SoShapeHints(3)

NAME
SoShapeHints - The SoShapeHints class is a node containing hints about how to render geometry.

SYNOPSIS

#include <Inventor/nodes/SoShapeHints.h>

Inherits SoNode.

  Public Types
enum VertexOrdering { UNKNOWN_ORDERING = SoShapeHintsElement::UNKNOWN_ORDERING, CLOCKWISE = SoShapeHintsElement::CLOCKWISE, COUNTERCLOCKWISE = SoShapeHintsElement::COUNTERCLOCKWISE }
enum ShapeType { UNKNOWN_SHAPE_TYPE = SoShapeHintsElement::UNKNOWN_SHAPE_TYPE, SOLID = SoShapeHintsElement::SOLID }
enum FaceType { UNKNOWN_FACE_TYPE = SoShapeHintsElement::UNKNOWN_FACE_TYPE, CONVEX = SoShapeHintsElement::CONVEX }
enum WindingType { NO_WINDING_TYPE = 0 }

  Public Member Functions
virtual SoType getTypeId (void) const
SoShapeHints (void)
virtual void doAction (SoAction *action)
virtual void GLRender (SoGLRenderAction *action)
virtual void callback (SoCallbackAction *action)
virtual void getBoundingBox (SoGetBoundingBoxAction *action)
virtual void pick (SoPickAction *action)

  Static Public Member Functions
static SoType getClassTypeId (void)
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.

  Public Attributes
SoSFEnum vertexOrdering
SoSFEnum shapeType
SoSFEnum faceType
SoSFEnum windingType
SoSFBool useVBO
SoSFFloat creaseAngle

  Protected Member Functions
virtual const SoFieldData * getFieldData (void) const
virtual ~SoShapeHints ()

  Static Protected Member Functions
static const SoFieldData ** getFieldDataPtr (void)

  Additional Inherited Members

Detailed Description

The SoShapeHints class is a node containing hints about how to render geometry. The SoShapeHints node is used to set up clues to the rendering subsystem about how particular aspects of the subsequent geometry in the scene graph should be drawn.

Here is an example on how to display a scene in wireframe mode which only shows the triangles facing towards the camera.

#include <Inventor/nodes/SoCone.h>
#include <Inventor/nodes/SoDrawStyle.h>
#include <Inventor/nodes/SoLightModel.h>
#include <Inventor/nodes/SoSeparator.h>
#include <Inventor/nodes/SoShapeHints.h>
#include <Inventor/Win/SoWin.h>
#include <Inventor/Win/viewers/SoWinExaminerViewer.h>
int main(int, char * argv[])
{
  HWND window = SoWin::init(argv[0]);
  SoSeparator * root = new SoSeparator;
  // wireframe
  SoDrawStyle * drawStyle = new SoDrawStyle;
  drawStyle->style = SoDrawStyle::LINES;
  root->addChild(drawStyle);
  // back-face culling
  SoShapeHints * shapeHints = new SoShapeHints;
  shapeHints->vertexOrdering = SoShapeHints::COUNTERCLOCKWISE;
  shapeHints->shapeType = SoShapeHints::SOLID;
  root->addChild(shapeHints);
  // keep a solid color
  SoLightModel * lightModel = new SoLightModel;
  lightModel->model = SoLightModel::BASE_COLOR;
  root->addChild(lightModel);
  // a cone
  SoCone * cone = new SoCone;
  root->addChild(cone);
  // set up a window
  SoWinExaminerViewer * viewer = new SoWinExaminerViewer(window);
  SbViewportRegion vpRegion = viewer->getViewportRegion();
  viewer->setSceneGraph(root);
  viewer->show();
  SoWin::mainLoop();
  delete viewer;
  return 0;
}

The default settings of the rendering system are tuned towards programmer convenience. For instance, the default is to render both sides of all polygons to make sure we avoid any 'holes' in the geometry if the vertex ordering should happen to differ for different polygons.

If the programmer gives up some of this convenience and uses SoShapeHints nodes to more closely specify information about the scene graph geometry, the clues given by the SoShapeHints node(s) will then be used by the rendering subsystem to avoid certain costly operations. Significant gains in rendering speed could be seen as a result.

Be aware that the way backface culling and two-sided lighting is decided by the rendering system is not at all intuitive. Here are the common rules of how primitive shapes will render themselves with regard to how the SoShapeHints::vertexOrdering and SoShapeHints::shapeType fields are set:

•   vertexOrdering == CLOCKWISE or COUNTERCLOCKWISE, shapeType == SOLID: causes primitives to be backface culled and rendered with one-sided lighting.
•   vertexOrdering == CLOCKWISE or COUNTERCLOCKWISE, shapeType == UNKNOWN_SHAPE_TYPE: primitives are not backface culled, and they are rendered with two-sided lighting.
•   vertexOrdering == UNKNOWN_ORDERING, any shapeType: primitives are not backface culled, and they are rendered with one-sided lighting. The OpenGL vertex ordering will be set to counterclockwise ordering.
The UNKNOWN_ORDERING enum has a special and non-intuitive meaning. The ordering is really set to counterclockwise -- in OpenGL and when generating normals. However, if you want to render your geometry with one-sided lighting and backface culling disabled, you have to use this enum value, and your polygons need to be in counterclockwise ordering.
FILE FORMAT/DEFAULTS:
ShapeHints {
    vertexOrdering UNKNOWN_ORDERING
    shapeType UNKNOWN_SHAPE_TYPE
    faceType CONVEX
    creaseAngle 0
}

