Szenengraph

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Revision as of 19:05, 25 November 2019 by Maker (talk | contribs) (Created page with "Image:Scenegraph.gif Bild von [http://www-evasion.imag.fr/~Francois.Faure/doc/inventorMentor/sgi_html/index.html Inventor mentor]")

Die Geometrie, die in den 3D-Ansichten von FreeCAD angezeigt wird, wird von der Bibliothek Coin3d dargestellt. Coin3D ist eine Implementierung des StandardsOpenInventor. Die Software unter [ http://en.wikipedia.org/wiki/Open_CASCADE openCascade] bietet ebenfalls die gleiche Funktionalität, aber schon zu Beginn von FreeCAD wurde entschieden, nicht den eingebauten openCascade Viewer zu verwenden, sondern auf die leistungsfähigere coin3D-Software zu wechseln. Eine gute Möglichkeit, etwas über diese Bibliothek zu erfahren, ist das BuchOpen Inventor Mentor.

OpenInventor ist eigentlich eine 3D Szenenbeschreibungssprache. Die in openInventor beschriebene Szene wird dann in OpenGL auf Ihrem Bildschirm dargestellt. Coin3D kümmert sich darum, so dass Programmierer sich nicht um komplexe openGL Aufrufe kümmern müssen und nur gültigen OpenInventor Code bereitstellen können. Der große Vorteil ist, dass openInventor ein sehr bekannter und gut dokumentierter Standard ist.

Eine der großen Aufgaben, die FreeCAD für Dich erledigt, ist die Übersetzung von openCascade Geometrieinformationen in die openInventor Sprache.

OpenInventor beschreibt eine 3D Szene in Form einer Szenegraph, wie die folgende:

Bild von Inventor mentor

An openInventor scenegraph describes everything that makes part of a 3D scene, such as geometry, colors, materials, lights, etc, and organizes all that data in a convenient and clear structure. Everything can be grouped into sub-structures, allowing you to organize your scene contents pretty much the way you like. Here is an example of an openInventor file:

#Inventor V2.0 ascii
 
Separator { 
    RotationXYZ {	
       axis Z
       angle 0
    }
    Transform {
       translation 0 0 0.5
    }
    Separator {	
       Material {
          diffuseColor 0.05 0.05 0.05
       }
       Transform {
          rotation 1 0 0 1.5708
          scaleFactor 0.2 0.5 0.2
       }
       Cylinder {
       }
    }
}

As you can see, the structure is very simple. You use separators to organize your data into blocks, a bit like you would organize your files into folders. Each statement affects what comes next, for example the first two items of our root separator are a rotation and a translation, both will affect the next item, which is a separator. In that separator, a material is defined, and another transformation. Our cylinder will therefore be affected by both transformations, the one who was applied directly to it and the one that was applied to its parent separator.

We also have many other types of elements to organize our scene, such as groups, switches or annotations. We can define very complex materials for our objects, with color, textures, shading modes and transparency. We can also define lights, cameras, and even movement. It is even possible to embed pieces of scripting in openInventor files, to define more complex behaviours.

If you are interested in learning more about openInventor, head directly to its most famous reference, the Inventor mentor.

In FreeCAD, normally, we don't need to interact directly with the openInventor scenegraph. Every object in a FreeCAD document, being a mesh, a part shape or anything else, gets automatically converted to openInventor code and inserted in the main scenegraph that you see in a 3D view. That scenegraph gets updated continuously when you do modifications, add or remove objects to the document. In fact, every object (in App space) has a view provider (a corresponding object in Gui space), responsible for issuing openInventor code.

But there are many advantages to be able to access the scenegraph directly. For example, we can temporarily change the appearence of an object, or we can add objects to the scene that have no real existence in the FreeCAD document, such as construction geometry, helpers, graphical hints or tools such as manipulators or on-screen information.

FreeCAD itself features several tools to see or modify openInventor code. For example, the following python code will show the openInventor representation of a selected object:

obj = FreeCAD.ActiveDocument.ActiveObject
viewprovider = obj.ViewObject
print viewprovider.toString()

But we also have a python module that allows complete access to anything managed by Coin3D, such as our FreeCAD scenegraph. So, read on to Pivy.

Mesh to Part/de
Pivy/de