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SoPolygonOffset Class Reference

The SoPolygonOffset class is a node type for "layering" rendering primitives. More...

#include <Inventor/nodes/SoPolygonOffset.h>

Inheritance diagram for SoPolygonOffset:
SoNode SoFieldContainer SoBase

Public Types

enum  Style { FILLED = SoPolygonOffsetElement::FILLED, LINES = SoPolygonOffsetElement::LINES, POINTS = SoPolygonOffsetElement::POINTS }
 
- Public Types inherited from SoNode
enum  NodeType {
  INVENTOR = 0x0000, VRML1 = 0x0001, VRML2 = 0x0002, INVENTOR_1 = 0x0004,
  INVENTOR_2_0 = 0x0008, INVENTOR_2_1 = 0x0010, INVENTOR_2_5 = 0x0020, INVENTOR_2_6 = 0x0040,
  COIN_1_0 = 0x0080, COIN_2_0 = 0x0100, EXTENSION = 0x0200, COIN_2_2 = 0x0400,
  COIN_2_3 = 0x0800, COIN_2_4 = 0x1000, INVENTOR_5_0 = 0x2000, COIN_2_5 = 0x4000,
  COIN_3_0 = 0x8000, INVENTOR_6_0 = 0x10000, COIN_4_0 = 0x20000
}
 

Public Member Functions

virtual SoType getTypeId (void) const
 Returns the type identification of an object derived from a class inheriting SoBase. This is used for runtime type checking and "downward" casting. More...
 
 SoPolygonOffset (void)
 
virtual void doAction (SoAction *action)
 
virtual void callback (SoCallbackAction *action)
 
virtual void GLRender (SoGLRenderAction *action)
 
- Public Member Functions inherited from SoNode
void setOverride (const SbBool state)
 
SbBool isOverride (void) const
 
void setNodeType (const NodeType type)
 
NodeType getNodeType (void) const
 
virtual SoNodecopy (SbBool copyconnections=FALSE) const
 
virtual SbBool affectsState (void) const
 
virtual void GLRenderBelowPath (SoGLRenderAction *action)
 
virtual void GLRenderInPath (SoGLRenderAction *action)
 
virtual void GLRenderOffPath (SoGLRenderAction *action)
 
virtual void getBoundingBox (SoGetBoundingBoxAction *action)
 
virtual void getMatrix (SoGetMatrixAction *action)
 
virtual void handleEvent (SoHandleEventAction *action)
 
virtual void pick (SoPickAction *action)
 
virtual void rayPick (SoRayPickAction *action)
 
virtual void search (SoSearchAction *action)
 
virtual void write (SoWriteAction *action)
 
virtual void audioRender (SoAudioRenderAction *action)
 
virtual void getPrimitiveCount (SoGetPrimitiveCountAction *action)
 
virtual void grabEventsSetup (void)
 
virtual void grabEventsCleanup (void)
 
virtual void startNotify (void)
 
virtual void notify (SoNotList *l)
 
SbUniqueId getNodeId (void) const
 
virtual SoChildListgetChildren (void) const
 
virtual void writeInstance (SoOutput *out)
 
virtual SoNodeaddToCopyDict (void) const
 
virtual void copyContents (const SoFieldContainer *from, SbBool copyconnections)
 
virtual SoFieldContainercopyThroughConnection (void) const
 
- Public Member Functions inherited from SoFieldContainer
void setToDefaults (void)
 
SbBool hasDefaultValues (void) const
 
SbBool fieldsAreEqual (const SoFieldContainer *container) const
 
void copyFieldValues (const SoFieldContainer *container, SbBool copyconnections=FALSE)
 
SbBool set (const char *const fielddata)
 
void get (SbString &fielddata)
 
virtual int getFields (SoFieldList &l) const
 
virtual int getAllFields (SoFieldList &l) const
 
virtual SoFieldgetField (const SbName &name) const
 
virtual SoFieldgetEventIn (const SbName &name) const
 
virtual SoFieldgetEventOut (const SbName &name) const
 
SbBool getFieldName (const SoField *const field, SbName &name) const
 
SbBool enableNotify (const SbBool flag)
 
