## Objective¶

For more Geant4 exporter implementation specifics Geant4 Geometry in Collada

## Mapping between Collada model and Geant4 model¶

Solid instance_geometry OR geometry ?
LogicalVolume node
PhysicalVolume instance_node

## input¶

input declares the input connections to a data source that a consumer requires. A data source is a container of raw data that lacks semantic meaning so that the data can be reused within the document. To use the data, a consumer declares a connection to it with the desired semantic information.

<source id="cubeverts-array">
<float_array count="24" id="cubeverts-array-array">-50 50 50 50 50 50 -50 -50 50 50 -50 50 -50 50 -50 50 50 -50 -50 -50 -50 50 -50 -50</float_array>
<technique_common>
<accessor count="8" source="#cubeverts-array-array" stride="3">
<param type="float" name="X"/>
<param type="float" name="Y"/>
<param type="float" name="Z"/>
</accessor>
</technique_common>
</source>
<vertices id="cubeverts-array-vertices">
<input source="#cubeverts-array" semantic="POSITION"/>
</vertices>

## library_node¶

Child elements: One or more nodes.

## node¶

Parent elements: library_nodes, node, visual_scene Child elements:

• transformational: lookat/matrix/rotate/scale/skew/translate
• instance_camera/../instance_geometry/instance_node/node
• extra

The node element embodies the hierarchical relationship of elements in a scene by declaring a point of interest in a scene. A node denotes one point on a branch of the scene graph. The node element is essentially the root of a subgraph of the entire scene graph.

Within the scene graph abstraction, there are arcs and nodes. Nodes are points of information within the graph. Arcs connect nodes to other nodes. Nodes are further distinguished as interior (branch) nodes and exterior (leaf) nodes. COLLADA uses the term node to denote interior nodes. Arcs are also called paths.

The node element represents a context in which the child transformation elements are composed in the order that they occur. All the other child elements are affected equally by the accumulated transformations in the scope of the node element.

The transformation elements transform the coordinate system of the node element. Mathematically, this means that the transformation elements are converted to matrices and postmultiplied in the order in which they are specified within the node to compose the coordinate system.

## instance_node¶

An instance_node creates an instance of an object described by a node element. Each instance of a node element refers to an element in the node hierarchy that has its own local coordinate system defined for placing objects in the scene.

Parent elements: node

<library_nodes>
<node id="myNode"/>
</library_nodes>
<node>
<node>
<translate>11.0 12.0 13.0</translate>
<instance_node url="#myNode"/>
</node>
</node>

### Thoughts¶

Looks like Geant4 logical/physical. But is it beneficial to stay at such a high level into the Collada file, perhaps should collapse to global coordinate space, like VRML2 does.

If do not do that in the export, then have to do that “flattening” within whatever interprets the Collada. Does pycollada do that ? OR is pycollada just giving access to the Collada content.

## matrix¶

Parent element: node

## library_geometries¶

One or more geometry.

## geometry¶

• only a single convex_mesh, mesh, spline, brep
<library_geometries>
<geometry id="geometry0" name="mycube">
<mesh>
<source id="cubenormals-array">
<float_array count="72" id="cubenormals-array-array">0 0 1 0 0 1 0 0 1 0 0 1 0 1 0 0 1 0 0 1 0 0 1 0 0 -1 0 0 -1 0 0 -1 0 0 -1 0 -1 0 0 -1 0 0 -1 0 0 -1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 0 0 -1 0 0 -1 0 0 -1 0 0 -1</float_array>
<technique_common>
<accessor count="24" source="#cubenormals-array-array" stride="3">
<param type="float" name="X"/>
<param type="float" name="Y"/>
<param type="float" name="Z"/>
</accessor>
</technique_common>
</source>
...
<triangles count="12" material="materialref">
<input source="#cubenormals-array" semantic="NORMAL" offset="1"/>
<input source="#cubeverts-array-vertices" semantic="VERTEX" offset="0"/>
<p>0 0 2 1 3 2 0 0 3 2 1 3 0 4 1 5 5 6 0 4 5 6 4 7 6 8 7 9 3 10 6 8 3 10 2 11 0 12 4 13 6 14 0 12 6 14 2 15 3 16 7 17 5 18 3 16 5 18 1 19 5 20 7 21 6 22 5 20 6 22 4 23</p>
</triangles>
</mesh>
</geometry>
</library_geometries>

## mesh¶

Parent element: geometry

Child elements:

• source (1+)
• vertices 1
• lines/linestrips/polygons/polylist/triangles/trifans/tristrips 0+

The vertices element under mesh is used to describe mesh-vertices. Polygons, triangles, and so forth index mesh-vertices, not positions directly. Mesh-vertices must have at least one input(unshared) element with a semantic attribute whose value is POSITION.

