NAME
SoPolygonOffset -
The SoPolygonOffset class is a node type for ’layering’ rendering
primitives.
A common problem with realtime 3D rendering systems is that rendered
primitives which are at approximately the same depth with regard to the
camera viewpoint will appear to flicker. I.e.: from one angle one
primitive will appear to be closer, while at another angle, another
primitive will appear closer. When this happens, the rendered graphics
at that part of the scene will of course look a lot less visually
pleasing.
SYNOPSIS
#include <Inventor/nodes/SoPolygonOffset.h>
Inherits SoNode.
Public Types
enum Style { FILLED = SoPolygonOffsetElement::FILLED, LINES =
SoPolygonOffsetElement::LINES, POINTS =
SoPolygonOffsetElement::POINTS }
Public Member Functions
SoPolygonOffset (void)
virtual void doAction (SoAction *action)
virtual void callback (SoCallbackAction *action)
virtual void GLRender (SoGLRenderAction *action)
Static Public Member Functions
static void initClass (void)
Public Attributes
SoSFFloat factor
SoSFFloat units
SoSFBitMask styles
SoSFBool on
Protected Member Functions
virtual ~SoPolygonOffset ()
Detailed Description
The SoPolygonOffset class is a node type for ’layering’ rendering
primitives.
A common problem with realtime 3D rendering systems is that rendered
primitives which are at approximately the same depth with regard to the
camera viewpoint will appear to flicker. I.e.: from one angle one
primitive will appear to be closer, while at another angle, another
primitive will appear closer. When this happens, the rendered graphics
at that part of the scene will of course look a lot less visually
pleasing.
One common situation where this problem often occurs is when you
attempt to put a wireframe grid as an outline on top of filled
polygons.
The cause of the problem described above is that the Z-buffer of any
render system has a limited resolution, often at 16, 24 or 32 bits.
Because of this, primitives which are close will sometimes get the same
depth value in the Z-buffer, even though they are not actually at the
same depth-coordinate.
To rectify the flickering problem, this node can be inserted in the
scene graph at the proper place(s) to explicitly define how polygons,
lines and/or points should be offset with regard to other primitives.
As for the details of how the SoPolygonOffset::factor and
SoPolygonOffset::units should be set, we quote the OpenGL
documentation:
The value of the offset is
factor * DZ + r * units
where DZ is a measurement of the change in depth relative to the
screen area of the polygon, and r is the smallest value that is
guaranteed to produce a resolvable offset for a given
implementation. The offset is added before the depth test is
performed and before the value is written into the depth buffer.
.fi
One word of notice with regard to the above quote from the OpenGL documentation: it doesn’t really make sense to set ’factor’ and ’units’ to values with different signs, i.e. ’factor’ to a negative value and ’units’ to a positive value, or vice versa.
The pixels would then be ’pushed back’ in z-order by one part of the equation, but at the same time be ’pushed forward’ by the other part of the equation. This would most likely give very inconsistent results, but which may at first look ok.
We mention this potential for making a mistake, as it seems to be a quite common error.
Below is a simple, correct usage example:
#Inventor V2.1 ascii
Separator {
Coordinate3 { point [ -1 -1 0, 1 -1 0, 1 1 0, -1 1 0 ] }
Separator {
BaseColor { rgb 1 1 0 }
# needs to draw polygons-as-line, and not "real" lines -- see
# documentation below on why this is so:
DrawStyle { style LINES }
# draw two triangles, to get a line crossing the face of the
# polygon:
IndexedFaceSet { coordIndex [ 0, 1, 2, -1, 0, 2, 3 -1 ] }
}
PolygonOffset {
styles FILLED
factor 1.0
units 1.0
}
BaseColor { rgb 0 0.5 0 }
FaceSet { numVertices [ 4 ] }
}
.fi
Without the polygonoffset node in the above example, the lines may look irregularly stippled with some graphics card drivers, as parts of it will show through the faceset, others not. This happen on seemingly random parts, as the z-buffer floating point calculations will be fickle with regard to whether or not the polygon or the line will be closer to the camera.
