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Element Classes


Detailed Description

The element classes in Coin are the containers of state information during action traversals of scene graphs. One element usually corresponts to one item of information, or sometimes a group of related information values. The elements work like a stack that is pushed and popped as the action traverses in and out of SoSeparator nodes, and the action will always just inspect the top of the stack when it needs to know a value.

Elements are internal implementation details of the workings of nodes and actions, and is not something one needs to worry about before writing ones own extension nodes. Writing extension elements is even more removed from plain Open Inventor usage, but is fully possible for the experienced Open Inventor developer.


Classes

class  SoElement
 SoElement is the abstract base class for all elements. More...
class  SoAccumulatedElement
 The SoAccumulatedElement class is an abstract class for storing accumulated state.

This is the superclass of elements where new element data accumulates with older data. More...

class  SoOverrideElement
 The SoOverrideElement maintains a list of overridable elements and a list over which elements should be overridden.

Only certain elements can be overridden. More...

class  SoFloatElement
 SoFloatElement is an abstract base class for elements that consists of a single float value.

This is the superclass of elements where the new element data replaces the old data, and where the data the element stores is a simple single precision floating point value. More...

class  SoInt32Element
 The SoInt32Element class is the base class for elements that simply store a 32-bit integer.

This is the superclass of elements where the new element data replaces the old data, and where the data the element stores is a simple 32-bit integer value. More...

class  SoGLColorIndexElement
 The SoGLColorIndexElement class sets the current OpenGL color.

This element is only used when the OpenGL canvas is in colorindex mode, ie where colors for individual pixels are fetched from a color lookup table ("CLUT"). The usual thing to do is to set up a canvas in RGBA truecolor mode. More...

class  SoGLLineWidthElement
 The SoGLLineWidthElement class changes the linewidth setting of the OpenGL render state.

Requests from the scenegraph to change the linewidth when rendering OpenGL line primitives will be made through this element, which forwards it to the appropriate native OpenGL call. More...

class  SoGLPointSizeElement
 The SoGLPointSizeElement class changes the pointsize setting of the OpenGL render state.

Requests from the scenegraph to change the pointsize when rendering point primitives will be made through this element, which forwards it to the appropriate native OpenGL call. More...

class  SoLineWidthElement
 The SoLineWidthElement class changes the linewidth setting of the render state.

Requests from the scenegraph to change the linewidth when rendering line primitives will be made through this element, which forwards it to the appropriate native call in the underlying rendering library. More...

class  SoListenerPositionElement
 The SoListenerPositionElement holds the position of the current listener.

This position is set by SoListener nodes and SoCamera Nodes during audio rendering. When a SoListener is visited by the SoAudioRenderAction, it will add a new SoListenerPositionElement to the state, holding it's position and with the setbylistener flag set. When a SoCamera is visited by SoAudioRenderAction, it will add a new SoListenerPositionElement only if there are no previous elements with the setbylistener flag set. More...

class  SoListenerOrientationElement
 The SoListenerOrientationElement holds the orientation of the current listener.

This orientation is set by SoListener nodes and SoCamera Nodes during audio rendering. When a SoListener is visited by the SoAudioRenderAction, it will add a new SoListenerOrientationElement to the state, holding it's orientation and with the setbylistener flag set. When a SoCamera is visited by SoAudioRenderAction, it will add a new SoListenerOrientationElement only if there are no previous elements with the setbylistener flag set. More...

class  SoListenerGainElement
 The SoListenerGainElement class stores the SoListener gain during a scene graph traversal.

This gain is set by SoListener nodes during audio rendering. The SoListenerGainElement is used when the SoVRMLSound nodes render themselves. More...

class  SoListenerDopplerElement
 The SoListenerDopplerElement holds the doppler velocity and factor of the current listener.

The dopplerVelocity and dopplerFactor is set by SoListener nodes during audio rendering. The SoListenerDopplerElement is used when the SoVRMLSound nodes render themselves. More...

class  SoPointSizeElement
 The SoPointSizeElement changes the pointsize setting of the render state.

