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Name Strings
Gert Wollny (gert.wollny 'at'
Copyright (c) 2019 Collabora LTD
Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at
Version 1, 2019/01/24.
Based on ARB_depth_clamp version 4, modified 2009/08/02.
OpenGL ES 2.0 is required.
Written based on the wording of the OpenGL ES 3.2 specification.
Conventional OpenGL clips geometric primitives to a clip volume
with six faces, two of which are the near and far clip planes.
Clipping to the near and far planes of the clip volume ensures that
interpolated depth values (after the depth range transform) must be
in the [0,1] range.
In some rendering applications such as shadow volumes, it is useful
to allow line and polygon primitives to be rasterized without
clipping the primitive to the near or far clip volume planes (side
clip volume planes clip normally). Without the near and far clip
planes, rasterization (pixel coverage determination) in X and Y
can proceed normally if we ignore the near and far clip planes.
The one major issue is that fragments of a primitive may extend
beyond the conventional window space depth range for depth values
(typically the range [0,1]). Rather than discarding fragments that
defy the window space depth range (effectively what near and far
plane clipping accomplish), the depth values can be clamped to the
current depth range.
This extension provides exactly such functionality. This
functionality is useful to obviate the need for near plane capping
of stenciled shadow volumes. The functionality may also be useful
for rendering geometry "beyond" the far plane if an alternative
algorithm (rather than depth testing) for hidden surface removal is
applied to such geometry (specifically, the painter's algorithm).
Similar situations at the near clip plane can be avoided at the
near clip plane where apparently solid objects can be "seen through"
if they intersect the near clip plane.
New Procedures and Functions
New Tokens
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv,
Additions to Chapter 12 of the OpenGL ES 3.2 Specification (Fixed-Function Vertex Post-Processing)
-- Section 12.5 "Primitive Clipping"
Add to the end of the 1st paragraph:
"Depth clamping is enabled with the generic Enable command and
disabled with the Disable command. The value of the argument to
either command is DEPTH_CLAMP_EXT. 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)."
Additions to Chapter 15 of the OpenGL ES 3.2 Specification (Writing Fragments and Samples to the Framebuffer)
-- Section 15.1.3 "Depth buffer test"
Add to the end of the 2nd paragraph:
"If depth clamping (see section 12.15) is enabled, before the
incoming fragment's z_w is compared z_w is 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 12.6.1)."
Additions to the AGL/GLX/WGL Specifications
GLX Protocol
New State
Add to table 6.4, transformation state
Get Value Type Get Command Initial Value Description Sec
-------------- ---- ----------- ------------- -------------- ------
DEPTH_CLAMP_EXT B IsEnabled False Depth clamping 12.5
New Implementation Dependent State
See the issue list in GL_ARB_depth_clamp.
Can fragments with w_c <=0 be generated when this extension is supported?
RESOLUTION: No. The inequalities in OpenGL ES Specification 12.5 clarify
that only primitives that lie in the region w_c >= 0 can be produced by
clipping and the vertex normalization in 12.6 clarifies that values
w_c = 0 are prohibited. Hence fragments with w_c <= 0 should also never
be generated when this extension is supported.
How does this extension differ from ARB_depth_clamp?
Instead of DEPTH_CLAMP the parameter is called DEPTH_CLAMP_EXT.
Push/pop attrib bits are not relevant for OpenGL ES.
Revision History
Version 1, 2019/01/25 (Gert Wollny) - rewrite ARB_depth_clamp against
OpenGL ES 3.2 instead of OpenGL 3.1.