Hough line.

Namespace:  AForge.Imaging
Assembly:  AForge.Imaging (in AForge.Imaging.dll) Version: 2.2.5.0 (2.2.5.0)

Syntax

C#
public class HoughLine : IComparable

Remarks

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

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:

CopyC#
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.

Inheritance Hierarchy

System..::.Object
  AForge.Imaging..::.HoughLine

See Also