Member Enumeration Documentation

  enum SoShapeHints::VertexOrdering
Enumeration of available ways to specify ordering of vertices for a polygon.
Enumerator
UNKNOWN_ORDERING
    Ordering not known, render both sides of the polygon.
CLOCKWISE
    Vertices are specified in a clockwise order.
COUNTERCLOCKWISE
    Vertices are specified in a counterclockwise order.

  enum SoShapeHints::ShapeType
Enumeration of different shape types.
Enumerator
UNKNOWN_SHAPE_TYPE
    Nothing known about the shape, be conservative when rendering.
SOLID
    The subsequent shapes in the graph are all known to be completely 'closed', solid 3D shapes. Backface culling will be done if vertexOrdering is known.

  enum SoShapeHints::FaceType
Enumeration of polygon face types.
Enumerator
UNKNOWN_FACE_TYPE
    Signifies: nothing is known about subsequent polygon data, be conservative when rendering.
All polygons in the scene will be analyzed to see if they need to be tessellated (broken up) into triangles before passed on to the underlying immediate mode rendering system.
The OpenGL rendering system handles most complex polygon types, but not all: it can for instance have problems with many-sided, concave polygons (concave polygons are 'hollow', that is: rounded inwards). Coin's internal tessellator will most often handle the cases that OpenGL fails on.
So if you are seeing weird artifacts in how complex polygons are rendered, try to change the SoShapeHints::faceType field to this value and see if they are then rendered correctly.
Beware that turning on this functionality might have the effect of making the rendering performance worse. If it has a noticeable effect on your particular scene graph, we advise that you investigate whether you could change how the polygons are generated for Coin rendering and then avoid using this flag.
CONVEX
    Subsequent faces are all convex, so turn off the check for and tessellation of not convex faces.
Subsequent polygons from faceset-type nodes (like SoFaceSet and SoIndexedFaceSet) will be sent unmodified to OpenGL, thereby assuming that the polygons are in a form handled by OpenGL.

  enum SoShapeHints::WindingType
Enumeration of different winding types.
Enumerator
NO_WINDING_TYPE
    Draw without holes.

Constructor & Destructor Documentation

  SoShapeHints::SoShapeHints (void)
Constructor.

  SoShapeHints::~SoShapeHints () [protected], [virtual]
Destructor.

Member Function Documentation

  SoType SoShapeHints::getClassTypeId (void) [static]
This static method returns the SoType object associated with objects of this class.

  SoType SoShapeHints::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.

Implements SoBase.

  const SoFieldData ** SoShapeHints::getFieldDataPtr (void) [static], [protected]
This API member is considered internal to the library, as it is not likely to be of interest to the application programmer.
 Returns the SoFieldData class which holds information about fields in this node.

  const SoFieldData * SoShapeHints::getFieldData (void) const [protected], [virtual]
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 SoShapeHints::doAction (SoAction * action) [virtual]
This function performs the typical operation of a node for any action.
Reimplemented from SoNode.

  void SoShapeHints::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.

  void SoShapeHints::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 SoShapeHints::getBoundingBox (SoGetBoundingBoxAction * action) [virtual]
Action method for the SoGetBoundingBoxAction.
Calculates bounding box and center coordinates for node and modifies the values of the action to encompass the bounding box for this node and to shift the center point for the scene more towards the one for this node.
Nodes influencing how geometry nodes calculate their bounding box also override this method to change the relevant state variables.
Reimplemented from SoNode.

  void SoShapeHints::pick (SoPickAction * action) [virtual]
Action method for SoPickAction.
Does common processing for SoPickAction action instances.
Reimplemented from SoNode.

Member Data Documentation

  SoSFEnum SoShapeHints::vertexOrdering
Specifies how vertices are ordered for polygon faces.
Set this field to SoShapeHints::CLOCKWISE or SoShapeHints::COUNTERCLOCKWISE if possible to turn on backface culling and thereby optimize rendering.
Default value is SoShapeHints::UNKNOWN_ORDERING.

  SoSFEnum SoShapeHints::shapeType
Hint about whether or not shapes are known to be 'closed'. Default value is SoShapeHints::UNKNOWN_SHAPE_TYPE.

  SoSFEnum SoShapeHints::faceType
Hint about whether or not polygon faces are known to be convex. Default value is SoShapeHints::CONVEX.

  SoSFEnum SoShapeHints::windingType
Hint about the winding type that is used to define holes in a polygon. Default value is SoShapeHints::NO_WINDING_TYPE, i.e. no winding type is used and therefore no holes are drawn. Up to now Coin does not support other winding types than the default.

  SoSFFloat SoShapeHints::creaseAngle
When normals are automatically generated by Coin (i.e. SoNormal nodes are not used), this is the smallest angle between two faces where we would still calculate normals to do flat shading.
If the angle between the normals of two neighboring faces is less than the value of this field, the faces will be smooth shaded around their common edge.
The angle is specified in radians, and the default value is 0.0, meaning no smoothing will be done by default.

Author
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