Name
KHR_config_attribs
Name Strings
EGL_KHR_config_attribs
Contributors
Jon Leech
Contacts
Jon Leech (jon 'at' alumni.caltech.edu)
Status
Complete
Version
Version 5, April 5, 2007
Number
EGL Extension #1
Dependencies
Requires EGL 1.2
Some of the extended config attributes defined by this extension are
only relevant when specific client APIs are supported.
This extension is written against the wording of the EGL 1.2
Specification. It exists for backwards compatibility with
functionality introduced in EGL 1.3.
Overview
This extension adds new EGL config attributes and attribute bits
that express limitations of configs on a per-API basis, including
whether client APIs created with respect to a config are expected to
pass conformance, and which optional OpenVG color space and alpha
mask format attributes are valid at surface creation time.
New Types
None
New Procedures and Functions
None
New Tokens
New EGLConfig bitmask attribute name:
EGL_CONFORMANT_KHR 0x3042
Valid bitfields in the EGL_SURFACE_TYPE bitmask attribute
of EGLConfig:
EGL_VG_COLORSPACE_LINEAR_BIT_KHR 0x0020
EGL_VG_ALPHA_FORMAT_PRE_BIT_KHR 0x0040
Additions to Chapter 3 of the EGL 1.2 Specification (EGL Functions and Errors)
Add to table 3.1, "EGLConfig attributes":
Attribute Type Notes
--------- ---- -----
EGL_CONFORMANT_KHR bitmask whether contexts created with
this config are conformant
Add to table 3.2, "Types of surfaces supported by an EGLConfig":
EGL Token Name Description
-------------- -----------
EGL_VG_COLORSPACE_LINEAR_BIT_KHR EGLConfig supports OpenVG rendering
in linear colorspace
EGL_VG_ALPHA_FORMAT_PRE_BIT_KHR EGLConfig supports OpenVG rendering
with premultiplied alpha
Add following the second paragraph of "Other EGLConfig Attribute
Descriptions" in section 3.4 on p. 16:
"If EGL_VG_COLORSPACE_LINEAR_BIT_KHR is set in EGL_SURFACE_TYPE,
then the EGL_COLORSPACE attribute may be set to
EGL_COLORSPACE_LINEAR when creating a window, pixmap, or pbuffer
surface (see section 3.5)."
"If EGL_VG_ALPHA_FORMAT_PRE_BIT_KHR is set in EGL_SURFACE_TYPE,
then the EGL_ALPHA_FORMAT attribute may be set to
EGL_ALPHA_FORMAT_PRE when creating a window, pixmap, or pbuffer
surface (see section 3.5)."
Add at the end of the fourth paragraph (description of
EGL_CONFIG_CAVEAT) on p. 17:
"... required OpenGL ES conformance tests (note that
EGL_NON_CONFORMANT_CONFIG is obsolete, and the same information
can be obtained from the EGL_CONFORMANT_KHR attribute on a
per-client-API basis, not just for OpenGL ES."
"EGL_CONFORMANT_KHR is a mask indicating if a client API context
created with respect to the corresponding EGLConfig will pass
the required conformance tests for that API. The valid bit
settings are the same as for EGL_RENDERABLE_TYPE, as defined in
table 3.3, but the presence or absence of each client API bit
determines whether the corresponding context will be conformant
or non-conformant(fn1)."
"(fn1) most EGLConfigs should be conformant for all supported
client APIs. Conformance requirements limit the number of
non-conformant configs that an implementation can define."
Add to the last paragraph of section 3.5.1 on p. 24 (describing
eglCreateWindowSurface):
"If <config> does not support the colorspace or alpha format
attributes specified in <attrib_list> (e.g. if EGL_COLORSPACE is
specified as EGL_COLORSPACE_LINEAR but the EGL_SURFACE_TYPE
attribute of <config> does not include
EGL_VG_COLORSPACE_LINEAR_BIT_KHR, or if EGL_ALPHA_FORMAT is
specified as EGL_ALPHA_FORMAT_PRE but EGL_SURFACE_TYPE does not
include EGL_VG_ALPHA_FORMAT_PRE_BIT_KHR), an EGL_BAD_MATCH error
is generated."
