Name
EXT_clip_control
Name Strings
GL_EXT_clip_control
Contact
Chad Versace, Google
Contributors
David Reveman, Google
Daniel Koch, NVIDIA
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: 2017-10-10
Revision: 2
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 parameter of ClipControlEXT:
LOWER_LEFT_EXT 0x8CA1
UPPER_LEFT_EXT 0x8CA2
Accepted by the parameter of ClipControlEXT:
NEGATIVE_ONE_TO_ONE_EXT 0x935E
ZERO_TO_ONE_EXT 0x935F
Accepted by the parameter of GetBooleanv, GetIntegerv, and
GetFloatv:
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
ClipControlEXT(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 ClipControlEXT if origin is not
LOWER_LEFT_EXT or UPPER_LEFT_EXT.
The error INVALID_ENUM is generated by ClipControlEXT 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
------------------- ---- ----------- ----------------------- --------------- ----
CLIP_ORIGIN_EXT Z2 GetIntegerv LOWER_LEFT_EXT Clip origin 12.4
CLIP_DEPTH_MODE_EXT Z2 GetIntegerv NEGATIVE_ONE_TO_ONE_EXT Clip depth mode 12.4
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.
2. 2017-10-10 (Daniel Koch)
- Added some missing EXT suffixes, remove mention of GetDoublev