measure_thresh — Extracting points with a particular gray value along a rectangle or an annular arc.
measure_thresh extracts points for which the gray value within an one-dimensional gray value profile is equal to the specified threshold Threshold. The gray value profile is projected onto the major axis of the measure rectangle which is passed with the parameter MeasureHandle, so the threshold points calculated within the gray value profile correspond to certain image coordinates on the rectangle's major axis. These coordinates are returned as the operator results in RowThresh and ColumnThresh.
For an explanation of the concept of 1D measuring see the introduction of chapter 1D Measuring.
If the gray value profile intersects the threshold line for several times, the parameter Select determines which values to return. Possible settings are 'first', 'last', 'first_last' (first and last) or 'all'. For the last two cases Distance returns the distances between the calculated points.
The gray value profile is created by averaging the gray values along all line segments, which are defined by the measure rectangle as follows:
The segments are perpendicular to the major axis of the rectangle,
they have an integer distance to the center of the rectangle,
the rectangle bounds the segments.
For every line segment, the average of the gray values of all points with an integer distance to the major axis is calculated. Due to translation and rotation of the measure rectangle with respect to the image coordinates the input image Image is in general sampled at subpixel positions.
Since this involves some calculations which can be used repeatedly in several projections, the operator gen_measure_rectangle2 is used to perform these calculations only once in advance. Here, the measure object MeasureHandle is generated and different interpolation schemes can be selected.
measure_thresh only returns meaningful results if the assumptions that the edges are straight and perpendicular to the major axis of the rectangle are fulfilled. Thus, it should not be used to extract edges from curved objects, for example. Furthermore, the user should ensure that the rectangle is as close to perpendicular as possible to the edges in the image. Additionally, Sigma must not become larger than approx. 0.5 * Length1 (for Length1 see gen_measure_rectangle2).
It should be kept in mind that measure_thresh ignores the domain of Image for efficiency reasons. If certain regions in the image should be excluded from the measurement a new measure object with appropriately modified parameters should be generated.
Measure object handle.
Sigma of gaussian smoothing.
Default value: 1.0
Suggested values: 0.0, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 7.0, 10.0
Typical range of values: 0.4 ≤ Sigma ≤ 100 (lin)
Minimum increment: 0.01
Recommended increment: 0.1
Restriction: Sigma >= 0.0
Default value: 128.0
Typical range of values: 0 ≤ Threshold ≤ 255 (lin)
Minimum increment: 0.5
Recommended increment: 1
Selection of points.
Default value: 'all'
List of values: 'all', 'first', 'first_last', 'last'
Row coordinates of points with threshold value.
Column coordinates of points with threshold value.
Distance between consecutive points.
If the parameter values are correct the operator measure_thresh returns the value 2 (H_MSG_TRUE). Otherwise, an exception is raised.
measure_pos, edges_sub_pix, measure_pairs