Name

    MESA_clip_disable

Name Strings

    GL_MESA_clip_disable

Contact

    Stefan Dösinger (stefan 'at' codeweavers.com)

Status

    Draft
    TODO: Token values
    TODO: Formal language defining interaction with gl_FragCoord
    TODO: Implementation dependent state
    TODO: Number

Version

    Version 1, 2012/12/9

Number

    ???

Dependencies

    Written based on the wording of the OpenGL 3.1 specification.

    Some wording taken from ARB_depth_clamp.

    ARB_depth_clamp must not be supported.

Overview

    This extension exposes the ability to disable the near and far clipping
    planes in a way simmilar to ARB_depth_clamp without specifying clamping
    behavior. It is intended to be used with orthogonal transformation matrices
    and the depth test disabled. If ARB_depth_clamp is supported by the
    driver, this extension must not be exposed.

    This extension guarantees that geometry outside the near and far planes of
    the clip volume are rendered if the depth test is disabled. If the depth
    test is enabled the behavior is undefined. Reading gl_FragCoord inside a
    fragment shader is undefined.

    This functionality is provided for porting applications that use another
    popular 3D API and draw pre-transformed and pre-clipped geometry with the
    depth test disabled.

Issues

    Should we reuse the DEPTH_CLAMP token?

        UNRESOLVED: Probably not. The behavior is different and the two should
        not be confused.

        Applications that detect the presence of ARB_depth_clamp by enabling
        DEPTH_CLAMP and check for an error may be misguided.

    What are the interactions with the raster position?

        UNRESOLVED: Probably the same as in ARB_depth_clamp for the reasons
        listed there.

New Procedures and Functions

    None

New Tokens

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

    CLIP_ENABLE_MESA              0x????

Additions to Chapter 2 of the OpenGL 3.1 Specification (OpenGL Operation)

 --  Section 2.17 "Clipping"

    Add to the end of the 3rd paragraph:

    "Clipping is enabled with the generic Enable command and
    disabled with the Disable command. The value of the argument to
    either command is CLIP_ENABLE_MESA. If depth clamping is enabled, the
    "-w_c <= z_c <= w_c" plane equation are ignored by view volume
    clipping (effectively, there is no near or far plane clipping)."

    If and only if the ARB_compatibility extension is supported:

 --  Section 2.22 "Current Raster Position"

    Add to the end of the 4th paragraph:

    "If clipping (see section 2.17) is disabled, then raster
    position z_w is first clamped to the range [min(n,f),max(n,f)],
    where n and f are the current near and far depth range values (see
    section 2.12.1)."

Additions to Chapter 3 of the OpenGL 3.1 Specification (Rasterization)

    None

Additions to Chapter 4 of the OpenGL 3.1 Specification (Per-Fragment Operations
and the Framebuffer)

 --  Section 4.1.5 "Depth buffer test"

    Add to the end of the 2nd paragraph:

    "If clipping (see section 2.17) is disabled, and the depth test
    is enabled, the results are undefined."

Additions to Chapter 5 of the OpenGL 3.1 Specification (Special Functions)

    None

Additions to Chapter 6 of the OpenGL 3.1 Specification (State and State Requests)

    None

Additions to the AGL/GLX/WGL Specifications

    None

GLX Protocol

    None

Errors

    None

New State

Add to table 6.7, transformation state

Get Value   Type  Get Command  Initial Value  Description     Sec      Attribute
----------  ----  -----------  -------------  --------------  ------   ----------------
CLIP_MESA   B     IsEnabled    True           Depth clipping  2.12.2   transform/enable
                                              on/off

New Implementation Dependent State

    ???

Interactions with ARB_compatibility

    If ARB_compatibility is not supported, the language about current
    raster position in section 2.22 is removed.