw-away EGL surface just to get
a current context.
The state of an OpenGL ES context with no default framebuffer provided
by EGL is the same as a context with an incomplete framebuffer object
bound.
New Procedures and Functions
None
New Tokens
None
Additions to the EGL Specification section "3.7.3 Binding Contexts and
Drawables"
Replace the following two error conditions in the
list of eglMakeCurrent errors:
" * If <ctx> is not a valid context, an EGL_BAD_CONTEXT error is
generated.
* If either <draw> or <read> are not valid EGL surfaces, an
EGL_BAD_SURFACE error is generated."
with the following error conditions:
" * If <ctx> is not a valid context and is not EGL_NO_CONTEXT, an
EGL_BAD_CONTEXT error is generated.
* If either <draw> or <read> are not valid EGL surfaces and are
not EGL_NO_SURFACE, an EGL_BAD_SURFACE error is generated.
* If <ctx> is EGL_NO_CONTEXT and either <draw> or <read> are not
EGL_NO_SURFACE, an EGL_BAD_MATCH error is generated.
* If either of <draw> or <read> is a valid surface and the other
is EGL_NO_SURFACE, an EGL_BAD_MATCH error is generated.
* If <ctx> does not support being bound without read and draw
surfaces, and both <draw> and <read> are EGL_NO_SURFACE, an
EGL_BAD_MATCH error is generated."
Replace the paragraph starting "If <ctx> is EGL_NO_CONTEXT and
<draw> and <read> are not EGL_NO_SURFACE..." with
"If both <draw> and <read> are EGL_NO_SURFACE, and <ctx> is a context
which supports being bound without read and draw surfaces, then no error
is generated and the context is made current without a
<default framebuffer>. The meaning of this is defined by the API of the
supporting context. (See chapter 4 of the OpenGL 3.0 Specification, and
the GL_OES_surfaceless_context OpenGL ES extension.)"
Append to the paragraph starting "The first time an OpenGL or OpenGL
ES context is made current..." with
"If the first time <ctx> is made current, it is without a default
framebuffer (e.g. both <draw> and <read> are EGL_NO_SURFACE), then
the viewport and scissor regions are set as though
glViewport(0,0,0,0) and glScissor(0,0,0,0) were called."
Interactions with other extensions
The semantics of having a current context with no surface for OpenGL ES
1.x and OpenGL ES 2.x are specified by the GL_OES_surfaceless_context
extension.
Issues
1) Do we need a mechanism to indicate which contexts may be bound with
<read> and <draw> set to NULL? Or is it ok to require that if this
extension is supported then any context of the particular API may be
made current with no surfaces?
RESOLVED. Because multiple API implementations may be available as
contexts we cannot guarantee that all OpenGL ES 1.x or OpenGL ES 2.x
contexts will support GL_OES_surfaceless_context. If the user attempts
to call eglMakeCurrent with EGL_NO_SURFACE on a context which does not
support it, this simply results in EGL_BAD_MATCH.
2) Do we need to include all of the relevant "default framebuffer" language
from the OpenGL specification to properly specify OpenGL ES behavior
with no default framebuffer bound?
RESOLVED. Yes, the behaviour of the GLES contexts when no default
framebuffer is associated with the context has been moved to the OpenGL
ES extension OES_surfaceless_context.
3) Since these EGL extensions also modify OpenGL ES behavior and introduce
a new error condition, do we want corresponding OpenGL ES extension
strings as well?
RESOLVED. Yes, see GL_OES_surfaceless_context extension.
4) How does this document interact with EGL_KHR_create_context and OpenGL
contexts?
RESOLVED. Some language defining the error conditions of eglMakeCurrent
have been imported from the draft specification of EGL_KHR_create_context
and the definitions of the behaviour of the GLES contexts without a
default framebuffer have been moved to GL_OES_surfaceless_context. Any
further interactions are left to the create_context extension to define
when it is completed.
Revision History
Version 5, 2014/01/07 (Jon Leech) - Correct references to
EXT_surfaceless_context with GL_OES_surfaceless_context.
Version 4, 2012/02/27 (Steven Holte) - Add language for error conditions
from EGL_KHR_create_context, and resolutions of issues. Combined API
specific extensions into a single extension.
Version 3, 2010/08/19 (Kristian Hoegsberg) - Move default framebuffer
language to new GLES extension (GL_OES_surfaceless_context) and make
this extension depend on that.
Version 2, 2010/08/03 (Jon Leech) - add default framebuffer language to
the OpenGL ES Specifications, including changes to initial GL state and
the FRAMEBUFFER_UNDEFINED incompleteness status when no default
framebuffer is bound.
Version 1, 2010/07/09 (Acorn Pooley) - Initial draft.
