Finding the points along a Polyline using ObjectARX, same concept in .NET too

by Fenton Webb

Any Curve derived object inside of AutoCAD uses Parameter values to define significant parts of that Curve. These Parameter values are defined as double values, ranging from 0.0 to n.

Lines define their Parameter values as 0.0 to Length. So a Line drawn from A to B would have a start parameter value of 0.0 and an end of Length – the midpoint, for instance, can be obtained by using :

double length = 0.0;
line->getEndParam(length);
line->getPointAtParam(length/2.0);

Circles define their Parameter values as 0.0 to 2*PI, as does Arcs and Ellipses. So, by obtaining the start and end params you can very easily get quadrant points using the same technique.

Using the Parameter methods of a Polyline you can get any point on a polyline; the Parameter values work roughly the same as a Line, except the Parameter values only define the vertexes as indexes, no length values – e.g. p1=Param 0.0, p2=Param 1.0, p3=Param 2.0, etc.

There are quite a few functions which work to obtain a point from a Parameter value or visa versa (search the ARX Reference for Param to see them all)…

Here’s a few which I use a lot when processing Curve geometry…

AcDbCurve::getParamAtPoint – get the parameter value at a given point, say the start point = vertex 3 and the end point = vertex 4.

// open a polyline and extract the vertex data from it
// start and end points for instance.....
ads_real fromParam = 0.0; 
// get the parameter value of vertex 3 start point 
pPoly->getParamAtPoint (startPoint, fromParam);       
ads_real toParam = 0.0; 
// get the parameter value of vertex 4 start point 
pPoly->getParamAtPoint (endPoint, toParam);       
 
 AcDbCurve::getPointAtParam - get a point using a parameter value. Using the fromParam and toParam above, we can obtain the midpoint between the startPoint (param 3) and the endPoint (param 4) like this

// get the mid point of the start and end points along the pline
pPoly->getPointAtParam (fromParam + ((toParam-fromParam) / 2.0), midPoint); 

AcDbCurve::getDistAtParam - get a distance from a Parameter value

Now because the parameter values of a Polyline are not length related, but vertex related, then the midpoint parameter value of the entire Polyline, and then its actual midpoint could be obtained like this…

double startParam=0.0, endParam=0.0, midParam=0.0, dist=0.0;
// always get the startParam, because it tends to be 
// defined slightly differently across different curve types
pPoly->getStartParam(startParam);
pPoly->getEndParam(endParam);
// now get the midpoint of the Polyline using params
AcGePoint3d midPoint;
pPoly->getDistAtParam(endParam, dist); 
pPoly->getParamAtDist(dist/2.0, midParam);
pPoly->getPointAtParam(midParam, midPoint);