Name

    EXT_clip_control

Name Strings

    GL_EXT_clip_control

Contact

    Chad Versace, Google <chadversary@google.com>

Contributors

    David Reveman, Google <reveman@google.com>
    See also GL_ARB_clip_control.

Notice

    Copyright (c) 2017 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete

Version

    Last Modified Date: 2017-08-08

Number

    OpenGL ES Extension #290

Dependencies

    Requires OpenGL ES 2.0.

    This extension is written against the OpenGL ES 3.2 Specification.

Overview

    This extension provides additional clip control modes to configure how
    clip space is mapped to window space.  This extension's goal is to 1)
    allow OpenGL to effectively match Direct3D's coordinate system
    conventions, and 2) potentially improve the numerical precision of the Z
    coordinate mapping.

    This extension is a port of GL_ARB_clip_control to OpenGL ES. For the
    complete overview of this extension, refer to the "Overview" section of
    GL_ARB_clip_control.

Differences from GL_ARB_clip_control

    Append the EXT suffix to all new symbols.

    Remove references to features not found in OpenGL ES 3.2: DEPTH_CLAMP and
    and Begin/End,

    Renumber sections to match the OpenGL ES 3.2 Specification.

New Procedures and Functions

    void ClipControlEXT(enum origin, enum depth);

New Tokens

    Accepted by the <origin> parameter of ClipControl:

        LOWER_LEFT_EXT                              0x8CA1
        UPPER_LEFT_EXT                              0x8CA2

    Accepted by the <depth> parameter of ClipControl:

        NEGATIVE_ONE_TO_ONE_EXT                     0x935E
        ZERO_TO_ONE_EXT                             0x935F

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

        CLIP_ORIGIN_EXT                             0x935C
        CLIP_DEPTH_MODE_EXT                         0x935D

Additions to the OpenGL ES 3.2 Specification, Chapter 12 (Fixed-Function Vertex
Post-Processing):

 -- Modify section 12.3 "Primitive Clipping"

    Insert before the 1st paragraph:

    "The command

        ClipControl(enum origin, enum depth);

    controls the clipping volume behavior.  /origin/ must be either
    LOWER_LEFT_EXT or UPPER_LEFT_EXT, otherwise the error INVALID_ENUM is
    generated.  /depth/ must be either NEGATIVE_ONE_TO_ONE_EXT or
    ZERO_TO_ONE_EXT, otherwise the error INVALID_ENUM is generated.

      These parameters update the clip control origin and
    depth mode respectively.  The state required for clip control is one
    bit for clip control origin and one bit for clip control depth mode.
    The initial value of the clip control origin is LOWER_LEFT_EXT and the
    initial value of the depth mode is NEGATIVE_ONE_TO_ONE_EXT.

    Replace the first paragraph with:

    "Primitives are clipped to the clip volume. In clip coordinates,
    the view volume is defined by

        -w_c <= x_c <= w_c
        -w_c <= y_c <= w_c
         z_m <= z_c <= w_c

     where z_m is -w_c when the clip control depth mode is
     NEGATIVE_ONE_TO_ONE_EXT and z_m is 0 when the mode is ZERO_TO_ONE_EXT."

 -- Modify section 12.5 "Coordinate Transformations"

    Replace the 3rd paragraph with (where ^T means transpose):

    "If a vertex in clip coordinates is given by (x_c y_c z_c w_c)^T
    then the vertex's normalized device coordinates are (x_d y_d z_d)^T =
    (x_c/w_c f*y_c/w_c z_c/w_c)^T where /f/ is +1 when the clip control
    origin is LOWER_LEFT_EXT and -1 when the origin is UPPER_LEFT_EXT."

 -- Modify section 12.5.1 "Controlling the Viewport"

    Replace the 2nd sentence of the 1st paragraph with (where ^T means
    transpose):

    "The vertex's window coordinates, (x_w y_w z_w)^T are given by:

        ( x_w )     ( p_x/2 x_d + o_x )
        ( y_w )  =  ( p_y/2 y_d + o_y )
        ( z_w )     (     s z_d + b   )

    where s is (f-n)/2 and b is (n+f)/2 when the clip control depth mode
    is NEGATIVE_ONE_TO_ONE_EXT; or s is (f-n) and b is n when the mode
    is ZERO_TO_ONE_EXT."

Additions to the OpenGL ES 3.2 Specification, Chapter 13 (Fixed-Function
Primitive Assembly and Rasterization):

 -- Modify section 13.7.1 "Basic Polygon Rasterization"

    Replace the 3rd sentence of the 1st paragraph with:

    "One way to compute this area is

               n-1
               ___
               \
      a = 1/2 f \  x^i_w * y^i(+)1_w - x^i(+)1_w * y^i_w
                /
               /__

    where f is +1 when the clip control origin is LOWER_LEFT_EXT and -1 when
    the origin is UPPER_LEFT_EXT, x^i_w and y^i_w are the x and y window
    coordinates of the ith vertex of the n-vertex polygon (vertices
    are numbered starting at zero for purposes of this computation),
    and i(+)1 is (i+1) mod n."

Errors

    The error INVALID_ENUM is generated by ClipControl if origin is not
    LOWER_LEFT_EXT or UPPER_LEFT_EXT.

    The error INVALID_ENUM is generated by ClipControl if depth is not
    NEGATIVE_ONE_TO_ONE_EXT or ZERO_TO_ONE_EXT.

New State

    Get Value            Type  Get Command  Initial Value            Description      Sec   Attribute
    -------------------  ----  -----------  -----------------------  ---------------  ----  ---------
    CLIP_ORIGIN_EXT        Z2  GetIntegerv  LOWER_LEFT_EXT           Clip origin      12.4  xform
    CLIP_DEPTH_MODE_EXT    Z2  GetIntegerv  NEGATIVE_ONE_TO_ONE_EXT  Clip depth mode  12.4  xform

Issues

    See the issue list in GL_ARB_clip_control.

Revision History

    1. 2017-08-08 (Chad Versace)
        - First draft. Port of GL_ARB_clip_control to OpenGL ES.