OES_sample_shading

GL_OES_sample_shading

Daniel Koch, NVIDIA Corporation (dkoch 'at' nvidia.com)

Pat Brown, NVIDIA
Eric Werness, NVIDIA
Graeme Leese, Broadcom
Contributors to ARB_sample_shading
Members of the OpenGL ES working group

Copyright (c) 2011-2013 The Khronos Group Inc. Copyright terms at
    http://www.khronos.org/registry/speccopyright.html

Khronos-approved extension specifications are updated in response to
issues and bugs prioritized by the Khronos OpenGL ES Working Group. For
extensions which have been promoted to a core Specification, fixes will
first appear in the latest version of that core Specification, and will
eventually be backported to the extension document. This policy is
described in more detail at
    https://www.khronos.org/registry/OpenGL/docs/update_policy.php

Complete. 
Ratified by the Khronos Board of Promoters on 2014/03/14.

Last Modified Date:         January 30, 2014
Revision:                   8

OpenGL ES Extension #169

OpenGL ES 3.0 and GLSL ES 3.00 required.

This extension is written against the OpenGL ES 3.0.2 (April 8, 2013)
and the OpenGL ES Shading Language Specification Revision 4
(March 6, 2013) specifications.

This extension requires OES_sample_variables.

In standard multisample rendering, an implementation is allowed to
assign the same sets of fragment shader input values to each sample.
This can cause aliasing where the fragment shader input values are
used to generate a result that doesn't antialias itself, for example
with alpha-tested transparency.

This extension adds the ability to explicitly request that an
implementation use a minimum number of unique set of fragment
computation inputs when multisampling a pixel. Specifying such a
requirement can reduce aliasing that results from evaluating the
fragment computations too few times per pixel.

This extension adds new global state that controls the minimum
number of samples for which attribute data is independently
interpolated. When enabled, all fragment-shading operations
are executed independently on each sample.

void MinSampleShadingOES(float value);

Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv,
and GetInteger64v:

    SAMPLE_SHADING_OES                              0x8C36

Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
GetInteger64v, and GetFloatv:

    MIN_SAMPLE_SHADING_VALUE_OES                    0x8C37

None.

Modify Section 3.3 Multisampling p. 95

(add a new subsection at the end of the section, p. 96)

3.3.1 Sample Shading

Sample shading can be used to specify a minimum number of unique samples
to process for each fragment.  Sample shading is controlled by calling
Enable or Disable with the symbolic constant SAMPLE_SHADING_OES.

If the value of SAMPLE_BUFFERS is zero or SAMPLE_SHADING_OES is
disabled, sample shading has no effect.  Otherwise, an implementation must
provide a minimum of

    max(ceil(<mss> * <samples>),1)

unique sets of fragment shader inputs for each
fragment, where <mss> is the value of MIN_SAMPLE_SHADING_VALUE_OES and
<samples> is the number of samples (the values of SAMPLES). These are
associated with the samples in an implementation-dependent manner. The
value of MIN_SAMPLE_SHADING_VALUE_OES is specified by calling

    void MinSampleShadingOES(float value);

with <value> set to the desired minimum sample shading fraction.  <value>
is clamped to [0,1] when specified.  The sample shading fraction may be
queried by calling GetFloatv with pname set to
MIN_SAMPLE_SHADING_VALUE_OES.

When the sample shading fraction is 1.0, a separate set of fragment shader
input values are evaluated for each sample, and each set of values
is evaluated at the sample location.

None.

None.

None.

None.

None

None.

Add to Table 6.7 (Multisampling)
                                      Get      Initial
Get Value                     Type  Command     Value    Description         Sec.
---------                     ----  ---------  --------  -----------         ----
SAMPLE_SHADING_OES            B     IsEnabled  FALSE    sample coverage     3.3.1
                                                        enable
MIN_SAMPLE_SHADING_VALUE_OES  R+    GetFloatv  0        fraction of multi-  3.3.1
                                                        samples to use for
                                                        sample shading

None.

(0) This extension is based on ARB_sample_shading.  What are the major
    differences?

    1- rebased against ES 3.0
    2- various editing for consistency to GL 4.4/GLSL 440 specs
    3- parameter to MinSampleShading changed to float to match GL 4.x
    4- removed mention of SAMPLE_ALPHA_TO_ONE
    5- replaced mention of "color and texture coordinates" with more
       generic language about fragment shader inputs.
    6- removed mention of multisample enable.
    7- moved shading language sample variables to OES_sample_variables

    For historical issues, please see ARB_sample_shading.

(1) OpenGL has a MULTISAMPLE enable that was not included in OpenGL ES.
    Should we add it into this extension or base it purely on if the target
    surface is multisample?

    DISCUSSION:
    GL (4.4) says:
    "Multisample rasterization is enabled or disabled by calling Enable or
    Disable with the symbolic constant MULTISAMPLE."

    GL ES (3.0.2) says:
    "Multisample rasterization cannot be enabled or disabled after a GL
    context is created."

    RESOLVED. Multisample rasterization should be based on the target
    surface properties.  Will not pick up the explicit multisample
    enable, but the language for ES3.0.2 doesn't sound right either.
    Bug 10690 tracks this and it should be fixed in later versions
    of the ES3.0 specification.

Rev.    Date      Author    Changes
----  ----------  --------  -----------------------------------------
8     2014-01-20  dkoch     rename to OES, clean editing notess
7     2014-01-20  dkoch     Fix a typo, and dangling ARB suffix
6     2013-10-03  dkoch     Minor edits
5     2013-09-09  dkoch     Require OES_sample_variables.
                            Move gl_SampleMaskIn interaction base extension.
4     2013-09-03  gleese    Moved sample variables to OES_sample_variables
3     2013-08-26  dkoch     resolved issues 1&2 and supporting edits.
                            replaced discussion of fixed-function inputs
                            with generic language.
2     2013-08-13  dkoch     add missing suffices, follow extension template
1     2013-08-12  dkoch     Port ARB_sample_shading to ES 3.0