Name
MESA_configless_context
Name Strings
EGL_MESA_configless_context
Contact
Neil Roberts <neil.s.roberts@intel.com>
Status
Proposal
Version
Version 1, February 28, 2014
Number
EGL Extension #not assigned
Dependencies
Requires EGL 1.4 or later. This extension is written against the
wording of the EGL 1.4 specification.
Overview
This extension provides a means to use a single context to render to
multiple surfaces which have different EGLConfigs. Without this extension
the EGLConfig for every surface used by the context must be compatible
with the one used by the context. The only way to render to surfaces with
different formats would be to create multiple contexts but this is
inefficient with modern GPUs where this restriction is unnecessary.
IP Status
Open-source; freely implementable.
New Procedures and Functions
None.
New Tokens
Accepted as <config> in eglCreateContext
EGL_NO_CONFIG_MESA ((EGLConfig)0)
Additions to the EGL Specification section "2.2 Rendering Contexts and Drawing
Surfaces"
Add the following to the 3rd paragraph:
"EGLContexts can also optionally be created with respect to an EGLConfig
depending on the parameters used at creation time. If a config is provided
then additional restrictions apply on what surfaces can be used with the
context."
Replace the last sentence of the 6th paragraph with:
"In order for a context to be compatible with a surface they both must have
been created with respect to the same EGLDisplay. If the context was
created without respect to an EGLConfig then there are no further
constraints. Otherwise they are only compatible if:"
Remove the last bullet point in the list of constraints.
Additions to the EGL Specification section "3.7.1 Creating Rendering Contexts"
Replace the paragraph starting "If config is not a valid EGLConfig..."
with
"The config argument can either be a valid EGLConfig or EGL_NO_CONFIG_MESA.
If it is neither of these then an EGL_BAD_CONFIG error is generated. If a
valid config is passed then the error will also be generated if the config
does not support the requested client API (this includes requesting
creation of an OpenGL ES 1.x context when the EGL_RENDERABLE_TYPE
attribute of config does not contain EGL_OPENGL_ES_BIT, or creation of an
OpenGL ES 2.x context when the attribute does not contain
EGL_OPENGL_ES2_BIT).
Passing EGL_NO_CONFIG_MESA will create a configless context. When a
configless context is used with the OpenGL API it can be assumed that the
initial values of the context's state will be decided when the context is
first made current. In particular this means that the decision of whether
to use GL_BACK or GL_FRONT for the initial value of the first output in
glDrawBuffers will be decided based on the config of the draw surface when
it is first bound."
Additions to the EGL Specification section "3.7.3 Binding Contexts and
Drawables"
Replace the first bullet point with the following:
"* If draw or read are not compatible with ctx as described in section 2.2,
then an EGL_BAD_MATCH error is generated."
Add a second bullet point after that:
"* If draw and read are not compatible with each other as described in
section 2.2, then an EGL_BAD_MATCH error is generated."
Issues
1. What happens when an OpenGL context with a double-buffered surface and
draw buffer set to GL_BACK is made current with a single-buffered
surface?
NOT RESOLVED: There are a few options here. An implementation can
raise an error, change the drawbuffer state to GL_FRONT or just do
nothing, expecting the application to set GL_FRONT drawbuffer before
drawing. However, this extension deliberately does not specify any
required behavior in this corner case and applications should avoid
mixing single- and double-buffered surfaces with configless contexts.
Future extensions may specify required behavior in this case.
Revision History
Version 1, February 28, 2014
Initial draft (Neil Roberts)