KHR_no_config_context

Name

KHR_no_config_context

Name Strings

EGL_KHR_no_config_context

Contributors

Etay Meiri
Alon Or-bach
Jeff Vigil
Ray Smith
Michael Gold
James Jones
Daniel Kartch
Adam Jackson
Jon Leech
  

Contact

Etay Meiri (etay.meiri 'at' intel.com)

IP Status

No known IP claims.

Status

Approved by the EGL Working Group on April 27, 2016

Approved by the Khronos Board of Promoters on July 22, 2016

Version

Version 9, 2016/09/08

Number

EGL Extension #101

Extension Type

EGL display extension

Dependencies

EGL 1.4 is required. This extension is written against the EGL 1.5
Specification of August 27, 2014.

Some of the capabilities of these extensions are only available when
creating OpenGL or OpenGL ES contexts supporting specific versions or
capabilities. All such restrictions are documented in the body of this
extension specification.
  

Overview

Modern GPUs allow contexts to render to almost any combination of
supported color and auxiliary buffer formats. Traditionally EGL context
creation is done with respect to an EGLConfig specifying buffer formats,
and constrains contexts to only work with surfaces created with a
"compatible" config.

This extension allows creation of GL & ES contexts without specifying an
EGLConfig.
  

New Procedures and Functions

None.

New Tokens

Accepted as the <config> parameter of eglCreateContext:

    EGL_NO_CONFIG_KHR                   ((EGLConfig)0)

Additions to the EGL 1.5 Specification

Modify the 3rd paragraph of section 2.2 "Rendering Contexts and
Drawing Surfaces":
  

"Surfaces and contexts are both created with respect to an EGLConfig. The EGLConfig describes the depth of the color buffer components and the types, quantities and sizes of the ancillary buffers (i.e., the depth, multisample, and stencil buffers). It is also possible to create a context without using an EGLConfig, by specifying relevant parameters at creation time (see sections 3.5 and 3.7, respectively)."

Modify the sixth paragraph of section 2.2:

"A context can be used with any EGLSurface that it is with (subject to the restrictions discussed in the section on address space). A context and surface are compatible if they were created with respect to the same EGLDisplay, and if either of the following sets of conditions apply:

* The context was created without an EGLConfig. Such contexts match
any valid EGLSurface.

or,

* The context and surface support the same type of color buffer
  (RGB or luminance).

* They have color buffers and ancillary buffers of the same depth.

  ... replicate remainder of this bullet point ...

As long as the compatibility constraint and the address space ..."

Insert a new paragraph after paragraph 3 in section 3.7.1 "Creating
Rendering Contexts" on p. 51:

" specifies an EGLConfig defining properties of the context. If is EGL_NO_CONFIG_KHR, the resulting context is said to be created . In this case, the context must pass the required conformance tests for that client API and must support being made current without a rendering surface. Such support is guaranteed for OpenGL ES 2.0 implementations supporting the GL_OES_surfaceless_context extension, OpenGL ES 3.0 and later versions of OpenGL ES, and OpenGL 3.0 and later versions of OpenGL. Support for other versions and other client APIs is implementation dependent."

Replace the EGL_BAD_CONFIG error for eglCreateContext on p. 56, and add
a new errors:
  

"* An EGL_BAD_CONFIG error is generated if is neither EGL_NO_CONFIG_KHR nor a valid .

* An EGL_BAD_MATCH error is generated if <config> is EGL_NO_CONFIG_KHR,
  and the requested client API type and version do not support being
  made current without a rendering surface.

* An EGL_BAD_MATCH error is generated if <config> is EGL_NO_CONFIG_KHR,
  and the implementation does not support the requested client API and
  version."

Modify the first error for eglMakeCurrent in the list on p. 58:
  

"* An EGL_BAD_MATCH error is generated if or are not compatible with , as described in section 2.2."

Modify the description of eglQueryContext in section 3.7.4 on p. 63:

"Querying EGL_CONFIG_ID returns the ID of the EGLConfig with respect to which the context was created, or zero if created without respect to an EGLConfig."

