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SoPolygonOffset.3coin2

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::StyleEnumeration of the rendering primitives which can be influenced by an SoPolygonOffset node.

Constructor & Destructor Documentation

SoPolygonOffset::SoPolygonOffset (void)Constructor.

References factor, on, styles, and units.

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.

References factor, SoOverrideElement::getPolygonOffsetOverride(), SoAction::getState(), SoNode::isOverride(), on, SoFieldContainer::set(), SoOverrideElement::setPolygonOffsetOverride(), styles, and units.

Referenced by callback(), and GLRender().

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.

References doAction().

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.

References doAction().

Member Data Documentation

SoSFFloat SoPolygonOffset::factorOffset 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.

Referenced by doAction(), and SoPolygonOffset().

SoSFFloat SoPolygonOffset::unitsOffset 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.

Referenced by doAction(), and SoPolygonOffset().

SoSFBitMask SoPolygonOffset::stylesThe 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.

Referenced by doAction(), and SoPolygonOffset().

SoSFBool SoPolygonOffset::onWhether the offset is on or off. Default is for SoPolygonOffset::on to be TRUE.

Referenced by doAction(), and SoPolygonOffset().

Author

Generated automatically by Doxygen for Coin from the source code.