Aligning Clouds with Constraints

task target
 		 This task shows you how to align a cloud of points with a reference by defining constraints
(made of pairs of constraint elements) based on canonic shapes
(points, lines, planes, spheres, cylinders, circles and their inverted elements)
and applying a priority order on those constraints. 
See the glossary for the definition of Cloud to Align, reference, constraint and constraint element.
  This operation is useful with mechanical shapes, where canonic shapes can be defined.
When such shapes can be used, this kind of alignment is quicker than the other types
proposed by V5.

You need to recognize canonic shapes 

  • on the Cloud to Align (Quick Surface Reconstruction offers a Basic Surface Recognition command),
  • then on the References (by extracting faces or creating points, lines and planes representing fixed constraints). 

These canonic shapes are not necessarily the same (any association of points/lines/planes is possible).
Then match the canonic shapes by pair, one on the Cloud to Align, the other on the References. 
It is possible to match one constraint element of the Cloud to Align with several constraint elements
(whatever their type) of the References, or vice versa
(a plane can be matched with three different points in three different constraints, for example).

Be careful to have consistent constraints, regarding the geometry. For example:

  • do not match three points with two different planes, 
  • if you match two normals to planes, be careful that they have the same orientation, 
  • when you select a cylinder or a circle, the constraint element taken into account is their center axis, i.e. a line.
  • when you select a sphere, the constraint element taken into account is its center point.
  • even if overconstraints are accepted, in some cases it might be necessary that the constraints form an isostatic system:
    A 3D object has 6 degrees of freedom, i.e. 3 translations and 3 rotations.
    Creating an isostatic system means that the 3 translations and the 3 rotations are locked.
    Here are the degrees of freedom locked by each pair of constraints:
      point

     

    		 line

     

    		 plane

     
    point 3 translations 2 translations 1 translation
    line 2 translations 2 translations and 2 rotations 1 translation and 1 rotation

     
    plane 1 translation 1 translation and 1 rotation 1 translation and 2 rotations

In a general case, you should combine the constraints to lock all the degrees of freedom.
aprereq.gif (1231 bytes) Open the AlignConstraint1.CATPart model from the samples directory:
  • In the Geometrical Set Input, you will find a Cloud to Align (CloudToAlign) and a set of constraint elements computed on it,
  • In the Geometrical Set Reference, you will find a set of constraint elements computed on Reference.
  • You can use the Geometrical Set New as the Define In Work Object.
scenario

 

  1. Select the Geometrical Set where you want to create the output cloud, if any, and select Define In Work Object.

  2. Click Align with Constraints in the Reposit toolbar. 
    The Align with Constraints dialog box appears.

  3. Select a Cloud to Align (we have selected CloudToAlign).
    is available to hide or show the Cloud to Align
    Once the Cloud to Align is selected, the button Add becomes available.

  4. Click Add.
    Select the large cylinder recognized on the Cloud to Align, then the large cylinder on Reference. 

  5. Repeat step 4. with the small cylinder, and once again with the sphere.
    The colors of the constraint elements picked change:

    • A color is associated to each couple of constraint elements that define a constraint.
    • The constraint element selected on the Cloud to Align is displayed in a solid color,
    • The constraint element selected on the Reference is displayed in a transparent color,
    • Another color is associated to the next couple of constraint elements, etc
  • The orientation of lines or axes selected on the Cloud to Align can be set automatically or manually using the Orientation options Automatic or Manual.
    • when Automatic is selected,
      the orientation of the constraint element is not displayed and is automatically optimized.
    • when Manual is selected,
      the orientation of the constraint element is represented by a red arrow.
      It can be inverted by clicking the arrow.
    • The Automatic option may be more time consuming.
    • This functionality is not available for constraint elements on the reference.
      If you need to invert the orientation of constraint elements on the reference,
      you can use the Invert Orientation command.
 

  1. The constraints created are listed in the dialog box.

    • Clear All deletes all the constraints.
    • Delete deletes the constraint selected.
    • Add adds new constraints.
    • In the In Cloud to Align column, you see which element has been picked on the Cloud to Align.
    • In the In Reference column, you see which element has been picked on the reference.
    • In the Priority column, you see which kind of priority is applied to this constraint.