Add to the next-to-last paragraph of section 3.5.2 on p. 26
(describing eglCreatePbufferSurface):
"If <config> does not support the colorspace or alpha format
attributes specified in <attrib_list> (as defined for
eglCreateWindowSurface), an EGL_BAD_MATCH error is generated."
Add to the last paragraph of section 3.5.4 on p. 29 (describing
eglCreatePixmapSurface):
"If <config> does not support the colorspace or alpha format
attributes specified in <attrib_list> (as defined for
eglCreateWindowSurface), an EGL_BAD_MATCH error is generated."
Issues
1) How should colorspace and alpha format restrictions be specified?
OpenVG implementations may not allow linear colorspace or
premultiplied alpha rendering to all configs they support.
RESOLVED: To maximize compatibility with EGL 1.3, we continue to
specify the desired colorspace and alpha format at surface
creation time. However, surface creation may fail if if the
specified colorspace or alpha format are not supported.
To allow apps to detect this situation, this extension adds
EGLConfig attributes specifying *if* linear colorspace and/or
premultiplied alpha formats are supported. If they are not
supported, surface creation with the corresponding attributes
set will fail with an EGL_BAD_MATCH error.
2) How should the colorspace and alpha format capabilities be
exposed in EGLConfigs?
RESOLVED: as bitfields of the existing EGL_SURFACE_TYPE bitmask
attribute.
A separate bitmask might be more orthogonal, but there are
plenty of unused bits in EGL_SURFACE_TYPE and this minimizes API
and programming complexity.
3) Are support for linear colorspace and and premultiplied alpha
formats orthogonal?
RESOLVED: Yes, according to the OpenVG Working Group. If they
were not orthogonal, we could not specify them as independent
bitfields.
4) Should information about conformance be specified on a
per-client-API basis?
RESOLVED: Yes. This is needed for conformance testing and cannot
be expressed by the EGL_CONFIG_CAVEAT attribute, which is OpenGL
ES-specific.
5) Should there also be a config attribute which specifies whether
EGL_RENDER_BUFFER will be respected?
UNRESOLVED: it would be consistent to add this attribute. but
it's not clear if there's a requirement for doing so yet.
6) Does this extension introduce a regression against EGL 1.2?
RESOLVED: Yes. This is unavoidable, since we're allowing failure
of surface creation that was required to succeed in the past.
However, implementations that could not support the required
colorspace or alpha mask format were effectively non-conformant
(e.g. broken) in any event. The new EGL_SURFACE_TYPE attributes
at least allow apps to know that their request will not be
satisfied.
Dependencies on OpenGL ES
If OpenGL ES is not supported, the EGL_OPENGL_ES_BIT in the
EGL_CONFORMANT_KHR is irrelevant.
Dependencies on OpenVG
If OpenVG is not supported, the EGL_OPENVG_BIT bit in
EGL_CONFORMANT_KHR, and the EGL_VG_COLORSPACE_LINEAR_BIT_KHR and
EGL_VG_ALPHA_FORMAT_PRE_BIT_KHR bits in EGL_SURFACE_TYPE, are
irrelevant.
Revision History
Version 5, 2007/04/05 - add enum values corresponding to EGL 1.3
core features.
Version 4, 2006/10/24 - prefix the bitfield names with "VG" to
clarify that they only apply to OpenVG rendering to surfaces
(although the corresponding core EGL_COLORSPACE and
EGL_ALPHA_FORMAT attribute names do not currently include this
prefix). Use "KHR" suffix instead of "OES".
Version 3, 2006/10/15 - add new config attribute to express whether
configs are conformant on a per-API basis. Correct sRGB
terminology to linear (sRGB is the default, linear colorspace
rendering may not be supported). Change extension name
accordingly.
Version 2, 2006/09/26 - add _OES extension suffix to bitfield names.
Version 1, 2006/09/26 - first draft.