<mesh>
<source id="cubenormals-array">
<float_array count="72" id="cubenormals-array-array">0 0 1 0 0 1 0 0 1 0 0 1 0 1 0 0 1 0 0 1 0 0 1 0 0 -1 0 0 -1 0 0 -1 0 0 -1 0 -1 0 0 -1 0 0 -1 0 0 -1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 0 0 -1 0 0 -1 0 0 -1 0 0 -1</float_array>
<technique_common>
<accessor count="24" source="#cubenormals-array-array" stride="3">
<param type="float" name="X"/>
<param type="float" name="Y"/>
<param type="float" name="Z"/>
</accessor>
</technique_common>
</source>
<source id="cubeverts-array">
<float_array count="24" id="cubeverts-array-array">-50 50 50 50 50 50 -50 -50 50 50 -50 50 -50 50 -50 50 50 -50 -50 -50 -50 50 -50 -50</float_array>
<technique_common>
<accessor count="8" source="#cubeverts-array-array" stride="3">
<param type="float" name="X"/>
<param type="float" name="Y"/>
<param type="float" name="Z"/>
</accessor>
</technique_common>
</source>
<vertices id="cubeverts-array-vertices">
<input source="#cubeverts-array" semantic="POSITION"/>
</vertices>
<triangles count="12" material="materialref">
<input source="#cubenormals-array" semantic="NORMAL" offset="1"/>
<input source="#cubeverts-array-vertices" semantic="VERTEX" offset="0"/>
<p>0 0 2 1 3 2 0 0 3 2 1 3 0 4 1 5 5 6 0 4 5 6 4 7 6 8 7 9 3 10 6 8 3 10 2 11 0 12 4 13 6 14 0 12 6 14 2 15 3 16 7 17 5 18 3 16 5 18 1 19 5 20 7 21 6 22 5 20 6 22 4 23</p>
</triangles>
</mesh>
In [36]: mesh.geometries[0].__class__

In [37]: geom = mesh.geometries[0]

In [38]: geom.
geom.bind               geom.createLineSet      geom.createPolylist     geom.double_sided       geom.load               geom.primitives         geom.sourceById
geom.collada            geom.createPolygons     geom.createTriangleSet  geom.id                 geom.name               geom.save               geom.xmlnode

In [39]: geom.primitives
Out[39]: [<TriangleSet length=12>]

In [40]: geom.primitives[0]
Out[40]: <TriangleSet length=12>

In [41]: geom.primitives[0][0]
Out[41]: <Triangle ([-50.  50.  50.], [-50. -50.  50.], [ 50. -50.  50.], "materialref")>

In [42]: geom.primitives[0][-1]
Out[42]: <Triangle ([ 50.  50. -50.], [-50. -50. -50.], [-50.  50. -50.], "materialref")>

## triangles/trifans/tristrips¶

For all of them,

• Each triangle described by the mesh has three vertices.
• The first triangle is formed from the first, second, and third vertices.
triangles
The second triangle is formed from the fourth, fifth, and sixth vertices, and so on.
trifans
Each subsequent triangle is formed from the current vertex, reusing the first and the previous vertices.
tristrips
Each subsequent triangle is formed from the current vertex, reusing the previous two vertices.

## triangles¶

The p (stands for primitive) element index values indicate the order in which the input values are used.

The indices in a p element refer to different inputs depending on their order. The first index in a p element refers to all inputs with an offset of 0. The second index refers to all inputs with an offset of 1. Each vertex of the triangle is made up of one index into each input. After each input is used, the next index again refers to the inputs with offset of 0 and begins a new vertex.

The winding order of vertices produced is counterclockwise and describes the front side of each triangle. If the primitives are assembled without vertex normals then the application may generate per-primitive normals to enable lighting.

<mesh>
<source id="position"/>
<source id="normal"/>
<vertices id="verts">
<input semantic="POSITION" source="#position"/>
</vertices>
<triangles count="2" material="Bricks">
<input semantic="VERTEX" source="#verts" offset="0"/>
<input semantic="NORMAL" source="#normal" offset="1"/>
<p>
0 0  1 3  2 1
0 0  2 1  3 2
</p>
</triangles>
</mesh>

## instance_geometry¶

Child elements: bind_material and extra

The binding of the geometry to material happens at instance_geometry allowing the same geometry to be bound to different materials.

The extra can provides arbitrary additional information.

<library_visual_scenes>
<visual_scene id="myscene">
<node name="node0" id="node0">
<instance_geometry url="#geometry0">
<bind_material>
<technique_common>
<instance_material symbol="materialref" target="#material0"/>
</technique_common>
</bind_material>
</instance_geometry>
</node>
</visual_scene>
</library_visual_scenes>
<library_geometries>
<geometry id="cube"/>
</library_geometries>
<node>
<node>
<translate>11.0 12.0 13.0</translate>
<instance_geometry url="#cube"/>
</node>
</node>