SoExtrusion Class Reference
[Shapes]
Geometric shape formed by extruding a 2D cross section along a 3D spine.
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#include <Inventor/nodes/SoExtrusion.h>
Public Member Functions | |
| virtual SoType | getTypeId () const |
| SoExtrusion () | |
Static Public Member Functions | |
| static SoType | getClassTypeId () |
Public Attributes | |
| SoMFVec2f | crossSection |
| SoMFRotation | orientation |
| SoMFVec2f | scale |
Detailed Description
Geometric shape formed by extruding a 2D cross section along a 3D spine.
The SoExtrusion node specifies geometric shapes based on a two-dimensional cross section extruded along a three-dimensional spine. The cross section can be scaled and rotated at each spine point to produce a wide variety of shapes.
An SoExtrusion is defined by:
- crossSection
The shape that will be extruded, defined by a 2D piecewise linear curve in the XZ plane (described as a series of connected vertices).
- spine
The cross-section is extruded along this path, defined by a 3D piecewise linear curve (described as a series of connected vertices). This field is inherited from the parent class SoBaseExtrusion.
- scale values
The cross-section curve is scaled by this value on the X and Z axes. If one value is specified it applies to every spine point, else there should be as many values as there are points in the spine. All scale values must be > 0.
- orientation values, The cross-section curve is rotated by this value relative to a local reference system with origin at the current spine point and X / Z axes in the plane containing the cross-section curve. If one value is specified it applies to every spine point, else there should be as many values as there are points in the spine.
Shapes are constructed as follows - For each point in the spine, the cross-section curve, which is a curve in the XZ plane, is scaled about the origin by the corresponding scale parameter (first value scales in X, second value scales in Z), rotated about the origin by the corresponding orientation parameter and translated by the vector defined by the corresponding vertex of the spine curve. Each instance of the cross-section is then connected to the following instance.
The scaleMode field is used to select the points that will be scaled by the current transformation (for example SoTransform), if any. Translation and rotation are applied in all cases. The options are:
- DEFAULT : Every point in the generated geometry is scaled.
- SPINE_ONLY : Only the spine points are scaled. The cross section points are not scaled, so the cross section shape is undistorted even is scaling is non-uniform.
- SECTION_ONLY : Only the cross section points are scaled. Not currently implemented.
- NONE : SoExtrusion ignores the scaling state. Not currently implemented.
A transformed cross section is found for each joint (that is, at each vertex of the spine curve, where segments of the extrusion connect), and the joints and segments are connected to form the surface. No check is made for self-penetration. Each transformed cross section is determined as follows:
1. Start with the cross section as specified, in the XZ plane.
2. Scale it about (0, 0, 0) by the value for scale given for the current joint.
3. Apply a rotation so that when the cross section is placed at its proper location on the spine it will be oriented properly. Essentially, this means that the cross section's Y axis ( up vector coming out of the cross section) is rotated to align with an approximate tangent to the spine curve.
For all points other than the first or last: The tangent for spine[ i ] is found by normalizing the vector defined by (spine[ i +1] - spine[ i -1]).
If the spine curve is closed: The first and last points need to have the same tangent. This tangent is found as above, but using the points spine[0] for spine[ i ], spine[1] for spine[ i +1] and spine[ n -2] for spine[ i -1], where spine[ n -2] is the next to last point on the curve. The last point in the curve, spine[ n -1], is the same as the first, spine[0].
If the spine curve is not closed: The tangent used for the first point is just the direction from spine[0] to spine[1], and the tangent used for the last is the direction from spine[ n -2] to spine[ n -1].
In the simple case where the spine curve is flat in the XY plane, these rotations are all just rotations about the Z axis. In the more general case where the spine curve is any 3D curve, you need to find the destinations for all 3 of the local X, Y, and Z axes so you can completely specify the rotation. The Z axis is found by taking the cross product of:
(spine[ i -1] - spine[ i ]) and (spine[ i +1] - spine[ i ]).
If the three points are collinear then this value is zero, so take the value from the previous point. Once you have the Z axis (from the cross product) and the Y axis (from the approximate tangent), calculate the X axis as the cross product of the Y and Z axes.
4. Given the plane computed in step 3, apply the orientation to the cross-section relative to this new plane. Rotate it counterclockwise about the axis and by the angle specified in the orientation field at that joint.
