Measurement and Fitting Parameters

Finding Sample Points

All the gauges and ESamplePath ^{.NET Python} contain all the point measurement parameters, with default values that detect reasonably contrasted edges:

• Transition strength: SetThreshold ^{.NET Python}, SetMinAmplitude ^{.NET Python} and SetMinArea ^{.NET Python}.

• Transition type: SetTransitionType ^{.NET Python}

• Transition choice: SetTransitionChoice ^{.NET Python} and SetTransitionIndex ^{.NET Python}.

• Local filtering for noise immunity: SetThickness ^{.NET Python} and GetSmoothing ^{.NET Python}.

  • Sampling area (rectangular by default) for transverse filtering SetRectangularSamplingArea ^{.NET Python}.

Transition Strength

The threshold ^{.NET Python} level is very important:

• Too high and significant transitions can be missed.

• Too low and noise can cause false transitions.

To resolve this dilemma, the EasyGauge transition selection mechanism can reject low contrast or false edges:

• The transition strength is measured by the peak amplitude and the area.

• Every edge measurement determine the peak amplitude and the area.

  • The peak amplitude is the difference between the peak value and the threshold.

  • The peak area is the area under the peak curve and the horizontal line at the threshold value.

• If either value falls below the minimum amplitude ^{.NET Python} or the minimum area ^{.NET Python}, the peak is disregarded and no point is assumed at that location.

Example of good and bad thresholds.

Peak amplitude and area.

Transition Type

Select the type of transition to find with SetTransitionType ^{.NET Python}:

• Simple edges:

  • White to Black (Wb ^{.NET Python})

  • Black to White (Bw ^{.NET Python})

  • indifferent polarities (BwOrWb ^{.NET Python})

  

  

  

  Black to white, white to black and indifferent polarities

• Complex edges:

  • Black to white to black (Bwb ^{.NET Python})

  • White to black to white (Wbw ^{.NET Python})

  

  Complex “Black to white to black” transition type.

  Hint

  You cannot normally detect transitions using the default threshold value (20) as BWB or WBW transitions base the peak analysis on the gray level profile along the sample path and not its first derivative.

Selecting the Most Relevant Transition

There can be multiple transitions that match the transition detection parameters. The parameter SetTransitionChoice ^{.NET Python} can be used to select the single most relevant transition. With point gauges, this parameter can have the value ETransitionChoice_All ^{.NET Python}, which will return all the transitions matching the other parameter of the detection.

Multiple transitions measured (left) and single transition measured (right).

You can select the single most relevant transition based on 4 criteria:

The highest peak ETransitionChoice_LargestAmplitude	The peak with the largest area ETransitionChoice_LargestArea
The peak closest to the gauge center ETransitionChoice_Closest	The nth peak encountered starting from one tip of the gauge (n=3) ETransitionChoice_NthFromBegin or ETransitionChoice_NthFromEnd

Note

When several peaks have the same value (same area, same amplitude…), the last peak is selected.

Filtering noise

Filtering the image locally can reduce the noise effects.

There are 2 types of filtering:

• The transverse (lengthwise) filtering averages several parallel lines when sampling the image.

  • The thickness ^{.NET Python} determines the number of parallel lines.

    

  • The parallel lines can be contained in a parallelogram or a rectangle (SetRectangularSamplingArea ^{.NET Python}).

        

• The longitudinal (crosswise) uniform filtering can also be applied to the resulting profile curve (SetSmoothing ^{.NET Python})

Configuring Sample Paths

• Use SetSamplingStep ^{.NET Python} to set the spacing between the paths.

  • Use GetNumSamples ^{.NET Python} to get the number of paths that are sampled.

• Use SetTolerance ^{.NET Python} to set the length of these paths that is the half-length of the line segment. The tolerance is how far to look for edge crossings around the nominal shape.

  Note

  Note that the EPointGauge ^{.NET Python} has no tolerance because it consists in the measurement of edge-crossing on its nominal line.

EasyGauge measurement concepts

Fitting Parameters

• Outlier rejection parameters:

  • The number of iterations ^{.NET Python} in the algorithm.

  • The threshold ^{.NET Python} used to tag a sample point as an outlier.

• Skip ranges:

  • The skip ranges define the exclusion ranges for the paths. The sample point indexes in the skip ranges are not taken into account to fit the model.

    Note

    The skip ranges apply to all gauge models but the polygon gauge.

  • Use AddSkipRange ^{.NET Python} to define several skip ranges.

  • Use RemoveSkipRange ^{.NET Python}, RemoveAllSkipRanges ^{.NET Python} and GetSkipRange ^{.NET Python} to manage the skip ranges.

  • Use the attribute EDrawingMode_PointsInSkipRange ^{.NET Python} with the method Draw ^{.NET Python} to display the skipped points.