The Calibrator tool handles lens distortion, lens magnification and camera magnification using an Nth order nonlinear calibration based on a precision grid. The Calibrator tool is very important in 2D and 3D gauging applications.

Application areas:

• High Precision Gauging
• 3D Machine Vision
• 2D and 3D Robot Vision

Demo Profile

• Calibrator - T1 - WideAngle Lens_001.zip

Please note the following:

• In high precision gauging use the highest polynomial order possible - increasing the polynomial order from 5 to 6 or even 7 have significant effect on the precision. With Calibrator Scorpion support sub-pixel measurement with precisions from 1/10th of pixel to 1/40th of a pixel. Orders from 3 to 7 is supported by Scorpion. Order 7 may fail due to lacking number of grid points - reduce order to 5 or 6 if this happens.
• In robot vision use 4 or 5 polynomial order. The primary purpose for Calibrator in Robot Vision is to remove the lens distortion. Using Calibrator Scorpion users can go closer and have pixel precision even using wide-angle lenses of 4.8 mm or 6 mm
• Increasing the number of dots in the calibration grid will normally increase the precision - decreasing the number of dots will make it a little bit easier to get Calibrator to locate the dots.
• Outside the dot area the polynomial position estimate are unstable - this effect will increase with the polynomial order.
• The Calibrator tool starts a reference chain and is the mother of all reference systems in Scorpion.

The image below is captured with 4.8 mm wide-angle lens. The grid has the following properties:

• 15 rows and 22 columns
• Dot spacing of 50 mm
• Pixel resolution is 1,01 mm

The following corrections are performed with lens calibration:

• The RMS position deviation is 0,3 mm or pixels
• The maximum deviation is 0,8 mm
• The edge points are moved 58,7 mm

The effect of using the calibrator is that the optical precision is 0,3 mm. Before correction the maximum error 2 * 58 mm = 116 mm. The maximum error is thus reduced with a factor close to 300.

Using the ImageResampler tool the following perfect image can be created:

 See also the Calibrated Measurement example.

Test profiles on the Scorpion CD and online - click the links - are:

• Calibrator - demonstration of sub-pixel measurement using calibration
• Calibrator - WideAngle Lens - demonstration of resampling and wide angle lens calibration

Setup

Calibration image

• Load activates a file browser
• Save (...) saves the current image to file
• Get current captures the current image and creates a unique name

ROI

• Active - used to select the grid area. If unchecked, the whole image is used
• Whole - Press to select the whole image as ROI (as a starting point)

• The ROI can be managed by the buttons

  • Paste - paste the ROI from the image to the scorpion clipboard
  • Copy - copy the ROI to the image from the scorpion clipboard

Grid

• Rows/Columns - specifies the number of elements in the grid
• Row/Col spacing - physical distance between grid elements
• Unit - specified to document the physical unit
• Auto - if checked, automatically count the number of grid elements when computing
• Count - if Auto is unchecked, counting is not done automatically. Press Count to run the counting algorithm manually

Polynomial order - Specifies the order of the Calibrator tool. Normally 5th order calibration is used - to improve accuracy an order of six or seven may be tried. However, too high an order can also model imperfections in the grid itself. In addition, the computation may fail with a "Computational error" message.

Compute - Press this button to recompute the calibration file.

Deviation Visualisation - Yellow arrow displays the Non-linear distortion - describes how the lens looks. The red arrows show the Model-fit residues - these arrows shall be random - white noise. The radio button sets the visualisation gain.

Gauging - indicates in green, yellow or red, the result of the calibration for gauging applications (great need for accuracy)

Robot vision - indicates in green, yellow or red, the result of the calibration for robot vision applications (less accuracy needed)

Result of model-fit

• Med deviation, RMS - specifies the average model fit given in pixels - normally 0.1 - 0.5 pixels
• Max deviation - the maximum error in pixels
• Missing points - number of grid points missing
• Rejected points - number of points not accepted
• Max correction, pixels - displays the largest correction - with wide angle lenses this can be as high as 25 pixels

Error messages

The calibrator may sometimes fail locating the grid points. If only a few grid points are missing, the calibrator will ignore the corresponding grid positions and still succeed. Likewise, extra grid points are in many cases also rejected. In some cases the algorithm will fail, and no calibration will be performed. Note that the grid points in the image must be regular enough to form natural rows and columns. Only a limited amount of perspective, skew or rotation can be corrected.

