F_VN_LocateEdge

Locate the points of the first occuring edge inside a specified search window, using an interpolation method.
Can use available TwinCAT Job Tasks for executing parallel code regions.
Can return partial results when canceled by Watchdog.

Syntax

Definition:

FUNCTION F_VN_LocateEdge : HRESULT
VAR_INPUT
    ipSrcImage     : ITcVnImage;
    ipEdgePoints   : Reference To  ITcVnContainer;
END_VAR
VAR_IN_OUT
    aStartPoint    : TcVnPoint2_REAL;
    aEndPoint      : TcVnPoint2_REAL;
END_VAR
VAR_INPUT
    eEdgeDirection : ETcVnEdgeDirection;
    fMinStrength   : REAL;
    nSearchLines   : UDINT;
    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_TcVnPoint2_REAL)
eEdgeDirection	ETcVnEdgeDirection	Specification of the edge direction to search 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
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 edge

Return value

HRESULT

Further information

The function F_VN_LocateEdge localizes an edge in a defined search window.

Parameter

Input image

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

Localized points ^{(Return value)}

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

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 with the parameters aStartPoint, aEndPoint and nSearchLines. 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_LocateEdge 6: — S: StartPoint, E: EndPoint, W: WindowWidth

Edge strength ^{(Return value)}

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

Parameters for edge localization

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

Expert parameters

The expert version F_VN_LocateEdgeExp contains additional parameters.

Application

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

hr := F_VN_LocateEdge(
    ipSrcImage              :=  ipImageIn,
    ipEdgePoints            :=  ipEdgePoints,
    aStartPoint             :=  aStartPoint,
    aEndPoint               :=  aEndPoint,
    eEdgeDirection          :=  eDirection,
    fMinStrength            :=  fMinStrength,
    nSearchLines            :=  nSearchLines,
    hrPrev                  :=  hr,
    fAvgStrength            =>  fAvgStrength
);

The localized edge points can be visualized with the function F_VN_DrawContours.

F_VN_DrawContours(ipEdgePoints, -1, ipImageRes, aColorGreen, 5, hr);

Samples

Locate Edge

Related functions

LocateCircularArc for localizing proportional circular arcs
LocateEdge for localizing linear edges
LocateEdges for localizing several linear edges
LocateEllipse for localizing complete ellipses

Required License

TC3 Vision Metrology 2D

System Requirements

Development environment	Target platform	PLC libraries to include
TwinCAT V3.1.4024.54 or later	PC or CX (x64) with PL50, e.g. Intel 4-core Atom CPU	Tc3_Vision