Requests from the scenegraph to change the pointsize when rendering point primitives will be made through this element. More...

class  SoReplacedElement
 The SoReplacedElement class is an abstract element superclass.

This is the superclass of all elements where the new element data replaces the old data, and where the data the element stores is not just a simple float or integer value. More...

The Element Classes

Elements are mostly internal to Coin, unless you create new extension nodes over Coin. Then you will probably need to know about them.

Elements are part of the design for scenegraph traversal in Coin.

It works like this: any traversal action instantiates and keeps a single SoState instance during traversal. The SoState instance uses SoElement objects as "memory units" to keep track of the current state for any feature of the scenegraph nodes.

As an example, consider the SoPointSize node: when the SoPointSize node is traversed by for instance a SoGLRenderAction, it will itself push a SoPointSizeElement onto the SoGLRenderAction's SoState stack. Later, when a SoPointSet node occurs in the scenegraph, it will request the current pointsize value from the SoState by reading off the value of it's SoPointSizeElement.

SoSeparator nodes will push and pop elements on and off the state stack, so anything that changes state below a SoSeparator node will not influence anything above the SoSeparator.

For more information on the theoretical underpinnings of this traversal design, you should consider reading available literature on the so-called "Visitor pattern". We recommend "Design Patterns", by Gamma, Helm, Johnson, Vlissides (aka the "Gang Of Four"). This book actually uses the Inventor API traversal mechanism as the case study for explaining the Visitor pattern.

For extending the Coin library with your own classes, we strongly recommend that you make yourself acquainted with the excellent «The Inventor Toolmaker» book (ISBN 0-201-62493-1), which describes the tasks involved in detail. This book was written by the original SGI Inventor designers and explains many of the underlying design ideas, aswell as having lots of hands-on examples on how to extend the Coin toolkit in ways that are true to the fundamental design ideas. («The Inventor Toolmaker» is also available at SGI's online library, at no cost. See Download The Inventor Toolmaker.) Reading the sourcecode of the built-in classes in Coin should also provide very helpful.

The following is a complete example on how to extend Coin with your own traversal elements. First, the class declaration of the new element (ie the header include file):

  // [texturefilenameelement.h]
  #ifndef TEXTUREFILENAMEELEMENT_H
  #define TEXTUREFILENAMEELEMENT_H

  #include <Inventor/elements/SoReplacedElement.h>
  #include <Inventor/SbString.h>

  class TextureFilenameElement : public SoReplacedElement {
    typedef SoReplacedElement inherited;

    SO_ELEMENT_HEADER(TextureFilenameElement);
  public:
    static void initClass(void);

    virtual void init(SoState * state);
    static void set(SoState * const state, SoNode * const node,
                    const SbString & filename);
    static const SbString & get(SoState * const state);
    static const TextureFilenameElement * getInstance(SoState * state);

  protected:
    virtual ~TextureFilenameElement();
    virtual void setElt(const SbString & filename);

  private:
    SbString filename;
  };

  #endif // !TEXTUREFILENAMEELEMENT_H

The implementation of the element:

  // [texturefilenameelement.cpp]
  //
  // The purpose of the code in this file is to demonstrate how you can
  // make your own elements for scene graph traversals.
  //
  // Code by Peder Blekken <pederb@sim.no>, 1999-12-09. Copyright
  // Systems in Motion.

  #include "texturefilenameelement.h"


  SO_ELEMENT_SOURCE(TextureFilenameElement);


  void
  TextureFilenameElement::initClass(void)
  {
    SO_ELEMENT_INIT_CLASS(TextureFilenameElement, inherited);
  }

  void
  TextureFilenameElement::init(SoState * state)
  {
    this->filename = "<none>";
  }