Errors

As described in the body of the extension above.

Conformance Tests

None

Sample Code

None

Dependencies On EGL 1.4

If implemented on EGL 1.4, interactions with EGL 1.5-specific features
are removed.
  

Issues

1) Should non-conformant no-config contexts be allowed to be created?

RESOLVED: No. We are not encouraging non-conformant contexts.

2) Are no-config contexts constrained to those GL & ES implementations which can support them?

RESOLVED: Yes. ES2 + OES_surfaceless_context, ES 3.0, and GL 3.0 all
support binding a surface without a context. This implies that they
don't need to know surface attributes at context creation time.
  

3) For an OpenGL or OpenGL ES context created with no config, what is the initial state of GL_DRAW_BUFFER and GL_READ_BUFFER for the default framebuffer?

RESOLVED: This is an implementation detail rather than a spec issue.
glReadBuffer/glDrawBuffer have undefined results if called without a
current context. The GL_DRAW_BUFFER and GL_READ_BUFFER are set on the
first eglMakeCurrent call and can be updated in glReadBuffer and
glDrawBuffers calls after that. Therefore, the attribute value with
which the context is created is irrelevant from the point of view of the
spec and is left up to the implementation.
  

4) Can eglMakeCurrent alter the GL_DRAW_BUFFER and GL_READ_BUFFER state of the default framebuffer?

RESOLVED: Yes, but only on the first call to eglMakeCurrent. The two
relevant excerpts from the OpenGL 3.2 Core Profile Specification.
From Section 4.2.1 Selecting a Buffer for Writing:

    For the default framebuffer, in the initial state the draw buffer
    for fragment color zero is BACK if there is a back buffer; FRONT if
    there is no back buffer; and NONE if no default framebuffer is
    associated with the context.

From 4.3.3 Pixel Draw/Read State:

    For the default framebuffer, in the initial state the read buffer is
    BACK if there is a back buffer; FRONT if there is no back buffer;
    and NONE if no default framebuffer is associated with the context.

Based on the above excerpts on the first call to eglMakeCurrent the
GL_DRAW_BUFFER and GL_READ_BUFFER are set to: GL_NONE if the surface is
NULL, GL_BACK if the surface is double buffered, GL_FRONT if the surface
is single buffered. Following calls to glReadBuffer and glDrawBuffers
change the GL_DRAW_BUFFER and GL_READ_BUFFER attributes and these values
persist even when the application change the current context.
  

5) Should we add an eglCreateGenericContext which is the same as eglCreateContext but without the config parameter?

RESOLVED: No.

6) Can no-config contexts share state with contexts that has a config?

RESOLVED: Yes. This extension implies that the dependency of the context
on the config is quite minimal so no restriction w.r.t sharing should be
enforced.
  

7) What surface types can be made current with a no-config context?

RESOLVED: any surface type supported by the implementation can be made
current with a no-config context.
  

Revision History

Version 9. 2016/09/08 (Jon Leech) - Modify cast of EGL_NO_CONFIG_KHR to
(EGLConfig) per bug 15473.

Version 8. 2016/08/09 (Jon Leech) - Assign extension number, reflow
text, and publish.

Version 7. 2016/05/09 - Recorded vote at working group and sent to
Promoters for ratification.

Version 6. 2016/04/27 - Updated issue #6. Added an EGL_BAD_MATCH case to
eglCreateContext.

Version 5. 2016/04/20 - White space cleanup. Added extension type.
Cleaned up issues #1, #2, #4 and #6.

Version 4. 2016/03/24 - Added a list of contributers. Fixed resolution
of issue #3 and #4.

Version 3. 2016/03/10 - removed restriction to window surfaces only.
Removed comment on EGL_RENDERABLE_TYPE. Resolved issues 3 and 4. Added
issue 7.

Version 2, 2016/03/09 - querying EGL_CONFIG_ID on a context created
without a config returns zero. Contexts created without a config can
share state with contexts which were created with a config.

Version 1, 2016/01/27 - branch from draft EGL_KHR_no_config specification.
  