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Name
ARM_pixmap_multisample_discard
Name Strings
EGL_ARM_pixmap_multisample_discard
Contributors
Arne Bergene Fossaa
Tom Cooksey
Endre Sund
David Garbett
Contacts
Tom Cooksey (tom 'dot' cooksey 'at' arm 'dot' com)
Status
Complete.
Version
Version 1, March 5, 2013
Number
EGL Extension #54
Dependencies
EGL 1.0 is required.
This extension is written against the wording of the EGL 1.4 Specification.
Overview
ARM_pixmap_multisample_discard adds an attribute to eglCreatePixmapSurface
that allows the client API implementation to resolve a multisampled pixmap
surface, therefore allowing the multisample buffer to be discarded.
Some GPU architectures - such as tile-based renderers - are capable of
performing multisampled rendering by storing multisample data in internal
high-speed memory and downsampling the data when writing out to external
memory after rendering has finished. Since per-sample data is never written
out to external memory, this approach saves bandwidth and storage space. In
this case multisample data gets discarded, however this is acceptable in
most cases.
The extension provides the EGL_DISCARD_SAMPLES_ARM attribute that allows
for implicit resolution when rendering to a pixmap surface. This complements
the OpenGL ES EXT_multisampled_render_to_texture extension which provides
similar functionality for rendering to an OpenGL ES texture.
New Types
None
New Procedures and Functions
None
New Tokens
Accepted as an attribute name in the <attrib_list> argument of
eglCreatePixmapSurface and by the <attribute> parameter of eglQuerySurface:
EGL_DISCARD_SAMPLES_ARM 0x3286
Changes to Chapter 3 of the EGL 1.4 Specification (EGL Functions and Errors)
Modify the second paragraph under "The Multisample Buffer" of Section 3.4,
page 18 (Configuration Management)
"Operations such as posting a surface with eglSwapBuffers (see section
3.9.1, copying a surface with eglCopyBuffers (see section 3.9.2), reading
from the color buffer using client API commands, binding a client API
context to a surface (see section 3.7.3), and flushing to a pixmap surface
created with the EGL_DISCARD_SAMPLES_ARM attribute enabled (see
section 3.5.4) may cause resolution of the multisample buffer to the color
buffer."
Modify the fifth paragraph under "The Multisample Buffer" of Section 3.4,
page 18 (Configuration Management)
"There are no single-sample depth or stencil buffers for a multisample
EGLConfig, or with a pixmap surface created with the
EGL_DISCARD_SAMPLES_ARM attribute (see section 3.5.4). The only depth and
stencil buffers are those in the multisample buffer. If the color samples
in the multisample buffer store fewer bits than are stored in the color
buffers, this fact will not be reported accurately. Presumably a
compression scheme is being employed, and is expected to maintain an
aggregate resolution equal to that of the color buffers."
Modify the fifth paragraph of Section 3.5.4, page 34 (Creating Native
Pixmap Rendering Surfaces)
"attrib list specifies a list of attributes for the pixmap. The list has the
same structure as described for eglChooseConfig. Attributes that can be
specified in attrib list include EGL_VG_COLORSPACE, EGL_VG_ALPHA_FORMAT and
EGL_DISCARD_SAMPLES_ARM."
Add the following between paragraphs eight and nine of Section 3.5.4,
page 34 (Creating Native Pixmap Rendering Surfaces)
"EGL_DISCARD_SAMPLES_ARM specifies whether the client API implementation is
allowed to implicitly resolve the multisample buffer. On some GPU
architectures - such as tile-based renderers - an implicit resolve can avoid
writing the multisample buffer back to external memory as the multisample
data is stored in internal high-speed memory.
The implicit resolve can occur when the client API uses the pixmap as the
source or destination of any operation, when flushing to the pixmap or when
the client API unbinds (or breaks) the pixmap. When these operations occur
is dependent on the client API implementation. They can occur as an explicit
part of client API functions (such as glFinish, glReadPixels and
glCopyTexImage) or they can occur implicitly.
Further rendering causes the implementation to read the surface buffer and
any ancillary buffers back in as single-sampled data.
Therefore use of this attribute may result in lower quality images.
Valid values are EGL_TRUE, in which case the multisample buffer can be
discarded, or EGL_FALSE, in which case the multisample buffer is preserved.
The default value is EGL_FALSE.
Note that the multisample buffer may be discarded during eglMakeCurrent
regardless of the value of the EGL_DISCARD_SAMPLES_ARM attribute (see
section 3.7.3)."
Modify the ninth paragraph of Section 3.5.4, page 34 (Creating Native
Pixmap Rendering Surfaces)
"On failure eglCreatePixmapSurface returns EGL_NO_SURFACE. If the attributes
of pixmap do not correspond to config, then an EGL_BAD_MATCH error is
generated. If config does not support rendering to pixmaps (the
EGL_SURFACE_TYPE attribute does not contain EGL_PIXMAP_BIT), an
EGL_BAD_MATCH error is generated. If config does not support the colorspace
or alpha format attributes specified in attriblist (as defined for
eglCreateWindowSurface), an EGL_BAD_MATCH error is generated. If config does
not specify non-zero EGL_SAMPLES and EGL_SAMPLE_BUFFERS and the
EGL_DISCARD_SAMPLES_ARM attribute is set to EGL_TRUE, then an EGL_BAD_MATCH
error is generated. If config is not a valid EGLConfig, an EGL_BAD_CONFIG
error is generated. If pixmap is not a valid native pixmap handle, then an
EGL_BAD_NATIVE_PIXMAP error should be generated. If there is already an
EGLSurface associated with pixmap (as a result of a previous
eglCreatePixmapSurface call), then a EGL_BAD_ALLOC error is generated.
Finally, if the implementation cannot allocate resources for the new EGL
pixmap, an EGL_BAD_ALLOC error is generated."
Add the following entry to Table 3.5, page 36
(Queryable surface attributes and types)
Attribute Type Description
------------------------- ------- ---------------------------------------
EGL_DISCARD_SAMPLES_ARM boolean Multisample resolve when flushing to
surface
Add the following paragraph before the last paragraph of Section 3.5.7,
page 38 (Surface Attributes)
"Querying EGL_DISCARD_SAMPLES_ARM returns whether a multisample resolve
is forced on every flush to the surface (see section 3.5.4). This will only
return EGL_TRUE for pixmap surfaces created with the EGL_DISCARD_SAMPLES_ARM
attribute set to EGL_TRUE. EGL_FALSE will be returned for window and
pbuffer surfaces."
Issues
1. Should eglSurfaceAttrib accept EGL_DISCARD_SAMPLES_ARM?
RESOLVED: No. The attribute should be decided at surface creation time.
2. Should eglCreateWindowSurface or eglCreatePbufferSurface accept
EGL_DISCARD_SAMPLES_ARM?
RESOLVED: No. While the attribute could equally apply to window and
pbuffer surfaces, no use case has been identified to justify the
additional maintenance this would require.
Revision History
Version 1, 2013/03/05 - Original release.
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