Name

    AMD_stencil_operation_extended

Name Strings

    GL_AMD_stencil_operation_extended

Contact

    Graham Sellers, AMD (graham.sellers 'at' amd.com)

Status

    Shipping

Version

    Last Modified Date:         01/11/2012
    Author Revision:            4

Number

    413

Dependencies

    OpenGL 1.0 is required.

    The definition of this extension is affected by EXT_stencil_wrap and
    OpenGL 2.0.

    This extension is written against the OpenGL Specification, Version 4.2
    (Core Profile).

Overview

    Stencil buffers are special buffers that allow tests to be made against an
    incoming value and action taken based on that value. The stencil buffer is
    updated during rasterization, and the operation used to update the stencil
    buffer is chosen based on whether the fragment passes the stencil test,
    and if it does, whether it passes the depth test. Traditional OpenGL
    includes support for several primitive operations, such as incrementing,
    or clearing the content of the stencil buffer, or replacing it with a
    specified reference value.

    This extension adds support for an additional set of operations that may
    be performed on the stencil buffer under each circumstance. Additionally,
    this extension separates the value used as the source for stencil
    operations from the reference value, allowing different values to be used
    in the stencil test, and in the update of the stencil buffer.

IP Status

    None.

New Procedures and Functions

    void StencilOpValueAMD(enum face, uint value);

New Tokens

    Accepted by the <sfail>, <dpfail> and <dppass> parameters of StencilOp
    and StencilOpSeparate:

        SET_AMD                                     0x874A
        AND                                         0x1501
        XOR                                         0x1506
        OR                                          0x1507
        NOR                                         0x1508
        EQUIV                                       0x1509
        NAND                                        0x150E
        REPLACE_VALUE_AMD                           0x874B

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

        STENCIL_OP_VALUE_AMD                        0x874C
        STENCIL_BACK_OP_VALUE_AMD                   0x874D

Additions to Chapter 2 of the OpenGL 4.2 (Core Profile) Specification
(OpenGL Operation)

    None.

Additions to Chapter 3 of the OpenGL 4.2 (Core Profile) Specification
(Rasterization)

    None.

Additions to Chapter 4 of the OpenGL 4.2 (Core Profile) Specification
(Per-Fragment Operations and the Framebuffer)

    Add to the functions introduced in Section 4.1.4 "Stencil Test", p.287

        The stencil test is controlled with

            <leave existing functions in place>
            void StencilOpValueAMD(enum face, uint value);

    Modify the paragraph beginning "StencilFuncSeparate and StencilOpSeparate
    take a <face> argument...", p.288

        StencilFuncSeparate, StencilOpSeparate and StencilOpValue take a
    <face> argument which can be FRONT, BACK or FRONT_AND_BACK and indicates
    which set of state is affected.

    Replace the second and third paragraphs on p.288, describing StencilOp and
    StencilOpSeparate with the following three paragraphs:

        StencilOp and StencilOpSeparate take three arguments that indicate what
    happens to the stored stencil value if this or certain subsequent tests fail
    or pass. sfail indicates what action is taken if the stencil test fails.
    The inputs to the stencil operation are the stencil reference value, the
    stencil operation source value, and the current content of the stencil
    buffer. The accepted symbolic constants are KEEP, ZERO, SET_AMD, REPLACE,
    REPLACE_VALUE_AMD, INCR, DECR, INVERT, INCR_WRAP, DECR_WRAP, AND, XOR, OR,
    NOR, EQUIV, and NAND. These correspond to keeping the current value, setting
    to zero, setting to the maximum representable value, replacing with the
    reference value, replacing with the operation source value, incrementing by
    the operation source  value with saturation, decrementing by the operation
    source value with saturation, bitwise inverting it, incrementing by the
    operation source value without saturation, decrementing by the operation
    source value without saturation, logically ANDing the operation source
    value value with it, logically XORing the operation source value value with
    it, logically ORing the operation source value with it, logically NORing
    the operation source value with it, logically XORing the operation source
    value with it and replacing the it with the logically inverted result of
    that computation, and logically NANDing the operation source value with it,
    respectively.

        For purposes of increment, decrement, the stencil bits are considered
    as an unsigned integer. Incrementing or decrementing with saturation clamps
    the stencil value between 0 and the maximum representable value.
    Incrementing without saturation will wrap such that incrementing the
    content of the stencil buffer in such a way that overflow occurs will cause
    the result of the operation to be masked by the number of bits representable
    by the stencil buffer. Decrementing without saturation will wrap such that
    decrementing the content of the stencil buffer in a manner such that the
    result of the subtraction would be negative causes the two's complement
    result to be interpreted as an unsigned integer and masked to the number of
    bits representable by the stencil buffer.

        The stencil operation source value is set by calling StencilOpValueAMD
    with <face> set to GL_FRONT, GL_BACK or GL_FRONT_AND_BACK, and <value> set
    to the new value of the stencil operation source value.

