NV_depth_clamp
GL_NV_depth_clamp
Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com)
Copyright NVIDIA Corporation, 2001.
Shipping
Last Modified Date: $Date: 2003/01/08 $
NVIDIA Revision: $Revision: #4 $
260
Written based on the wording of the OpenGL 1.2.1 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.
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
primitives?
RESOLUTION: All geometric primitives are affected by depth
clamping.
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?
RESOLUTION: GL_ENABLE_BIT and GL_TRANSFORM_BIT.
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?
RESOLUTION: No.
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 core OpenGL specification (section 2.11) is
worded to allow the possibility of generating fragments where wc<=0.
These should never be generated when this extension is supported.
   New Procedures and Functions |
None
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev:
DEPTH_CLAMP_NV 0x864F
| Additions to Chapter 2 of the OpenGL 1.2.1 Specification (OpenGL Operation) |
-- Section 2.11 "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_NV. If depth clamping is enabled, the
"-wc <= zc <= wc" plane equation are ignored by video volume clipping
(effectively, there is no near or far plane clipping)."
Change the 8th paragraph to indicate that only wc>0 fragments should
be generated rather than even allowing the posibility that wc<=0
fragments may be generated:
"A line segment or polygon whose vertices have wc values of differing
signs may generate multiple connected components after clipping.
GL implementations are not required to handle this situation.
That is, only the portion of the primitive that lies in the region
of wc>0 should be produced by clipping."
-- Section 2.12 "Current Raster Position"
Add to the end of the 4th paragraph:
"If depth clamping (see section 2.11) is enabled, then raster position
zw is first clamped as follows. If the raster postition's wc>0,
then zw is clamped the range [min(n,f),max(n,f)] where n and f are
the current near and far depth range values (see section 2.10.1)."
| Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization) |
None
| Additions to Chapter 4 of the OpenGL 1.2.1 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 2.11) is enabled, before the incoming
fragment's zw is compared, zw must first be clamped as follows: If the
fragment's wc>0, then zw 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 2.10.1)."
| Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions) |
None
| Additions to Chapter 6 of the OpenGL 1.2.1 Specification (State and State Requests) |
None
| Additions to the AGL/GLX/WGL Specifications |
None
None
None
(table 6.7)
Get Value Type Get Command Initial Value Description Sec Attribute
-------------- ---- ----------- ------------- -------------- ------ ----------------
DEPTH_CLAMP_NV B IsEnabled False Depth clamping 2.10.2 transform/enable
on/off
| New Implementation Dependent State |
None
None
None
 |  | Last update: November 14, 2006.
Cette page doit être lue avec un navigateur récent respectant le standard XHTML 1.1.
|
