F_VN_LocateEdgesExp2

F_VN_LocateEdgesExp2 1:

Locate the points of multiple occuring edges inside a specified search window with subpixel accuracy. (expert function)
Can use available TwinCAT Job Tasks for executing parallel code regions.
Can return partial results when canceled by Watchdog.

Syntax

Definition:

FUNCTION F_VN_LocateEdgesExp2 : HRESULT
VAR_INPUT
    ipSrcImage           : ITcVnImage;
    ipEdgePoints         : Reference To  ITcVnContainer;
END_VAR
VAR_IN_OUT
    aStartPoint          : TcVnPoint2_REAL;
    aEndPoint            : TcVnPoint2_REAL;
END_VAR
VAR_INPUT
    nNumEdges            : UDINT;
    eEdgeDirection       : ETcVnEdgeDirection;
    bAlternateDirection  : BOOL;
    fMinStrength         : REAL;
    nSearchLines         : UDINT;
    fSearchLineDist      : REAL;
    nMaxThickness        : UDINT;
    nSubpixelsIterations : UDINT;
    fApproxPrecision     : REAL;
    eAlgorithm           : ETcVnEdgeDetectionAlgorithm;
    fRangeCutoffFactor   : REAL;
    ipEdgeStrengths      : Reference To  ITcVnContainer;
    bAvgEdgeStrengths    : BOOL;
    hrPrev               : HRESULT;
END_VAR
VAR_OUTPUT
    fAvgStrength         : REAL;
END_VAR

 Inputs

Name

Type

Description

ipSrcImage

ITcVnImage

Source image (1 channel)

ipEdgePoints

Reference To ITcVnContainer

Returns the detected edge points (ContainerType_Vector_Vector_TcVnPoint2_REAL)

nNumEdges

UDINT

The (maximum) number of edges to search for

eEdgeDirection

ETcVnEdgeDirection

Specification of the edge direction to search for

bAlternateDirection

BOOL

If true, eEdgeDirection is alternated after each detected edge. Else, only edges with eEdgeDirection are searched for.

fMinStrength

REAL

Specification of the minimum strength (intensity difference) of the edge to search for

nSearchLines

UDINT

Width of the search window, i.e. the number of search lines (1, 3, 5, 7, ...), centered around the line specified by aStartPoint and aEndPoint

fSearchLineDist

REAL

Distance between the search lines in pixels (> 0)

nMaxThickness

UDINT

Specification of the maximum thickness of the edge to search for, which means fMinStrength must be reached within nMaxThickness pixels

nSubpixelsIterations

UDINT

Specifies the number of subpixels (for INTERPOLATION, 10 - 20 usually is sufficient) or maximum number of iterations for optimizing the parameters (for APPROX_ERF and APPROX_GAUSSIAN, 50 - 100 usually is sufficient)

fApproxPrecision

REAL

Specifies the approximation precision for APPROX_ERF and APPROX_GAUSSIAN (0.001 usually is sufficient, unused for INTERPOLATION)

eAlgorithm

ETcVnEdgeDetectionAlgorithm

Selection of the edge detection algorithm

fRangeCutoffFactor

REAL

Relative edge strength factor to cut off the range of pixels used for approximation. Negative numbers are valid to include noise, i.e. intensity differences in the other direction. At most nMaxThickness pixels are used in each direction.

ipEdgeStrengths

Reference To ITcVnContainer

Returns the detected edge strengths (optional, set to 0 if not required; ContainerType_Vector_Vector_REAL or ContainerType_Vector_REAL, depending on bAvgEdgeStrengths)

bAvgEdgeStrengths

BOOL

If true, ipEdgeStrengths returns the average strength per edge instead of individual strengths for each search line

hrPrev

HRESULT

HRESULT indicating the result of previous operations (If SUCCEEDED(hrPrev) equals false, no operation is executed.)

 In/Outputs

Name

Type

Description

aStartPoint

TcVnPoint2_REAL

Position from which to start the search process (in the direction of aEndPoint)

aEndPoint

TcVnPoint2_REAL

Position where the search process ends

 Outputs

Name

Type

Description

fAvgStrength

REAL

Returns the average strength of the detected edges

 Return value

HRESULT

Further information

The F_VN_LocateEdgesExp2 function is the expert version of F_VN_LocateEdges. It contains additional parameters.

Parameter

Input image

The ipSrcImage input image must be a 1-channel grayscale image.

Localized points (Return value)

The ipEdgePoints container returns the localized points of the linear edges.

The search window

First, a search window has to be defined to determine where and in which direction to search for an edge. This is done using the aStartPoint, aEndPoint and nSearchLines parameters. The search window results from the search line from aStartPoint to aEndPoint and parallel to it (nSearchLines - 1) / 2 search lines in each direction, each with 1 pixel distance. Thus nSearchLines defines the number of search lines and must always have an odd value.

F_VN_LocateEdgesExp2 2:Fig.15: S: StartPoint, E: EndPoint, W: WindowWidth

Edge strength (Return value)

The fAvgStrength return value displays the actual contrast of the localized edges (averaged over all search lines).

Parameters for edge localization

The remaining parameters are explained in detail in the chapter Edge localization.

Edge thicknesses (Return value)

The detected edge thicknesses are returned via ipEdgeStrengths. Depending on bAvgEdgeStrengths, either the average thickness per edge is returned as the container type ContainerType_Vector_REAL or the individual thicknesses for each search line are returned as ContainerType_Vector_Vector_REAL. This parameter is optional and can be set to 0 if it is not required.

Application

The localization of a linear edge with expert parameters looks like this, for example:

hr := F_VN_LocateEdgesExp2(
    ipSrcImage              := ipImageIn,
    ipEdgePoints            := ipEdgePoints,
    aStartPoint             := aStartPoint,
    aEndPoint               := aEndPoint,
    nNumEdges               := nNumEdges,
    eEdgeDirection          := eDirection,
    bAlternateDirection     := bAlternateDirection,
    fMinStrength            := fMinStrength,
    nSearchLines            := nSearchLines,
    fSearchLineDist         := fSearchLineDist,
    nMaxThickness           := nMaxThickness,
    nSubpixelsIterations    := nSubpixIter,
    fApproxPrecision        := 0.001,
    eAlgorithm              := eAlgorithm, 
    fRangeCutoffFactor      := 0.135,
    ipEdgeStrengths         := ipEdgeStrengths,
    bAvgEdgeStrengths       := bAvgEdgeStrengths,
    hrPrev                  := hr,
    fAvgStrength            => fAvgStrength);

Related functions

Required License

TC3 Vision Metrology 2D

System Requirements