F_VN_LocateEllipse

Locate an ellipse, using an interpolation method for locating the edges.
Can use available TwinCAT Job Tasks for executing parallel code regions.
Can return partial results when canceled by Watchdog.

Syntax

Definition:

FUNCTION F_VN_LocateEllipse : HRESULT
VAR_INPUT
    ipSrcImage     : ITcVnImage;
END_VAR
VAR_IN_OUT
    stEllipse      : TcVnRotatedRectangle;
    aCenter        : TcVnPoint2_REAL;
END_VAR
VAR_INPUT
    fSearchRadius  : REAL;
    eEdgeDirection : ETcVnEdgeDirection;
    fMinStrength   : REAL;
    hrPrev         : HRESULT;
END_VAR

Inputs

Name	Type	Description
ipSrcImage	ITcVnImage	Source image (1 channel)
fSearchRadius	REAL	Radius around aCenter to search for edges (aCenter + fSearchRadius in all directions must be inside image borders!)
eEdgeDirection	ETcVnEdgeDirection	Specification of the edge direction to search for (from center to outside ellipse)
fMinStrength	REAL	Specification of the minimum strength (intensity difference) of the edge to search for
hrPrev	HRESULT	HRESULT indicating the result of previous operations (If SUCCEEDED(hrPrev) equals false, no operation is executed.)

In/Outputs

Name	Type	Description
stEllipse	TcVnRotatedRectangle	Returns the detected ellipse
aCenter	TcVnPoint2_REAL	The expected ellipse center

Return value

HRESULT

Further information

The F_VN_LocateEllipse function localizes and measures an ellipse. It is based on the edge localization and uses normal parameters. It returns the localized ellipse as a structure of the type TcVnRotatedRectangle (center point, size and angle).

Parameter

Input image

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

Ellipse ^{(Return value)}

The stEllipse parameter returns the localized ellipse as structure of the type TcVnRotatedRectangle (center point, size and angle).

Search circle

The circular search area is defined via the expected aCenter center point and a fSearchRadius search radius. The search radius should be set somewhat larger than the expected ellipse radius in order to securely localize the ellipse.

Parameters for edge localization

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

Expert parameters

The F_VN_LocateEllipseExp expert version contains additional parameters.

Application

The localization and measurement of an ellipse with an estimated center point of [360, 240] and a radius of 100px looks like this, for example:

VAR
    aGuessedCenter      :   TcVnPoint2_REAL := [360, 240];
    fGuessedRadius      :   REAL := 100;
END_VAR

hr := F_VN_LocateEllipse(
    ipSrcImage          := ipImageIn,
    stEllipse           := stEllipse,
    aCenter             := aGuessedCenter,
    fSearchRadius       := (fGuessedRadius + 15), // increase search radius to make sure ellipse is found!
    eEdgeDirection      := TCVN_ED_LIGHT_TO_DARK,
    fMinStrength        := fMinStrength,
    hrPrev              := hr);

The properties of the localized ellipse can then be accessed via the stEllipse.aCenter, stEllipse.stSize and stEllipse.fAngle variables. In addition, the ellipse can be visualized with the F_VN_DrawEllipse function:

hr := F_VN_DrawEllipse(stEllipse, ipImageRes, aColorGreen, 5, hr);

Samples

Locate Ellipse

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