OpenTK pick matrix calculation. Transforms viewmatrix according to pick location and picking region. Modified matrix can be used for object pick determination.

pickingmatrixcalculation.cs

/// <summary>
/// OpenTK/OpenGl pick matrix calculation. Takes pick location,pick region to
// modify the current viewport matrix.
/// </summary>
/// <param name="x">The x pick location.</param>
/// <param name="y">The y pick location.</param>
/// <param name="deltax">The pick region x bounds.</param>
/// <param name="deltay">The pick region y bounds.</param>
/// <param name="viewport">The viewport matrix.</param>
public void PickMatrix(double x, double y, double deltax, double deltay, int[] viewport)
{
    if (deltax <= 0 || deltay <= 0) { 
      return;
    }
 
    // Translate and scale the picked region to cover the
    // canvas 
    GL.Translate((viewport[2] - 2 * (x - viewport[0])) / deltax,
	    (viewport[3] - 2 * (y - viewport[1])) / deltay, 0);
    GL.Scale(viewport[2] / deltax, viewport[3] / deltay, 1.0);
}