[Qgis-developer] calculate distance from two points along line in pyqgis

[gene]

It is a pure problem of analytical geometry or vector calculus (look at 
PyQGIS: geometry, vectors, vector algebra or direction cosines...(in French
<http://www.portailsig.org/content/pyqgis-des-geometries-des-vecteurs-de-l-algebre-vectorielle-ou-des-cosinus-directeurs> 
))

Is the line is straight the cartesian distance between two points is given
by

    def distance(point1,point2):
        #Theoretical cartesian Euclidean distance between 2 points (x,y)
        return math.sqrt((point2.x()-point1.x())**2 +
(point2.y()-point1.y())**2)

As PyQGIS has the sqrDist function, you can replace by

    def distance(point1,point2):
        return math.sqrt(point1.sqrDist(point2))

By definition, the point of intersection between two lines lies on the two
lines. It is easy to extract the start point or the end point of the lines
and calculate the distance.

If you want a confirmation, there are many algorithms in Python to verify
that 3 points are collinear

If the line has many segments, work at the straight segments level 



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