measure_pos — Extract straight edges perpendicular to a rectangle or annular arc.
measure_pos(Image : : MeasureHandle, Sigma, Threshold, Transition, Select : RowEdge, ColumnEdge, Amplitude, Distance)
measure_pos extracts straight edges which
lie perpendicular to the major axis of a rectangle or annular arc.
For an explanation of the concept of 1D measuring see the introduction of chapter 1D Measuring.
The algorithm of measure_pos
works by averaging the gray values in “slices”
perpendicular to the major axis of the rectangle or annular arc
in order to obtain
a one-dimensional edge profile. The sampling is done at subpixel
positions in the image Image at integer row and column
distances (in the coordinate frame of the rectangle) from the center
of the rectangle. Since this involves some calculations which can
be used repeatedly in several measurements, the operator
gen_measure_rectangle2 or gen_measure_arc is used to
perform these calculations
only once, thus increasing the speed of measure_pos
significantly. Since there is a trade-off between accuracy and
speed in the subpixel calculations of the gray values, and thus in
the accuracy of the extracted edge positions, different
interpolation schemes can be selected in
gen_measure_rectangle2. (The interpolation only influences
rectangles not aligned with the image axes.) The measure object
generated with gen_measure_rectangle2 is passed in
MeasureHandle.
After the one-dimensional edge profile has been calculated, subpixel
edge locations are computed by convolving the profile with the
derivatives of a Gaussian smoothing kernel of standard deviation
Sigma. Salient edges can be selected with the parameter
Threshold, which constitutes a threshold on the amplitude,
i.e., the absolute value of the first derivative of the edge.
Note that the amplitude values are scaled by the factor
.
Additionally, it is possible to select only positive edges, i.e.,
edges which constitute a dark-to-light transition in the direction
of the major axis of the rectangle or the arc (Transition =
'positive'), only negative edges, i.e., light-to-dark
transitions (Transition = 'negative'), or both
types of edges (Transition = 'all'). Finally, it
is possible to select which edge points are returned. If
Select is set to 'all', all edge points are
returned. If it is set to 'first', only the first of the
extracted edge points is returned, while it is set to
'last', only the last one is returned.
The extracted edges are returned as single points which lie on the
major axis of the rectangle or arc in (RowEdge,ColumnEdge).
The corresponding edge amplitudes are returned in
Amplitude. In addition, the distance between consecutive
edge points is returned in Distance. Here, Distance[i]
corresponds to the distance between Edge[i] and Edge[i+1], i.e., the
tuple Distance contains one element less than the tuples
RowEdge and ColumnEdge.
measure_pos only returns meaningful results if the
assumptions that the edges are straight and perpendicular to the
major axis of the rectangle or arc are fulfilled. Thus, it should
not be used to extract edges from curved objects, for example.
Furthermore, the user should ensure that the rectangle or arc 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_pos 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.
Image (input_object) singlechannelimage → object (byte / uint2 / real)
Input image.
MeasureHandle (input_control) measure → (handle)
Measure object handle.
Sigma (input_control) number → (real)
Sigma of gaussian smoothing.
Default value: 1.0
Suggested values: 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 >= 0.4
Threshold (input_control) number → (real)
Minimum edge amplitude.
Default value: 30.0
Suggested values: 5.0, 10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 90.0, 110.0
Typical range of values: 1
≤
Threshold
≤
255 (lin)
Minimum increment: 0.5
Recommended increment: 2
Transition (input_control) string → (string)
Light/dark or dark/light edge.
Default value: 'all'
List of values: 'all', 'negative', 'positive'
Select (input_control) string → (string)
Selection of end points.
Default value: 'all'
List of values: 'all', 'first', 'last'
RowEdge (output_control) point.y-array → (real)
Row coordinate of the center of the edge.
ColumnEdge (output_control) point.x-array → (real)
Column coordinate of the center of the edge.
Amplitude (output_control) real-array → (real)
Edge amplitude of the edge (with sign).
Distance (output_control) real-array → (real)
Distance between consecutive edges.
If the parameter values are correct the operator measure_pos
returns the value 2 (H_MSG_TRUE). Otherwise an exception is raised.
edges_sub_pix,
fuzzy_measure_pos
measure_pairs,
fuzzy_measure_pairs,
fuzzy_measure_pairing
1D Metrology