• Too few points found - not enough blobs found
• Too many points found - too many blobs found
• Too few points match - too few found blobs fit the grid points
• Bad grid - bad specification (rows, columns)
• Cannot find row - row cannot be built from blobs. (Too much perspective or rotation?)
• Found row too short - not enough points found to form a row
• Too many points in row - too many points found that could belong to a row
• Cannot estimate left blob position - could not deduce position of leftmost grid point in row
• Cannot estimate mid blob position - could not deduce position of middle point(s) in row
• Points too close in row - blobs too close to separate. (Maybe a blob found inside another blob?)
• Computational error. Wrong polynomial order? - reduce the polynomial order and try again

You can often rectify the situation by adjusting the blob search parameters under the Advanced page. Turning off the Sobel filter can often help with imperfect images.

Calibration Image Mouse Menu - activated by right clicking the image

• Add point to clipboard - Ctrl+LMouse - adds point to clipboard
• Reset Clipboard - Ctrl+Shift+RMouse - clears polygon on clipboard
• Copy objects - copies any overlay object into clipboard
  • useful when configuring polygon tools
• Clipboard -
  • Copy X - copies the x- cursor position to the clipboard
  • Copy Y - copies the y-cursor position to the clipboard
  • Copy point - copies x and y cursor position to the clipboard
    • can be used to paste the position into a tool
  • Remove last point - Ctrl-RMouse
  • Remove first point
  • Move point - Shift+Ctrl+RMouse - move selected point
    • it can be easier to remove and insert a new point than moving
  • Insert point - Shift+LMouse
  • Remove point -  Shift+RMouse
  • Smooth - smoothes the clipboard polygon
  • Simplify - removes points on a straight line
  • Undo - will undo last operation
  • Closed polygon - will open or close polygon
• Set measure origin - activates image measurements
• Measure - de/activates image measurements
• Freeze - freeze the image
• Show info - displays a yellow label image operation
• Copy image without graphics
• Save shown image without graphics

Advanced

Blob search parameters

• Active - activates the Advanced setup
• Restore default values - replace all values with defaults
• Use Sobel filtering (gradient search) - default on
  • Gradient threshold (minimum) - The threshold can be lowered in low contrast images if points are missing
  • Lower threshold - lower blob threshold when sobel filtering is off
  • Upper threshold - upper blob threshold when sobel filtering is off
  • Search outside thresholds - invert meaning of thresholds
• Morphology (e=erode, d=dilate) - ed can improve point location when sobel filtering is off
  • Hint: Morphology should be off when Sobel filtering is active
• Smooth factor - Specifies the level of smoothing of the grid points' blob contours
• Required circularity (max 1.0) - The value can be lowered if points are missing
• Minimum diameter (pixels) - Minimum diameter to accept a grid point
• Maximum diameter (pixels) - Maximum diameter to accept a grid point
• Minimum found blobs (percent of rows*columns) - Percent of grid points found to accept calibration image
• Minimum matched blobs (percent of found blobs) - Percent of found blobs accepted as grid points

Auto grid settings

• Sector angle for neighbour search - where to search for neighbour blobs when counting rows/columns. Turn off Auto and click Count to visualise the effect of this setting

Output reference system

These settings adjust the placement of the calculated reference system. The origin can be placed anywhere in the picture, regardless of the circle positions in the calibration image. The angle can also be adjusted.

• Position of origin:
  • Top left circle - default: the reference system origin is centered at the upper left circle in the calibration image
  • Image top left - origin is at the upper left corner of the image (see note below)
  • Image center - origin in the center of the image
  • Specify (pixel coordinates) - origin anywhere in the image (specified position)
• Zero angle - adjust for grid rotation - the angle of the reference system is adjusted so that on average, the x axis is pointing straight downwards in the picture. Note that with perspective in the calibration image this may not work very well.

Note: Placing the origin outside the calibration circles may introduce inaccuracies.

Hint: Using a fixed (pixel based) origin and a zero angle setting will in general cancel the effects of the direction and position of the calibration image. This will in general make recalibration easier.

Visualisation

Results

ExecuteCmd support (see also executeCmd)

def Recalibrate():
  cal=GetTool('calib')
  print cal.executeCmd('GetCurImage')
  if GetBoolValue('Setup.Small'):
     print cal.executeCmd('Compute','POLORDER=5;ROWS=17;COLS=22;ROWSPACING=10;COLSPACING=10')
  else:
     print cal.executeCmd('Compute','POLORDER=6;ROWS=22;COLS=30;ROWSPACING=10;COLSPACING=10')