CALIBRATE AND MEASURE EXTENSIONS TO
FOIL
CALIBRATE
This software suite can be used to calibrate the camera field and
measure objects in the camera field of view. The calibrate section
lists the routines used to calibrate the camera image as downloaded
from the framegrabber. The measure section allows the user to the
measure objects in these images.
Dark Field Subtraction
Used to balance the "black" values from the camera
Illumination correction
Imaging the inspection area without a part in it typically provides
the data for this. This measurement is used to develop thresholds
for later measurements.
Calibration Standard
Ideally a calibration standard is provided, such as an array of
dots, which span the whole view area of the camera. This allows
the calculation of scale and variations in scale across the field
of view. Three corrections are typically performed,
- Perspective correction,
- Color Aberration correction, and
- Scale correction.
Coordinate Systems
Raw Image Coordinates
(ix,iy) these are the coordinates of each pixel in the image. The
coordinate system origin is the upper left hand corner of the image,
x extends to the right, y extends down.
Corrected Image Coordinates
(cx,cy) these are the coordinates of each pixel in the corrected
image, after Perspective, Color aberration, and scale correction.
The origin is the center of the image, with x extending to the right,
and y extending up.
Object Coordinates
Object coordinates are used to place measurement tools and report
measurements. Object coordinates are reached by an affine transformation
from corrected image coordinates.
Orientation tools
An initial step in measuring an image is to determine the orientation
of the object in the field of view. Locating some prominent features
of the object and setting the origin and angle of the world coordinate
system to match the orientation of the object to be measured do
this.
Bounding Box
One method of locating the object is to compute its minimum-area-bounding
box. This algorithm finds the smallest box that can contain the
image of the object. The software can then use the long side of
the box to orient the system.
Algorithm.
- Find the centroid of the image of the object.
- Measure the diameter of the object along radials from the centroid.
- Take the maximum and the minimum diameters. Pick the bounding
box with less area determined by these two diameters.
Find Hole
Sometimes the bounding box is not enough, for example when the part
is square. Locating a hole often can remove the ambiguity.
Algorithm
- Collect a set of points that have no part with their convex
hull.
- Compute the center of the convex hull
Find Line
Finding a line of the required length can also be used to orient
the object.
Algorithm:
- From a collection of points partition the points in to sets,
which contain more than four points that are co-linear (with tolerance).
- Eliminate sets whose lines cross the interior of the part.
- Extend the sets to the end points.
Find Corners
Place the intersections of lines in the object. Select a corner
pair as the base line of the object. The outline of the Algorithm:
- Locate all the lines in the part, and extend them to their
endpoints.
- All the end points of the lines are at corners.
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MEASURE
Image Alignment Tools
After the orientation of the part is determined, an affine transformation
is defined which moves the part into the ‘standard’
position and orientation for measurement. This typically requires,
rotation, mirroring, and translation of the part. This step sets
orientation of the transformation of corrected-image coordinates
to object coordinates. The image is not actually transformed; the
measurement tools are transformed to align to the image. This reduces
the errors in measuring the object.
Measurement Tools
Creating a Point Set
- Defining a measurement tool. A measurement
tool is a collection of line segments, which are used to detect
edge points of the object. Each line segment detects one point.
A complete measurement tool collects a set of points, which are
used to perform measurements.
- Collecting points A line segment from
a measurement tool determines a point by sampling the image, and
finding the location of an edge in the image. The edge is determined
by the point where the image pixel values cross a threshold determined
by a percentage of the minimum and maximum values of the pixels
along the line.
- Extending point sets to the end points
The point set detected by a measurement tool, is extended by following
a line or circular arc, determined by the last three points in
the point set, until no edge is detected. This is done using a
provided step size, and threshold. A binary search is performed
to reach the desired accuracy dictated by the input parameters.
Photometric tools
Spatial Metric tools
Once a point set is determined by a measurement tool, that point
set can be measured in several ways.
- Linear Dimensions
- Vertical
- Horizontal
- Oblique
- Circular Dimensions
- Angular Dimensions
- Between Lines
- Circular Arc
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