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. More...

#include <Inventor/nodes/SoPolygonOffset.h>

Inheritance diagram for SoPolygonOffset:

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 run-time 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 SoNode *	copy (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)

uint32_t	getNodeId (void) const

virtual SoChildList *	getChildren (void) const

virtual void	writeInstance (SoOutput *out)

virtual SoNode *	addToCopyDict (void) const

virtual void	copyContents (const SoFieldContainer *from, SbBool copyconnections)

virtual SoFieldContainer *	copyThroughConnection (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 SoField *	getField (const SbName &name) const

virtual SoField *	getEventIn (const SbName &name) const

virtual SoField *	getEventOut (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 SoAuditorList &	getAuditors (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 SoNode *	getByName (const SbName &name)

static int	getByName (const SbName &name, SoNodeList &l)

static void	initClass (void)

static void	initClasses (void)

static uint32_t	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 SoFieldContainer *	checkCopy (const SoFieldContainer *orig)

static SoFieldContainer *	findCopy (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 SoBase *	getNamedBase (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 SoFieldData *	getFieldData (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
uint32_t	uniqueId

Protected Attributes inherited from SoFieldContainer
SbBool	isBuiltIn

Static Protected Attributes inherited from SoNode
static uint32_t	nextUniqueId = 1

static int	nextActionMethodIndex = 0

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.

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 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 ( )

protectedvirtual

Destructor.

Member Function Documentation

SoType SoPolygonOffset::getTypeId ( void ) const

virtual

Returns the type identification of an object derived from a class inheriting SoBase. This is used for run-time 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 pre-defined 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 toplevel superclasses for the various class groups.

Implements SoBase.

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.

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.

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

SoPolygonOffset.h
SoPolygonOffset.cpp

Public Types

Public Member Functions

Static Public Member Functions

Public Attributes

Protected Member Functions

Static Protected Member Functions

Additional Inherited Members

Detailed Description

Member Enumeration Documentation

Constructor & Destructor Documentation

Member Function Documentation

Member Data Documentation