FindCurve

Locates a single circular arc edge segment within a curved image region. FindCurve forms a one-dimensional projection of the image region by summing pixel values on concentric arc segments scanned in the positive x-direction relative to the region's local coordinate system. 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)

Region

Also known as the Region of Interest (ROI), specifies the region of the image that undergoes analysis. Double-click on the Region parameter to create an Interactive Graphics Mode that you can transform and rotate. Select this parameter and press the Maximize Region button on the Job Edit toolbar of the property sheet to automatically stretch the region 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.
Width	The dimension along the X-axis of the region.
Height	The dimension along the Y-axis of the region.
Angle	The orientation, in fixture coordinates.
Curve	The angular deviation between the x-axis and the opposing boundary line of the region.

Note: The Fixture and Region 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 = first edge	Specifies that the first edge detected receives the highest consideration.
2 = last edge	Specifies that the last edge 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.

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.

Angle Range

Specifies the tolerance of the function to edge rotation (0 to 10; default = 0). The function still finds edges that are outside of the specified range, but the accuracy may not be as high.

Note:

Setting this parameter to a value greater than 0 reduces the speed of the function.
This setting is currently ignored; the maximum Angle Range of 10 is always allowed within an annular segment.

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 FindCurve graphical overlays on top of the image.

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

Outputs

Returns

An Edge data structure containing a single curved edge segment, or #ERR if any of the input parameters are invalid.

Results

When FindCurve is initially inserted into a cell, a result table is created in the spreadsheet.

The following Edges Vision Data Access functions are automatically inserted into the spreadsheet to create the result table:

Label	Function	Description
CentX	GetX(Edges, 0, 0)	The x-coordinate of the center of curvature.
CentY	GetY(Edges, 0, 0)	The y-coordinate of the center of curvature.
StartX	GetX(Edges, 0, 1)	The x-coordinate of the first endpoint.
StartY	GetY(Edges, 0, 1)	The y-coordinate of the first endpoint.
EndX	GetX(Edges, 0, 2)	The x-coordinate of the second endpoint.
EndY	GetY(Edges, 0, 2)	The y-coordinate of the second endpoint.
Radius	GetRadius(Edges, 0)	The arc radius.
Score	GetScore(Edges, 0)	The edge contrast score. Score is positive for black-to-white transitions, negative for white-to-black transitions.