Concept – 2D Metrology

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This chapter contains operators for 2D metrology.

Concept of 2D Metrology

With 2D metrology, you can measure the dimensions of objects that can be represented by specific geometric primitives. The geometric shapes that can be measured comprise circles, ellipses, rectangles, and lines. You need approximate values for the positions, orientations, and dimensions of the objects to measure. Then, the real edge positions of the objects in the image are located near the boundaries of the approximate objects. With these edge positions, the parameters of the geometric shapes are optimized to better fit to the image data and are returned as measurement result.

The approximate values for the shape parameters of an object as well as some parameters that control the measurement are stored in a data structure that is called metrology object. The edges of the object in the image are located within so-called measure regions. These are rectangular regions that are arranged perpendicular to the boundaries of the metrology objects. Parameters that adjust the dimension and distribution of the measure regions are specified together with the approximate shape parameters for each metrology object. When the measurement is applied, the edge positions inside all measure regions are determined and fitted to geometric shapes using a RANSAC algorithm. All metrology objects, all further information that is necessary for the measurement, and the measurement results are stored in a data structure that is called metrology model.

(1)	(2)

The geometric shapes in (1) are measured using 2D Metrology (2): A metrology model with 4 metrology objects (blue contours) is created. Using the edge positions (cyan crosses) located within the measure regions (gray rectangles) for each metrology object, the geometric shapes (green contours) are fitted and their parameters can be queried. As shown for the circles, more than one instance per object can be found. This image is from the example program apply_metrology_model.hdev.

In the following, the steps that are required to use 2D metrology are described briefly.

• Create the metrology model and specify the image size:

  First, a metrology model must be created using

  • create_metrology_modelCreateMetrologyModel.

  The metrology model is used as a container for one or more metrology objects. For an efficient measurement, after creating the metrology model, the image size of the image in which the measurements will be performed should be specified using

  • set_metrology_model_image_sizeSetMetrologyModelImageSize.

• Provide approximate values:

  Then, metrology objects are added to the metrology model. Each metrology object consists of the approximate shape parameters for the corresponding object in the image and of the parameters that control the measurement. The parameters that control the measurement comprise, e.g., parameters that specify the dimension and distribution of the measure regions. Furthermore, several generic parameters can be adjusted for each metrology object. The metrology objects are specified with

  • add_metrology_object_circle_measureAddMetrologyObjectCircleMeasure for circles,

  • add_metrology_object_ellipse_measureAddMetrologyObjectEllipseMeasure for ellipses,

  • add_metrology_object_rectangle2_measureAddMetrologyObjectRectangle2Measure for rectangles, and

  • add_metrology_object_line_measureAddMetrologyObjectLineMeasure for lines.

  • add_metrology_object_genericAddMetrologyObjectGeneric allows to create metrology objects of different shapes (e.g., ellipse, circle, etc.) using one operator.

  To visually inspect the defined metrology objects, you can access their XLD contours with the operator get_metrology_object_model_contourGetMetrologyObjectModelContour. To visually inspect the created measure regions, you can access their XLD contours with the operator get_metrology_object_measuresGetMetrologyObjectMeasures.

• Modify the model parameters:

  If a camera calibration has been performed, the camera parameters and the pose of the measurement plane can be set with

  • set_metrology_model_paramSetMetrologyModelParam.

  Then, the result of the measurements returned by get_metrology_object_resultGetMetrologyObjectResult will be in world coordinates. The reference coordinate system in which the metrology objects are defined can also be changed with set_metrology_model_paramSetMetrologyModelParam.

• Modify object parameters:

  Many parameters can be set when adding the metrology objects to the metrology model. Some of them can also be modified afterwards using the operator

  • set_metrology_object_paramSetMetrologyObjectParam.

• Align the metrology model:

  To translate and rotate the metrology model before the next measurement is performed, you can use the operator

  • align_metrology_modelAlignMetrologyModel.

  An alignment is temporary and is replaced by the next alignment. The metrology model itself is not changed. Note that typically the alignment parameters are obtained using shape-based matching.

• Apply the measurement:

  The actual measurement in the image is performed with

  • apply_metrology_modelApplyMetrologyModel.

  The operator locates the edges within the measure regions and fits the specified geometric shape to the edge positions using a RANSAC algorithm. The edges are located internally using the operator measure_posMeasurePos or fuzzy_measure_posFuzzyMeasurePos (see also chapter 1D Measuring). The latter uses fuzzy methods and is used only if at least one fuzzy function was set via set_metrology_object_fuzzy_paramSetMetrologyObjectFuzzyParam before applying the measurement. If more than one instance of the returned object shape is needed (compare image above), the generic parameter 'num_instances'"num_instances" must be set to the number of instances that should be returned. The parameter can be set when adding the individual metrology objects or afterwards with the operator set_metrology_object_paramSetMetrologyObjectParam.

• Access the results:

  After the measurement, the results can be accessed. The parameters of the adapted geometric shapes of the objects are queried with the operator

  • get_metrology_object_resultGetMetrologyObjectResult.

  Querying only the edges used for the returned result and their amplitudes is also done using get_metrology_object_resultGetMetrologyObjectResult.

  The row and column coordinates of all located edges can be accessed with

  • get_metrology_object_measuresGetMetrologyObjectMeasures.

  To visualize the adapted geometric shapes, you can access their XLD contours with

  • get_metrology_object_result_contourGetMetrologyObjectResultContour.

Further operators

In addition to the operators mentioned above, you can copy the metrology handle with copy_metrology_modelCopyMetrologyModel, write the metrology model to file with write_metrology_modelWriteMetrologyModel, read a model from file again using read_metrology_modelReadMetrologyModel, and serialize or deserialize a metrology model using serialize_metrology_modelSerializeMetrologyModel or deserialize_metrology_modelDeserializeMetrologyModel.

Furthermore, you can query various information from the metrology model. For example, you can query the indices of the metrology objects with get_metrology_object_indicesGetMetrologyObjectIndices, query parameters that are valid for the entire metrology model with get_metrology_model_paramGetMetrologyModelParam, query a fuzzy parameter of a metrology model with get_metrology_object_fuzzy_paramGetMetrologyObjectFuzzyParam, query the number of instances of the metrology objects of a metrology model with get_metrology_object_num_instancesGetMetrologyObjectNumInstances, and query the current configuration of the metrology model with get_metrology_object_paramGetMetrologyObjectParam.

Additionally, you can reset all parameters of a metrology model using reset_metrology_object_paramResetMetrologyObjectParam or reset only all fuzzy parameters and fuzzy functions of a metrology model using reset_metrology_object_fuzzy_paramResetMetrologyObjectFuzzyParam.

Glossary

In the following, the most important terms that are used in the context of 2D Metrology are described.

• metrology model Data structure that contains all metrology objects, all information needed for the measurement, and the measurement results.

• metrology object Data structure for the object to be measured with 2D metrology. The metrology object is represented by a specific geometric shape for which the shape parameters are approximately known. Additionally, it contains parameters that control the measurement, e.g., parameters that specify the dimension and distribution of the measure regions.

• measure regions Rectangular regions that are arranged perpendicular to the boundaries of the approximate objects. Within these regions the edges that are used to get the exact shape parameters of the metrology objects are extracted.

• returned instance of a metrology object For each metrology object, different instances of the object can be returned by the measurement, e.g., if parallel structures of the same shape exist near to the boundaries of the approximated geometric shape (see image above). The sequence of the returned instances is arbitrary, i.e., it is no measure for the quality of the fitting.

Further Information

See also the “Solution Guide on 2D Measuring” for further details about 2D metrology.