  2. Right-click the line with CylinderA1.2 and select Strict.
    This contextual menu displays the priorities that can be applied on each constraint,
    from the highest priority (Strict) to the lowest (Low).

    The dialog box is updated accordingly.

  3. By default, the Keep Initial check box is selected:
    • the initial Cloud to Align is kept as such,
    • a copy of this initial Cloud to Align is made and appears in the specification tree,
    • this copy is aligned with the References.

    Clear this check box if you do not want to create a copy of the initial Cloud to Align, e.g. because of its size. The initial Cloud to Align itself will be aligned with the References.
    To avoid inconsistencies, you cannot clear the Keep Initial check box in the following two cases:
    • when the geometry of the Cloud to Align is shared by other cloud elements, e.g. when the Cloud to Align shares points or vertices with another cloud without any parent/child relationship between them. Clearing the Keep Initial check box would move the other cloud together with the Cloud to Align, which you do not want to.
    • when the Cloud to Align is a feature, e.g. a GSD transformation feature.

  4. Click Apply, then More.

    • The Statistics windows appears.
      Alignment #1 indicates this is the first computation you have launched since you have opened the dialog box,
      For each constraint (e.g. Constraint #1) is given:
      • the type of the constraint and the constraint element selected  type: line (CylinderA1.2) / line (CylinderR1.1).
      • the priority applied to this constraint (Strict),
      • the distance (dist.=0) and the angle (angl.=0) between the constraint element on the Cloud to Align
        and the constraint element on the reference.
    • You can see that the constraint with a strict priority is fully respected.
  • You can easily retrieve the history of the computations and revert to a given combination of constraints
    before creating the aligned model.
  • You will find the distance gaps for constraints of the types point/point, point/line, point/plane, line/line,
    line/plane and plane/plane, as well as the angular gaps for constraints of the types line/line, line/plane and plane/plane.
    Since those gaps are computed from the infinite support of lines and planes that make the constraints,
    the distance between two non parallel planes (or a line and a plane) will be null.
    Only significant gaps between a line and a plane or between two planes will be displayed
    (i.e. only when the two constraint elements are parallel).
  • The Statistics are given for the constraint elements, they do not refer to the real gaps between
    the Cloud to Align and the reference. Those gaps are given by the Distance Analysis.
  • A constraint with a Strict priority is not fully respected when the distance is greater than 0.001 or the angle is greater than 0.5.
    This may happen when a constraint with a Strict priority is applied simultaneously to other constraints.
    A message warns you that the constraint could not be fully respected.
  • If the constraints are not consistent, an information message is displayed.
    You can then decide whether the result is satisfactory or not.
 

  1. Once you are satisfied with the alignment computed, click OK to validate and exit the action.

    If Keep Initial is selected:

    • A new cloud, in fact the copy of the initial Cloud to Align (CloudToAlign.1 in New in our example), is created in the specification tree, and aligned with the Reference,
    • the new cloud has the same graphic properties than the initial Cloud to Align,
    • the initial Cloud to Align is hidden. 

    If Keep Initial is not selected:

    • The Cloud to Align is aligned with the Reference and no new cloud is created in the specification tree.
    In both cases, an Axis Systems is created with:
    • AxisTrs.x (system axis computed on the Cloud to Align) and 
    • AxisRef.x (axis system created on the output cloud). 
    Those Axis Systems can be used with the Axis to Axis action on other elements to align.
    Align with previous transformation is also available.
When the alignment cannot be computed, an information message is displayed (Update error). You can:
  • delete constraints
  • delete and add other constraints
  • check the consistency of the constraints
  • eventually, re-compute the basic shapes used.

The method we suggest is:

  1. Exit the action,

  2. Restart the alignment with one single constraint,

  3. Click Apply.

  4. If the computation is still impossible, restart the alignment with another constraint.

  5. If the computation is possible, add another constraint and repeat the process from step 3 in loops until the result is satisfactory.

If constraints are deleted, the intermediate computation is erased.