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".
VE Series Smart Camera
76
www.bannerengineering.com - Tel: 763.544.3164