  TextureFilenameElement::~TextureFilenameElement()
  {
  }

  void
  TextureFilenameElement::set(SoState * const state, SoNode * const node,
                              const SbString & filename)
  {
    TextureFilenameElement * elem = (TextureFilenameElement *)
      SoReplacedElement::getElement(state, classStackIndex, node);
    elem->setElt(filename);
  }

  const SbString &
  TextureFilenameElement::get(SoState * const state)
  {
    return TextureFilenameElement::getInstance(state)->filename;
  }

  void
  TextureFilenameElement::setElt(const SbString & filename)
  {
    this->filename = filename;
  }

  const TextureFilenameElement *
  TextureFilenameElement::getInstance(SoState * state)
  {
    return (const TextureFilenameElement *)
      SoElement::getConstElement(state, classStackIndex);
  }

And a small, stand-alone test application putting the new element to use:

  // [lstextures.cpp]
  //
  // The purpose of this file is to make a small wrapper "tool" around
  // the TextureFilenameElement extension element, just for showing
  // example code on how to make use of a user-defined custom element.
  //
  // The code goes like this:
  //
  // We initialize the element, enable it for the SoCallbackAction, read
  // a scene graph file, set callbacks on SoTexture2 and all shape nodes
  // and applies the SoCallbackAction. The callbacks will then print out
  // the texture filename information from the TextureFilenameElement
  // each time an interesting node is hit.
  //
  //
  // Code by Peder Blekken <pederb@sim.no>. Cleaned up, integrated in
  // Coin distribution and commented by Morten Eriksen <mortene@sim.no>.
  // 1999-12-09. Copyright Systems in Motion.

  #include <Inventor/SoDB.h>
  #include <Inventor/SoInput.h>
  #include <Inventor/actions/SoCallbackAction.h>
  #include <Inventor/nodes/SoSeparator.h>
  #include <Inventor/nodes/SoTexture2.h>
  #include <Inventor/nodes/SoShape.h>
  #include <Inventor/misc/SoState.h>
  #include <stdio.h>

  #include "texturefilenameelement.h"


  SoCallbackAction::Response
  pre_tex2_cb(void * data, SoCallbackAction * action, const SoNode * node)
  {
    const SbString & filename = ((SoTexture2 *)node)->filename.getValue();
    TextureFilenameElement::set(action->getState(), (SoNode *)node, filename);

    (void)fprintf(stdout, "=> New texture: %s\n",
                  filename.getLength() == 0 ?
                  "<inlined>" : filename.getString());

    return SoCallbackAction::CONTINUE;
  }

  SoCallbackAction::Response
  pre_shape_cb(void * data, SoCallbackAction * action, const SoNode * node)
  {
    const SbString & filename =
      TextureFilenameElement::get(action->getState());

    (void)fprintf(stdout, "   Texturemap on %s: %s\n",
                  node->getTypeId().getName().getString(),
                  filename.getLength() == 0 ?
                  "<inlined>" : filename.getString());

    return SoCallbackAction::CONTINUE;
  }

  void
  usage(const char * appname)
  {
    (void)fprintf(stderr, "\n\tUsage: %s <modelfile.iv>\n\n", appname);
    (void)fprintf(stderr,
                  "\tLists all texture filenames in the model file,\n"
                  "\tand on which shape nodes they are used.\n\n"
                  "\tThe purpose of this example utility is simply to\n"
                  "\tshow how to create and use an extension element for\n"
                  "\tscene graph traversal.\n\n");
  }

  int
  main(int argc, char ** argv)
  {
    if (argc != 2) {
      usage(argv[0]);
      exit(1);
    }

    SoDB::init();

    TextureFilenameElement::initClass();
    SO_ENABLE(SoCallbackAction, TextureFilenameElement);

    SoInput input;
    if (!input.openFile(argv[1])) {
      (void)fprintf(stderr, "ERROR: couldn't open file ``%s''.\n", argv[1]);
      exit(1);
    }

    SoSeparator * root = SoDB::readAll(&input);
    if (root) {
      root->ref();
      SoCallbackAction cbaction;
      cbaction.addPreCallback(SoTexture2::getClassTypeId(), pre_tex2_cb, NULL);
      cbaction.addPreCallback(SoShape::getClassTypeId(), pre_shape_cb, NULL);
      cbaction.apply(root);
      root->unref();
      return 0;
    }
    return 1;
  }

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