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Name Strings
Jon Leech (jon 'at'
Mark J. Kilgard, NVIDIA Corporation (mjk 'at'
Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at
Specification Update Policy
Khronos-approved extension specifications are updated in response to
issues and bugs prioritized by the Khronos OpenGL Working Group. For
extensions which have been promoted to a core Specification, fixes will
first appear in the latest version of that core Specification, and will
eventually be backported to the extension document. This policy is
described in more detail at
Complete. Approved by the ARB on July 3, 2009.
Version 3, 2009/08/02.
Based on NV_depth_clamp version 4, modified 2003/01/08.
ARB Extension #61
Written based on the wording of the OpenGL 3.1 specification.
ARB_compatibility affects the behavior of this extension.
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.
Another way to specify this functionality is to describe it in terms
of generating the equivalent capping geometry that would need to be
drawn at the near or far clip plane to have the same effect as not
clipping to the near and far clip planes and clamping interpolated
depth values outside the window-space depth range. Should the
functionality be described this way?
RESOLUTION: No. Describing the functionality as capping is
fairly involved. Eliminating far and near plane clipping and
clamping interpolated depth values to the depth range is much
simpler to specify.
Should depth clamping affect points or just line and polygon geometric
RESOLUTION: All geometric primitives are affected by depth
In the case of points, if you render a point "in front of" the
near clip plane, it should be rendered with the zw value min(n,f)
where n and f are the near and far depth range values if depth
clamping is enabled. Similarly, a point "behind" the far clip
plane should be rendered with the zw value max(n,f).
How should the setting of the raster position function when depth
clamping is enabled?
RESOLUTION: When setting the raster position, clamp the raster
position zw to the range [min(n,f),max(n,f)] where n and f are
the near and far depth range values.
This rule ensures that the raster position zw will never be outside
the [0,1] range (because n and far are clamped to the [0,1] range).
We specify the raster position to be updated this way because
otherwise a raster position zw could be specified outside the [0,1]
range when depth clamping is enabled, but then if depth clamping
is subsequently disabled, that out-of-range raster position zw
could not be written to the depth buffer.
This semantic can make for some unexpected semantics that are
described here. Say that depth clamping is enabled and the raster
position is set to point behind the far clip plane such that the
pre-clamped zw is 2.5. Because depth clamping is enabled the
raster position zw is clamped to the current near and far depth
range values. Say these values are 0.1 and 0.9. So 2.5 is clamped
to 0.9.
Now consider what happens if a bitmap (or image rectangle) is
rendered with depth testing enabled under various situations.
If depth clamping remains enabled and the depth range is unchanged,
the bitmap fragments are generated with a zw of 0.9.
However, if depth range is subsequently set to 0.2 and 0.8 and
depth clamping is enabled, the bitmap fragments will have their
zw depth component clamped to 0.8. But if the depth range was
changed to 0.2 and 0.8 but depth range clamped is disabled, the
bitmap fragments will have a 0.9 zw depth component since then
the depth clamping is then not applied.
What push/pop attrib bits should affect the depth clamp enable?
How does depth clamping interact with depth replace operations (say
from NV_texture_shader)?
RESOLUTION: The depth clamp operation occurs as part of the depth
test so depth clamping occurs AFTER any depth replace operation
in the pipeline. A depth replace operation can reassign the
fragment's zw, but depth clamping if enabled will subsequently
clamp this new zw.
Does depth clamping affect read/draw/copy pixels operations involving
depth component pixels?
Does depth clamping occur after polygon offset?
RESOLUTION: Yes. Depth clamping occurs immediately before the
depth test.
Can fragments with wc<=0 be generated when this extension is supported?
RESOLUTION: No. The OpenGL Specification was revised in 2006/08 to
remove explicit references to w_c <= 0, since the clip volume test
disallows all such vertices. Corresponding language has been
removed from this extension specification as well. Such fragments
should never be generated.
New Procedures and Functions
New Tokens
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev:
Additions to Chapter 2 of the OpenGL 3.1 Specification (OpenGL Operation)
-- Section 2.17 "Clipping"
Add to the end of the 3rd 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. 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 depth clamping (see section 2.11) is enabled, 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)
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 depth clamping (see section 2.17) 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 2.12.1)."
Additions to Chapter 5 of the OpenGL 3.1 Specification (Special Functions)
Additions to Chapter 6 of the OpenGL 3.1 Specification (State and State Requests)
Additions to the AGL/GLX/WGL Specifications
GLX Protocol
New State
Add to table 6.7, transformation state
Get Value Type Get Command Initial Value Description Sec Attribute
-------------- ---- ----------- ------------- -------------- ------ ----------------
DEPTH_CLAMP B IsEnabled False Depth clamping 2.12.2 transform/enable
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.
Revision History
Version 1, 2009/05/14 (Jon Leech) - rewrite NV_depth_clamp against
OpenGL 3.1 instead of OpenGL 1.2.1. Clarify which functionality only
applies to GL 3.1 + ARB_compatibility. Remove NV suffix and assign
ARB extension string in preparation for inclusion in OpenGL 3.2.
Version 2, 2009/05/15 (Jon Leech) - Note interaction with
Version 3, 2009/07/01 (Jon Leech) - Match core 3.2 spec updates,
removing w_c <= 0 language. Update corresponding issue to note this
was done to the core spec in 2006/08.
Version 4, 2009/08/02 (Jon Leech) - Assign ARB extension number.