5. Finally, the cross section is translated to the location of the spine point.
Surfaces of revolution: If the cross section is an approximation of a circle and the spine is straight, then the SoExtrusion is equivalent to a surface of revolution, where the scale parameters define the size of the cross section along the spine.
Cookie-cutter extrusions: If the scale is 1, 1 and the spine is straight, then the cross section acts like a cookie cutter, with the thickness of the cookie equal to the length of the spine.
Bend/twist/taper objects: These shapes are the result of using all fields. The spine curve bends the extruded shape defined by the cross section, the orientation parameters twist it around the spine, and the scale parameters taper it (by scaling about the spine).
SoExtrusion has three parts: the sides, the beginCap (the surface at the initial end of the spine) and the endCap (the surface at the final end of the spine). The caps have an associated SFBool field that indicates whether it exists (TRUE) or doesn't exist (FALSE).
When the beginCap or endCap fields are specified as TRUE, planar cap surfaces will be generated regardless of whether the crossSection is a closed curve. (If crossSection isn't a closed curve, the caps are generated as if it were -- equivalent to adding a final point to crossSection that's equal to the initial point. Note that an open surface can still have a cap, resulting (for a simple case) in a shape something like a soda can sliced in half vertically.) These surfaces are generated even if spine is also a closed curve. If a field value is FALSE, the corresponding cap is not generated.
SoExtrusion automatically generates its own normals. Orientation of the normals is determined by the vertex ordering of the quads generated by SoExtrusion. The vertex ordering is in turn determined by the crossSection curve. If the crossSection is counterclockwise when viewed from the +Y axis, then the polygons will have counterclockwise ordering when viewed from 'outside' of the shape (and vice versa for clockwise ordered crossSection curves).
Texture coordinates are automatically generated by extrusions. Textures are mapped so that the coordinates range in the U direction from 0 to 1 along the crossSection curve (with 0 corresponding to the first point in crossSection and 1 to the last) and in the V direction from 0 to 1 along the spine curve (again with 0 corresponding to the first listed spine point and 1 to the last). When crossSection is closed, the texture has a seam that follows the line traced by the crossSection's start/end point as it travels along the spine. If the endCap and/or beginCap exist, the crossSection curve is uniformly scaled and translated so that the largest dimension of the cross-section (X or Z) produces texture coordinates that range from 0.0 to 1.0. The beginCap and endCap textures' S and T directions correspond to the X and Z directions in which the crossSection coordinates are defined.
Also 3D texture coordinates are automatically generated, in a similar way to 2D textures.
NOTE: If your extrusion appears to twist unexpectedly, try setting environment variable OIV_EXTRUSION_EPSILON to a value slightly smaller number than the default, which is .998.
NOTE: If your crossSection is not convex, you must use a SoShapeHints and set the faceType field to UNKNOWN_FACE_TYPE.
FILE FORMAT/DEFAULT
- Extrusion {
| beginCap | TRUE |
| endCap | TRUE |
| spine | [ 0 0 0, 0 1 0 ] |
| scaleMode | DEFAULT |
| crossSection | [ 1 1, 1 -1, -1 -1, -1 1, 1 1 ] |
| orientation | 0 0 1 0 |
| scale | 1 1 |
SEE ALSO
- See related examples:
Constructor & Destructor Documentation
| SoExtrusion::SoExtrusion | ( | ) |
Constructor.
Member Function Documentation
| static SoType SoExtrusion::getClassTypeId | ( | ) | [static] |
Returns the type identifier for this class.
Reimplemented from SoBaseExtrusion.
| virtual SoType SoExtrusion::getTypeId | ( | ) | const [virtual] |
Returns the type identifier for this specific instance.
Reimplemented from SoBaseExtrusion.
Member Data Documentation
The shape that will be extruded, defined by a 2D piecewise linear curve in the XZ plane (described as a series of connected vertices).
Default is a square [ 1 1, 1 -1, -1 -1, -1 1, 1 1 ].
The cross-section curve is rotated by this value relative to a local reference system with origin at the current spine point and X / Z axes in the plane containing the cross-section curve.
If one value is specified it applies to every spine point, else there should be as many values as there are points in the spine. Default is no rotation.
The cross-section curve is scaled by this value on the X and Z axes.
If one value is specified it applies to every spine point, else there should be as many values as there are points in the spine. All scale values must be > 0. Default is (1,1) meaning no scaling.
The documentation for this class was generated from the following file:
- Inventor/nodes/SoExtrusion.h