FindCircle

Locates a single circular edge within an annular (circle or ring shape) image region. FindCircle forms one-dimensional projections of the image region by summing pixel values on concentric circles, scanned in the direction of increasing radius. Edge transitions are extracted from the projected image data.

Inputs

Parameter

Description

Image

This parameter must reference a spreadsheet cell that contains an Image data structure. By default, this parameter references cell A0 , containing the AcquireImage function. This parameter can also reference other Image data structures, such as those returned by the Image Vision Data Access functions or Coordinate Transforms Functions.

Fixture

Defines the Region of Interest (ROI) relative to a Fixture input or the output of the image coordinate system for the Vision Tools function. Setting the ROI relative to a Fixture ensures that if the Fixture is rotated or translated, the ROI is rotated or translated in relation to the Fixture.

Note: 2D projections use real world units but fixturing with 2D tools requires input in pixel values. When using a 2D projection from a 3D point cloud as a Fixture, convert the real world units to pixel values using the TransWorldToPixel function.

The default setting is (0,0,0), the top leftmost corner of the image.

X	The X-offset, in image coordinates.
Y	The Y-offset, in image coordinates.
Theta	The rotation from the image X-axis, in the image coordinate system. (+/-360 degrees clockwise)

Annulus

Also known as the Region of Interest (ROI), it specifies the region of the image that undergoes analysis. It creates an annular image region that can be translated and rotated. With this parameter selected, by pressing the Maximize Region button on the toolbar of the property sheet, the region is automatically stretched to cover the entire image.

X	The X-offset of the origin, in fixture coordinates.
Y	The Y-offset of the origin, in fixture coordinates.
Inner Radius	The size of the inner radius.
Outer Radius	The size of the outer radius.

Note: The Fixture and Annulus parameters must be defined within the bounds of the image. Otherwise, the function returns #ERR.

Polarity

Specifies the polarity of the edge to be located, relative to the scan direction of the Edge Region. The function only reports edges of the specified polarity.

0 = black-to-white	Specifies that the edge polarity is black-to-white.
1 = white-to-black	Specifies that the edge polarity is white-to-black.
2 = either (default)	Specifies that the edge polarity could be either black-to-white or white-to-black.

Find By

Specifies the edge selection technique the function uses to select an edge, amongst multiple edge candidates.

0 = best score (default)	Specifies that the edge with the highest score receives the highest consideration.
1 = smallest circle	Specifies that the smallest circle detected receives the highest consideration.
2 = largest circle	Specifies that the largest circle detected receives the highest consideration.

Accept Thresh

Specifies the minimum acceptable edge contrast (0 to 100; default = 25); the contrast value is normalized from the greyscale histogram within the Region to a scale of 0 to 100. The function rejects any edge whose contrast score falls below this value.

Normalize Score

Specifies whether the edge scores will be normalized by the greyscale histogram of the Region. When Normalize Score is enabled, raw edge scores are normalized by the maximum contrast found within the image region using the following formula:

Score = (Raw Edge Score) x (100/Max Contrast)

Where Raw Edge Score is the grey level difference across the edge and Max Contrast is the difference between the maximum and minimum grey level (0 to 255) within the image region.

When Normalize Score is disabled, low contrast edges will score lower than high contrast edges. The following formula is used:

Score = (Raw Edge Score) x (100/255)

0 = OFF	Do not normalize the score.
1 = ON (default)	Normalize the score.

Note: Tip: This parameter is useful for finding edges in a low contrast region. If the edges to be detected are expected to be only high contrast, disable this parameter.

Edge Width

Specifies the pixel distance (1 to 50; default = 3) over which an edge transition takes place. Edge Width is used to filter the image before edges are extracted.

Show

Specifies the display mode for FindCircle graphical overlays on top of the image.

0 = hide all (default)	All graphics will be hidden, except when the cell containing the FindCircle function is the active cell in the spreadsheet.
1 = result graphics only	The circular edge will be displayed at all times.
2 = input and result graphics	Both the input image region and the circular edge will be displayed at all times.
3 = show all: input, result, and chart	The input image region, the circular edge, and the edge response chart will be displayed at all times.

Outputs

Returns

An Edge containing a single circular edge, or #ERR if any of the input parameters are invalid.

Results

When FindCircle is initially inserted into a cell, a result table is created in the spreadsheet using the following Edges Vision Data Access functions

Label	Function	Description
CentX	GetX(Edges, 0, 0)	The center x-coordinate.
CentY	GetY(Edges, 0, 0)	The center y-coordinate.
Radius	GetRadius(Edges, 0)	The radius.
Score	GetScore(Edges, 0)	The edge contrast score. Score is positive for black-to-white transitions, and negative for white-to-black transitions.