Self adjusting Attribute values based on point along curve

[Strydaris]

Morning Everyone,

 

Was hoping someone could help me out a little bit with something.

I am more looking for an answer to "Can this be done".

 

What I am looking for is to create a lisp that has a self adjusting attribute value.
This value would be based on a few factors. I think the best way to describe what I am looking for is to show an image and explain that way.

 

Point A & B would be given points (Elevation heights) and also fixed in place.

Slope to A & B could be anywhere between 2% to 5%.

Point C is the value I need returned along that curved polyline.

This much I can do. Its a bit tricky and probably needs some code updates, but I have it working.

What I am looking to add is something like this....

Give the user the option to LOCK a Slope at a percentage. [Lock Slope] Then [Lock Left Slope][Lock Right Slope][None]

Once a slope is locked, I would like the user to be able  to drag Point C to their desired location, with the number for Point C and the opposite slope being updated as point C is moved along the path.

I have seen Lee Mac's Object Align lisp and know about the vlax-Curve functions that can do the "Move Point C along that path, get the distance From point A to Point C along a curve etc etc. SO I can figure out how to do the math and get the numbers.
What I havent found is whether or not Attributes can be dynamically updated values or not.

Has anyone done anything like this before?

Thanks

[BIGAL]

You can get the angle of a point along a pline, never tried in arcs.

Try this, not really tested.

(defun alg-ang (obj pnt)
  (angle '(0. 0. 0.)
     (vlax-curve-getfirstderiv
       obj
       (vlax-curve-getparamatpoint
         obj
         pnt
       )
     )
  )
)

(setq obj (vlax-ename->vla-object (car  (entsel "Pick obj"))))
(setvar 'osmode 512) ; nearest for testing

(while (setq pt (getpoint "\npick point on pline"))
(princ (setq ang (alg-ang obj pt)))
)