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HoughLine Class

Hough line.
Inheritance Hierarchy
SystemObject
  Accord.ImagingHoughLine

Namespace:  Accord.Imaging
Assembly:  Accord.Imaging (in Accord.Imaging.dll) Version: 3.7.0
Syntax
public class HoughLine : IComparable
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The HoughLine type exposes the following members.

Constructors
  NameDescription
Public methodHoughLine
Initializes a new instance of the HoughLine class.
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Methods
  NameDescription
Public methodCompareTo
Compare the object with another instance of this class.
Public methodEquals
Determines whether the specified object is equal to the current object.
(Inherited from Object.)
Protected methodFinalize
Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.
(Inherited from Object.)
Public methodGetHashCode
Serves as the default hash function.
(Inherited from Object.)
Public methodGetType
Gets the Type of the current instance.
(Inherited from Object.)
Protected methodMemberwiseClone
Creates a shallow copy of the current Object.
(Inherited from Object.)
Public methodToString
Returns a string that represents the current object.
(Inherited from Object.)
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Fields
  NameDescription
Public fieldIntensity
Line's absolute intensity, (0, +∞).
Public fieldRadius
Line's distance from image center, (−∞, +∞).
Public fieldRelativeIntensity
Line's relative intensity, (0, 1].
Public fieldTheta
Line's slope - angle between polar axis and line's radius (normal going from pole to the line). Measured in degrees, [0, 180).
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Extension Methods
  NameDescription
Public Extension MethodHasMethod
Checks whether an object implements a method with the given name.
(Defined by ExtensionMethods.)
Public Extension MethodIsEqual
Compares two objects for equality, performing an elementwise comparison if the elements are vectors or matrices.
(Defined by Matrix.)
Public Extension MethodToT
Converts an object into another type, irrespective of whether the conversion can be done at compile time or not. This can be used to convert generic types to numeric types during runtime.
(Defined by ExtensionMethods.)
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Remarks

Represents line of Hough Line transformation using polar coordinates. See Wikipedia for information on how to convert polar coordinates to Cartesian coordinates.

Note Note
Hough Line transformation does not provide information about lines start and end points, only slope and distance from image's center. Using only provided information it is not possible to draw the detected line as it exactly appears on the source image. But it is possible to draw a line through the entire image, which contains the source line (see sample code below).

Sample code to draw detected Hough lines:

HoughLineTransformation lineTransform = new HoughLineTransformation( );
// apply Hough line transofrm
lineTransform.ProcessImage( sourceImage );
Bitmap houghLineImage = lineTransform.ToBitmap( );
// get lines using relative intensity
HoughLine[] lines = lineTransform.GetLinesByRelativeIntensity( 0.5 );

foreach ( HoughLine line in lines )
{
    // get line's radius and theta values
    int    r = line.Radius;
    double t = line.Theta;

    // check if line is in lower part of the image
    if ( r < 0 )
    {
        t += 180;
        r = -r;
    }

    // convert degrees to radians
    t = ( t / 180 ) * Math.PI;

    // get image centers (all coordinate are measured relative
    // to center)
    int w2 = image.Width /2;
    int h2 = image.Height / 2;

    double x0 = 0, x1 = 0, y0 = 0, y1 = 0;

    if ( line.Theta != 0 )
    {
        // none-vertical line
        x0 = -w2; // most left point
        x1 = w2;  // most right point

        // calculate corresponding y values
        y0 = ( -Math.Cos( t ) * x0 + r ) / Math.Sin( t );
        y1 = ( -Math.Cos( t ) * x1 + r ) / Math.Sin( t );
    }
    else
    {
        // vertical line
        x0 = line.Radius;
        x1 = line.Radius;

        y0 = h2;
        y1 = -h2;
    }

    // draw line on the image
    Drawing.Line( sourceData,
        new IntPoint( (int) x0 + w2, h2 - (int) y0 ),
        new IntPoint( (int) x1 + w2, h2 - (int) y1 ),
        Color.Red );
}

To clarify meaning of Radius and Theta values of detected Hough lines, let's take a look at the below sample image and corresponding values of radius and theta for the lines on the image:

Detected radius and theta values (color in corresponding colors):

  • Theta = 90, R = 125, I = 249;
  • Theta = 0, R = -170, I = 187 (converts to Theta = 180, R = 170);
  • Theta = 90, R = -58, I = 163 (converts to Theta = 270, R = 58);
  • Theta = 101, R = -101, I = 130 (converts to Theta = 281, R = 101);
  • Theta = 0, R = 43, I = 112;
  • Theta = 45, R = 127, I = 82.

See Also