SbBool isNotifyEnabled (void) const
 
SbBool set (const char *fielddata, SoInput *input)
 
void get (SbString &fielddata, SoOutput *out)
 
virtual SbBool validateNewFieldValue (SoField *field, void *newval)
 
virtual void addWriteReference (SoOutput *out, SbBool isfromfield=FALSE)
 
SbBool getIsBuiltIn (void) const
 
virtual void getFieldsMemorySize (size_t &managed, size_t &unmanaged) const
 
void setUserData (void *userdata) const
 
void * getUserData (void) const
 
- Public Member Functions inherited from SoBase
void ref (void) const
 
void unref (void) const
 
void unrefNoDelete (void) const
 
int32_t getRefCount (void) const
 
void touch (void)
 
SbBool isOfType (SoType type) const
 Returns TRUE if the type of this object is either of the same type or inherited from type. More...
 
virtual SbName getName (void) const
 
virtual void setName (const SbName &newname)
 
void addAuditor (void *const auditor, const SoNotRec::Type type)
 
void removeAuditor (void *const auditor, const SoNotRec::Type type)
 
const SoAuditorListgetAuditors (void) const
 
SbBool shouldWrite (void)
 
void assertAlive (void) const
 

Static Public Member Functions

static SoType getClassTypeId (void)
 
static void initClass (void)
 
- Static Public Member Functions inherited from SoNode
static uint32_t getCompatibilityTypes (const SoType &nodetype)
 
static SoType getClassTypeId (void)
 
static SoNodegetByName (const SbName &name)
 
static int getByName (const SbName &name, SoNodeList &l)
 
static void initClass (void)
 
static void initClasses (void)
 
static SbUniqueId getNextNodeId (void)
 
static int getActionMethodIndex (const SoType type)
 
static void getBoundingBoxS (SoAction *action, SoNode *node)
 
static void GLRenderS (SoAction *action, SoNode *node)
 
static void callbackS (SoAction *action, SoNode *node)
 
static void getMatrixS (SoAction *action, SoNode *node)
 
static void handleEventS (SoAction *action, SoNode *node)
 
static void pickS (SoAction *action, SoNode *node)
 
static void rayPickS (SoAction *action, SoNode *node)
 
static void searchS (SoAction *action, SoNode *node)
 
static void writeS (SoAction *action, SoNode *node)
 
static void audioRenderS (SoAction *action, SoNode *node)
 
static void getPrimitiveCountS (SoAction *action, SoNode *node)
 
- Static Public Member Functions inherited from SoFieldContainer
static void initClass (void)
 
static SoType getClassTypeId (void)
 
static void cleanupClass (void)
 
static void initCopyDict (void)
 
static void addCopy (const SoFieldContainer *orig, const SoFieldContainer *copy)
 
static SoFieldContainercheckCopy (const SoFieldContainer *orig)
 
static SoFieldContainerfindCopy (const SoFieldContainer *orig, const SbBool copyconnections)
 
static void copyDone (void)
 
- Static Public Member Functions inherited from SoBase
static void initClass (void)
 Sets up initialization for data common to all instances of this class, like submitting necessary information to the Coin type system.
 
static SoType getClassTypeId (void)
 This static method returns the SoType object associated with objects of this class.
 
static void addName (SoBase *const base, const char *const name)
 
static void removeName (SoBase *const base, const char *const name)
 
static void incrementCurrentWriteCounter (void)
 
static void decrementCurrentWriteCounter (void)
 
static SoBasegetNamedBase (const SbName &name, SoType type)
 
static int getNamedBases (const SbName &name, SoBaseList &baselist, SoType type)
 
static SbBool read (SoInput *input, SoBase *&base, SoType expectedtype)
 
static void setInstancePrefix (const SbString &c)
 
static void setTraceRefs (SbBool trace)
 
static SbBool getTraceRefs (void)
 
static SbBool connectRoute (SoInput *input, const SbName &fromnodename, const SbName &fromfieldname, const SbName &tonodename, const SbName &tofieldname)
 
static SbBool readRoute (SoInput *input)
 

Public Attributes

SoSFFloat factor
 
SoSFFloat units
 
SoSFBitMask styles
 
SoSFBool on
 

Protected Member Functions

virtual const SoFieldDatagetFieldData (void) const
 
virtual ~SoPolygonOffset ()
 