    Modify the paragraph beginning "If the stencil test fails, the ... ", p.288
    as follows:

        If the stencil test fails, the incoming fragment is discarded. The state
    required consists of the most recent values passed to StencilFunc or
    StencilFuncSeparate, to StencilOp or StencilOpSeparate, and to
    StencilOpValueAMD, and a bit indicating whether stencil testing is enabled
    or disabled. In the initial state, stenciling is disabled, the front and
    back reference values are both zero, the front and back stencil comparison
    functions are both ALWAYS, the front and back stencil mask are both set to
    the value 2^S - 1, where S is greater than or equal to the number of bits
    in the deepest buffer supported by the GL implementation, and the front
    and back stencil operation values are both 1. Initially, all three front
    and back stencil operations are KEEP.

Additions to Chapter 5 of the OpenGL 4.2 (Core Profile) Specification
(Special Functions)

    None.

Additions to Chapter 6 of the OpenGL 4.2 (Core Profile) Specification
(State and State Requests)

    None.

Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.

Errors

    INVALID_ENUM is generated by StencilOpValueAMD if <face> is not FRONT,
    BACK or FRONT_AND_BACK.

New State

    Modify Table 6.20 "Pixel Operations", p.394:

    +--------------------------------+----------+---------------+-----------+-------------------------------------------+--------+
    |                                |          | Get           | Initial   |                                           |        |
    | Get Value                      | Type     | Command       | Value     | Description                               | Sec.   |
    +--------------------------------+----------+---------------+-----------+-------------------------------------------+--------+
    | STENCIL_FAIL                   | Z16      | GetIntegerv   | KEEP      | Front stencil fail action                 | 4.1.4  |
    | STENCIL_PASS_DEPTH_FAIL        | Z16      | GetIntegerv   | KEEP      | Front stencil depth buffer fail action    | 4.1.4  |
    | STENCIL_PASS_DEPTH_PASS        | Z16      | GetIntegerv   | KEEP      | Front stencil depth buffer pass action    | 4.1.4  |
    | STENCIL_BACK_FAIL              | Z16      | GetIntegerv   | KEEP      | Back stencil fail action                  | 4.1.4  |
    | STENCIL_BACK_PASS_DEPTH_FAIL   | Z16      | GetIntegerv   | KEEP      | Back stencil depth buffer fail action     | 4.1.4  |
    | STENCIL_BACK_PASS_DEPTH_PASS   | Z16      | GetIntegerv   | KEEP      | Back stencil depth buffer pass action     | 4.1.4  |
    | STENCIL_OP_VALUE_AMD           | Z+       | GetIntegerv   | 1         | Front stencil operation value             | 4.1.4  |
    | STENCIL_BACK_OP_VALUE_AMD      | Z+       | GetIngeterv   | 1         | Back stencil operation value              | 4.1.4  |
    +--------------------------------+----------+---------------+-----------+-------------------------------------------+--------+

    NOTE: The existing STENCIL{_BACK}_{*} enumerants  change have changed from Z8 to Z16.

Dependencies on EXT_stencil_wrap

    If EXT_stencil_wrap is not supported, remove references to INCR_WRAP and
    DECR_WRAP. Also, change the definition of the STENCIL_{*} state to Z14
    rather than Z16.

Dependencies on OpenGL 2.0

    If the GL version is less than 2.0, remove all references to
    StencilOpSeparate. Furthermore, the <face> parameter to StencilOpValueAMD
    must be FRONT_AND_BACK, otherwise an INVALID_ENUM error will be generated.

Issues

    1) Is the stencil mask applied to the stencil operation value?

    RESOLVED: No, only to the reference and current values before the test.
    The stencil operation is carried out on the full value. The stencil write
    mask is applied to the result during stencil buffer update, however.

    2) Is this really backwards compatible? Does it break OpenGL?

    RESOLVED: Yes, this is backwards compatible and does not break anything.
    The INCR{_WRAP} and DECR{_WRAP} operations have been semantically redefined
    to be generalized add and subtract operations. However, the stencil
    operation source value is used in the addition operation and its default is
    1. Therefore, in the default state (assuming the stencil operation source
    value is not changed), INCR and DECR still increment and decrement by 1.

    3) What if I want to apply a logical operation with the stencil reference
    value and the current stencil buffer contents?

    RESOLVED: Set the stencil operation source value to the same thing as the
    stencil reference value.

Revision History

    Rev.    Date      Author    Changes
    ----  ----------  --------  -----------------------------------------
    4     01/11/2012  gsellers  Update for OpenGL 4.2. Prepare for posting.
    3     12/17/2010  yunzhang  update enum definitions
    2     06/10/2010  gsellers  Add separate stencil op value. Add issues.
    1     06/08/2010  gsellers  Initial revision