8.4 Blob Tool
8.4.3 Blob Tool: Results
Tools & Results and All Results list information from the current and previous inspections.
A red box around a tool indicates that the tool failed. Status provides information about the specific failure.
All Results displays the Result, Time, Pass Count, and Fail Count information at a glance. Expand the inspection tool to see specific results for that tool.
Count
The number of blobs found.
Total Area
The total area of all of the blobs found.
Area Range
The minimum and maximum areas of the blobs found.
Expand Area Range to see the area and centroid (center point) for each blob.
Statistics
Advanced statistical information about each blob.
When enabled, Perimeter, Compactness, Major Axis Length, Minor Axis Length, Major Axis Angle, Eccentricity, Minimum Radius, Maximum Radius, Minimum Radius Position, and Maximum Radius Position will be calculated in addition to Area and Centroid.
Available when Caculate Blob Statistics is checked. See the following sections for more details.
Adaptive Threshold
The grayscale value for the software-selected threshold value used to generate the blobs.
This result is blank if the threshold type is set to Fixed.
Execution Time
The execution time, in milliseconds, for the currently selected tool in the current inspection.
Expand Execution Time to see the historical minimum and maximum execution times up to this point for the selected tool.
Use the reset button in the Inspection Summary to reset these historical values.
Status
Status and error messages display as appropriate.
Area
The area (A) is a count of the total number of pixels that belong to the blob.
Centroid
The centroid (xc, yc) is the point at the center of mass of the blob.
For simple blobs such as solid circles, ellipses, or rectangles, this is the center of the shape. For more complicated shapes it is helpful to imagine a piece of cardboard cut in the shape of the blob. The centroid is the point where you could balance the cardboard on the tip of a pencil. For complicated shapes—especially shapes with unfilled holes—the centroid might lie outside of the shape.
The x coordinate of the centroid is calculated by adding the x coordinates of each pixel in the blob and dividing by the area. The y coordinate is similar:
Perimeter
The perimeter (P) gives an approximate measurement of the length of the circumference of the blob.
Because blobs are built from individual pixels, it is most practical to estimate the perimeter by counting the contributions of individual pixels on the blob's boundary. The following describes the exact values that are added to the perimeter for each possible pixel configuration. In each example, the description refers to the center pixel in the corresponding pictures.
A pixel with no neighbors that belong to the same blob contributes 3.14 linear pixels to the perimeter of the blob. This can happen only in a blob that has an area of one. Since such small blobs are usually ignored, this circumstance is rare.
A pixel with one neighbor that belongs to the same blob contributes 2.571 linear pixels to the perimeter of the blob.
A pixel with two neighbors that belong to the same blob, forming a straight line, contributes exactly 2 linear pixels to the perimeter of the blob.
A pixel with two neighbors that belong to the same blob, forming a corner, contributes 1.414 linear pixels to the perimeter of the blob.
A pixel with three neighbors that belong to the same blob contributes exactly 1 linear pixel to the perimeter of the blob.
A pixel with four neighbors that belong to the same blob contributes nothing to the perimeter of the blob.
This method of counting slightly overestimates the "true" perimeter. For example, a circle with a radius of 100 pixels has a computed perimeter of approximately 660 pixels, compared with the expected value of 628 pixels.
If the sensor is configured to convert pixel distances to other units, (for example, millimeters), the perimeter is given in those units. If the blob contains holes that have not been filled, the length of the perimeter includes the points on the perimeters of these holes.
Compactness
Compactness is a measurement of the space a blob occupies.
The compactness is high for blobs that are nearly circular and low for blobs that are elongated or complicated.
compactness =
Where A is the area and P is the perimeter of the Blob in question. An idealized circle would have a compactness of 100, but because the perimeter is approximated (see above), the highest realistic value for most blobs is roughly 90. Very small blobs with just a handful of pixels may reach or even exceed the theoretical maximum of 100, again because of the approximations in the perimeter calculation.
Major Axis Length, Minor Axis Length, and Major Axis Angle
Major Axis Length, Minor Axis Length, and Major Axis Angle are determined using a best fit ellipse.
To understand Major Axis Length, Minor Axis Length, and Major Axis Angle, it is important to note that these are not measurements of the blob itself because the blob may be an irregular shape. Rather, these measurements are determined by a well-defined shape, a "best fit ellipse".
Figure 120. Best Fit Ellipse
These three results combine to give information about the elongation and orientation of a blob . The equations used to compute these statistics are complicated, but the results usually have an intuitively useful meaning, described below. The first step in computing these results is to compute the M2,0, M0,2 and M1,1 statistical moments:
where A is the area, (xc, yc) are the coordinates of the centroid, and (xi, yi) are the coordinates of pixel i. These values represent the variance with respect to x, the variance with respect to y, and the covariance, respectively. The final results can be calculated as follows:
The following table gives a more practical perspective on how to interpret these results. If the sensor is configured to convert pixels to other units, the major and minor axis lengths are given in those units. The major axis angle is always given in degrees.
Blob Shape Meaning of Major Axis
Length Meaning of Minor Axis
Length Meaning of Major Axis
angle Circular, no holes Diameter of the circle Equal to the major axis
length Unstable
Elliptical, no holes Length of the ellipse Width of the ellipse Orientation of the ellipse Square, no holes Diameter of a circle that best
approximates the square Equal to the major axis
length Unstable
Rectangular, no holes Length of an ellipse that best
approximates the rectangle Width of an ellipse that best
approximates the rectangle Orientation of the rectangle Complicated shape, no holes Length of an ellipse that best
approximates the shape Width of an ellipse that best
approximates the shape Orientation of the shape—
unstable if length and with are nearly equal
Any shape with holes Results vary depending on exact shape—experiment on your particular shape
Results vary depending on exact shape—experiment on your particular shape
Results vary depending on exact shape—experiment on your particular shape
Eccentricity
The eccentricity of a blob is the length of the major axis divided by the length of the minor axis.
For circular regions and other regions with radial symmetry (such as a square), the value will be very close to 1. For elongated regions, the value will increase.
Max Radius and Max Radius Position
The Maximum Radius of a blob is the distance from the centroid of the blob to the farthest pixel on that blob's perimeter.
The Maximum Radius Position gives the pixel coordinates of that farthest perimeter point.
If the sensor is configured to convert pixels to other units, the maximum radius will be given in those units.
Min Radius and Min Radius Position
The Minimum Radius of a blob is the distance from the centroid of the blob to the closest pixel on that blob's perimeter.
The Minimum Radius Position gives the pixel coordinates of that closest perimeter point.
If the blob contains unfilled holes, the minimum radius position may be on the perimeter of a hole. If the sensor is configured to convert pixels to other units, the minimum radius will be given in those units.