- Protected Member Functions inherited from SoNode
 SoNode (void)
 
virtual ~SoNode ()
 
virtual SbBool readInstance (SoInput *in, unsigned short flags)
 
- Protected Member Functions inherited from SoFieldContainer
 SoFieldContainer (void)
 
virtual ~SoFieldContainer ()
 
- Protected Member Functions inherited from SoBase
 SoBase (void)
 
virtual ~SoBase ()
 
virtual void destroy (void)
 
SbBool hasMultipleWriteRefs (void) const
 
SbBool writeHeader (SoOutput *out, SbBool isgroup, SbBool isengine) const
 
void writeFooter (SoOutput *out) const
 
virtual const char * getFileFormatName (void) const
 
virtual SoNotRec createNotRec (void)
 

Static Protected Member Functions

static const SoFieldData ** getFieldDataPtr (void)
 
- Static Protected Member Functions inherited from SoNode
static const SoFieldData ** getFieldDataPtr (void)
 
static void setNextActionMethodIndex (int index)
 
static int getNextActionMethodIndex (void)
 
static void incNextActionMethodIndex (void)
 
static void setCompatibilityTypes (const SoType &nodetype, const uint32_t bitmask)
 
- Static Protected Member Functions inherited from SoBase
static uint32_t getCurrentWriteCounter (void)
 
static void staticDataLock (void)
 
static void staticDataUnlock (void)
 

Additional Inherited Members

- Protected Types inherited from SoBase
enum  BaseFlags { IS_ENGINE = 0x01, IS_GROUP = 0x02 }
 
- Protected Attributes inherited from SoNode
SbUniqueId uniqueId
 
- Protected Attributes inherited from SoFieldContainer
SbBool isBuiltIn
 
- Static Protected Attributes inherited from SoNode
static SbUniqueId nextUniqueId = 1
 
static int nextActionMethodIndex = 0
 

Detailed Description

The SoPolygonOffset class is a node type for "layering" rendering primitives.

A common problem with real time 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.

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 ] }
}

Without the polygon offset 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 {
styles FILLED
on TRUE
}
Since
TGS Inventor 2.5
Coin 1.0

Member Enumeration Documentation

◆ Style

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

Constructor & Destructor Documentation

◆ SoPolygonOffset()

SoPolygonOffset::SoPolygonOffset ( void  )

Constructor.

◆ ~SoPolygonOffset()

SoPolygonOffset::~SoPolygonOffset ( )
protectedvirtual

Destructor.

Member Function Documentation

◆ getTypeId()

SoType SoPolygonOffset::getTypeId ( void  ) const
virtual

Returns the type identification of an object derived from a class inheriting SoBase. This is used for runtime type checking and "downward" casting.

Usage example:

void foo(SoNode * node)
{
if (node->getTypeId() == SoFile::getClassTypeId()) {
SoFile * filenode = (SoFile *)node; // safe downward cast, knows the type
}
}

For application programmers wanting to extend the library with new nodes, engines, nodekits, draggers or others: this method needs to be overridden in all subclasses. This is typically done as part of setting up the full type system for extension classes, which is usually accomplished by using the predefined macros available through for instance Inventor/nodes/SoSubNode.h (SO_NODE_INIT_CLASS and SO_NODE_CONSTRUCTOR for node classes), Inventor/engines/SoSubEngine.h (for engine classes) and so on.

For more information on writing Coin extensions, see the class documentation of the top level superclasses for the various class groups.

Implements SoBase.

◆ getFieldData()

const SoFieldData * SoPolygonOffset::getFieldData ( void  ) const
protectedvirtual

Returns a pointer to the class-wide field data storage object for this instance. If no fields are present, returns NULL.

Reimplemented from SoFieldContainer.

◆ doAction()

void SoPolygonOffset::doAction ( SoAction action)
virtual

This function performs the typical operation of a node for any action.

Reimplemented from SoNode.

◆ callback()

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.

◆ GLRender()

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 want to throw geometry primitives at OpenGL override this method.

Reimplemented from SoNode.

Member Data Documentation

◆ factor

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.

◆ units

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.

◆ styles

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 cannot 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.

◆ on

SoSFBool SoPolygonOffset::on

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


The documentation for this class was generated from the following files: