QCOM_alpha_test

GL_QCOM_alpha_test

Maurice Ribble (mribble 'at' qualcomm.com)

Benj Lipchak
Maurice Ribble

Complete.

Last Modified Date: October 11, 2010
Revision: #2

OpenGL ES Extension #89

OpenGL ES 2.0 is required.

This extension is written against the OpenGL ES 2.0 specification.

This extension reintroduces the alpha test per-fragment operation
from OpenGL ES 1.x.  Some hardware has a dedicated unit capable of
performing this operation, and it can save ALU operations in the fragment
shader by avoiding the conditional discard.

void AlphaFuncQCOM(enum func, clampf ref);

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

    ALPHA_TEST_QCOM                             0x0BC0

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

    ALPHA_TEST_FUNC_QCOM                        0x0BC1
    ALPHA_TEST_REF_QCOM                         0x0BC2

Accepted by the <func> parameter of AlphaFuncQCOM:

    NEVER
    LESS
    EQUAL
    LEQUAL
    GREATER
    NOTEQUAL
    GEQUAL
    ALWAYS

Add a new section 4.1.4 - Alpha Test

    The alpha test discards a fragment conditional on the outcome of a
    comparison between the incoming fragment's alpha value and a constant
    value.  The comparison is enabled or disabled with the generic Enable
    and Disable commands using the symbolic constant ALPHA_TEST.  When
    disabled, it is as if the comparison always passes.  The test is
    controlled with

        void AlphaFuncQCOM(enum func, clampf ref);

    <func> is a symbolic constant indicating the alpha test function; <ref>
    is a reference value.  <ref> is clamped to lie in [0,1], and then
    converted to a fixed-point value according to the rules given for an A
    component in section 2.12.8.  For purposes of the alpha test, the
    fragment's alpha value is also rounded to the nearest integer.  The
    possible constants specifying the test function are NEVER, ALWAYS, LESS,
    LEQUAL, EQUAL, GEQUAL, GREATER, or NOTEQUAL, meaning pass the fragment
    never, always, if the fragment's alpha value is less than, less than or
    equal to, equal to, greater than or equal to, greater than, or not equal
    to the reference value, respectively.

    The required state consists of the floating-point reference value, an
    eight-valued integer indicating the comparison function, and a bit
    indicating if the comparison is enabled or disabled.  The initial state
    is for the reference value to be 0 and the function to be ALWAYS.
    Initially, the alpha test is disabled.

None

(table 6.17, Pixel Operations) add the following entries:

                                       Initial
Get Value            Type  Get Command  Value   Description          Section
-------------------  ----  -----------  ------  -------------------  -------
ALPHA_TEST_QCOM      B     IsEnabled    False   Alpha test enabled   4.1.4
ALPHA_TEST_FUNC_QCOM Z8    GetIntegerv  ALWAYS  Alpha test function  4.1.4
ALPHA_TEST_REF_QCOM  R+    GetFloatv    0       Alpha test reference 4.1.4
                                                value

#01    11/04/2007    Benj Lipchak    Created based on ES 1.1 spec language.
#02    10/11/2010    Maurice Ribble  Updated to be QCOM extension and fixed
                                     some wording.