See the API documentation of the SoPolygonOffset::styles field below for a discussion of one important limitation of OpenGL’s Z-buffer offset mechanism: it only works with polygons or polygons rendered in line or point mode, using the SoDrawStyle::style field.
FILE FORMAT/DEFAULTS:
PolygonOffset {
factor 1
units 1
styles FILLED
on TRUE
}
Since:
TGS Inventor 2.5
Coin 1.0
Member Enumeration Documentation
enum SoPolygonOffset::Style Enumeration of the rendering primitives which
can be influenced by an SoPolygonOffset node.
Constructor & Destructor Documentation
SoPolygonOffset::SoPolygonOffset (void) Constructor.
SoPolygonOffset::~SoPolygonOffset () [protected, virtual] Destructor.
Member Function Documentation
void SoPolygonOffset::initClass (void) [static] Sets up initialization for
data common to all instances of this class, like submitting necessary
information to the Coin type system.
Reimplemented from SoNode.
void SoPolygonOffset::doAction (SoAction * action) [virtual] This function
performs the typical operation of a node for any action.
Reimplemented from SoNode.
void SoPolygonOffset::callback (SoCallbackAction * action) [virtual] Action
method for SoCallbackAction.
Simply updates the state according to how the node behaves for the
render action, so the application programmer can use the
SoCallbackAction for extracting information about the scene graph.
Reimplemented from SoNode.
void SoPolygonOffset::GLRender (SoGLRenderAction * action) [virtual] Action
method for the SoGLRenderAction.
This is called during rendering traversals. Nodes influencing the
rendering state in any way or who wants to throw geometry primitives at
OpenGL overrides this method.
Reimplemented from SoNode.
Member Data Documentation
SoSFFloat SoPolygonOffset::factor Offset multiplication factor. Scales the
variable depth in the z-buffer of the rendered primitives.
See SoPolygonOffset’s main class documentation above for detailed
information on how the factor value is used.
Default value is 1.0.
SoSFFloat SoPolygonOffset::units Offset translation multiplication factor.
Will be multiplied with the value which represents the smallest
discrete step that can be distinguished with the underlying Z-buffer
resolution.
See SoPolygonOffset’s main class documentation above for detailed
information on how the units value is used.
Note that positive values will push geometry ’away’ into the Z-buffer,
while negative values will ’move’ geometry closer.
Default value is 1.0.
SoSFBitMask SoPolygonOffset::styles The rendering primitive type to be
influenced by this node. This is a bitmask variable, so you can select
several primitive types (out of filled polygons, lines and points) be
influenced by the offset at the same time.
There is one very important OpenGL limitation to know about in this
regard: Z-buffer offsetting can only be done for either polygons, or
for polygons rendered as lines or as points.
So attempts at using this node to offset e.g. SoLineSet /
SoIndexedLineSet or SoPointSet primitives will not work.
See the comments in the scene graph below for a detailed example on
what SoPolygonOffset can and can not do:
#Inventor V2.1 ascii
Separator {
# render polygon:
Coordinate3 { point [ -1.1 -1.1 0, 1.1 -1.1 0, 1.1 1.1 0, -1.1 1.1 0 ] }
BaseColor { rgb 0 0.5 0 }
FaceSet { numVertices [ 4 ] }
# offset polygon-as-lines to be in front of above polygon:
PolygonOffset {
styles LINES
factor -2.0
units 1.0
}
# render lines:
Coordinate3 { point [ -1 -1 0, 1 -1 0, 1 1 0, -1 1 0 ] }
BaseColor { rgb 1 1 0 }
Switch {
# change this to ’0’ to see how glPolygonOffset() does *not* work
# with ’true’ lines
whichChild 1
DEF child0 Group {
# can *not* be offset
IndexedLineSet { coordIndex [ 0, 1, 2, 3, 0, 2, -1, 1, 3 -1 ]
}
}
DEF child1 Group {
# will be offset
DrawStyle { style LINES }
FaceSet { numVertices [ 4 ] }
}
}
}
Field default value is SoPolygonOffset::FILLED.
SoSFBool SoPolygonOffset::on Whether the offset is on or off. Default is
for SoPolygonOffset::on to be TRUE.
Author
Generated automatically by Doxygen for Coin from the source code.