select lwpolyline then move to next vertex to get the angle

I have select a lwpolyline by using GetSubEntity. This lwpolyline could
have an arc in them. I would like get next vertex after the location I
have selected. I did use the IntersectWith but I have to draft a dumbing
line. Is there a better way?

 

Bill,

Here's a little trick for adding a vertex at given point (or before/after
given vertex) along the pline:

1) Turn on OSNAP=Nearest to prevent picking a point outside a pline
2) Find all distances between picked point (or vertex) and every pline's
vertice
3) Assign an index to some variable during iterating the vertices
4) Compare them all to each other and find the shortest one - an index
assigned to that specific distance is a vertex _nearest_ to the given point.

Now, for finding if adding a vertex is supposed to happen "before" or
"after" a found one, do the following:

1) Calculate the original pline's length
2) Add a vertex "after" the found one using above described function
3) If new pline's length is different than the original one, then undo step
2) and add a vertex "before" the calculated one.

Regards,
Maksim Sestic



----- Original Message -----
From: "Bill Schooler" <>
Newsgroups: autodesk.autocad.customization.vba
Sent: Tuesday, June 08, 2004 18:42
Subject: select lwpolyline then move to next vertex to get the angle

 

Hmmm...how are you proposing that the distance be measured? If it is linear then wouldn't a polyline that turns back toward the selected spot potentially give a false answer?
[pre]
|____.____a
b____|
|
[/pre]
In the above example the next vertex from the selected point "." should be "a" but the closest is "b".

Regards

Wayne Ivory
IT Analyst Programmer
Wespine Industries Pty Ltd

 

True... I've been working with more or less "straight" polylines that don't
"bend" much toward one of it's vertices. There must be some more universal
solution to this... probably relying on measuring a PolarPoint between two
neighbouring vertices and comapring it to a given point along a pline...

Most of my time I spent solving similar things in VBA, so I wondered if some
of the solutions might be of interest to the wider auditorium. Maybe I
publish them on a Web site, as I certainly need assurance regarding
implemented approach and correctness (I'm a big fan of "universal"
solutions)

Regards,
Maksim Sestic

linear then wouldn't a polyline that turns back toward the selected spot
potentially give a false answer?

 

I would loop through the poly's segments, and just check which segment the
picked point is on (assuming the osnap was used). This should only give
ambiguous results when the user picks exactly at an intersection, or picks
overlapping segments, which are both ambiguous situations, anyways. If you
don't want to use snap, change the FUZZ loop to instead track the minimum
distance from the pick pt to each segment, and call that the picked segment.

HTH,
James


[pseudocode]
'coords() = polyline vertices
'pickPt = variant holding picked point
dim x() as double, y() as double, z() as double
numPts = ((UBound(coords) + 1) / 3) 'assumes 0-bounded array of 3-D
points
ReDim x(0 To numPts - 1)
ReDim y(0 To numPts - 1)
ReDim z(0 To numPts - 1)
For i = 0 To numPts - 1
x(i) = coords(3 * i)
y(i) = coords(3 * i + 1)
z(i) = coords(3 * i + 2)
Next 'i

'
const FUZZ as double = 0.001
dim dist_pt_to_seg as double
for i = 0 to numpts-2 'different for closed pline's ?
dist_pt_to_seg = Dist_Pt_Ln (pickpt, acPoint3D(x(i),y(i),z(i)),
acPoint3D(x(i+1),y(i+1),z(i+1)))
if abs(dist_pt_to_seg) < FUZZ then
'insert new vertex after i'th vertex and stop checking
exit for
endif
next 'i
[/pseudocode]

Code:

Function Dist_Pt_Ln(Pt As Variant, EndA As Variant, EndB As Variant) As
Double

'courtesy http://softsurfer.com/Archive/algorithm_0102/algorithm_0102.htm
'cross-product formulation used

Dim w(0 To 2) As Double
Dim u(0 To 2) As Double
Dim v(0 To 2) As Double
Dim UxW(0 To 2) As Double
Dim len_v As Double, w_dot_u As Double

v(0) = EndB(0) - EndA(0)
v(1) = EndB(1) - EndA(1)
v(2) = EndB(2) - EndA(2)
w(0) = Pt(0) - EndA(0)
w(1) = Pt(1) - EndA(1)
w(2) = Pt(2) - EndA(2)
len_v = Sqr(v(0) ^ 2 + v(1) ^ 2 + v(2) ^ 2)

u(0) = v(0) / len_v:  u(1) = v(1) / len_v
u(2) = v(2) / len_v

UxW(0) = u(1) * w(2) - u(2) * w(1)
UxW(1) = u(2) * w(0) - u(0) * w(2)
UxW(2) = u(0) * w(1) - u(1) * w(0)

Dist_Pt_Ln = Sqr(UxW(0) ^ 2 + UxW(1) ^ 2 + UxW(2) ^ 2)

End Function



Public Function acPoint3D(x As Double, y As Double, Optional z As Double =
0) As Variant
' courtesy www.acadx.com
Dim retVal(0 To 2) As Double
retVal(0) = x: retVal(1) = y: retVal(2) = z
acPoint3D = retVal
End Function

 

James,

What I had in mind is function that adds a vertex at picked point along the
polyline. Like BREAK and PLJOIN functions incombined. ACAD is generally
missing such generic function, while PEDIT is the only possible (but by far
complicated) exit. Many users I've been working with are mainly switching
from other vector-based applications (Corel, Visio...) to ACAD and they
usually miss some sort of Add-Vertex-At-Picked-Point functionality.

As I'm in development of tools that immitate Autodesk's MAP ones, I'm fully
aware of ACAD's backdraws when we talk about
precision/exactness/ease-of-use. There's also Autodesk's lean towards GIS
implementation, and it has a lot to do with it, but I guess this reply is
way way out of topic this time...

Regards,
Maksim Sestic

 

OOPS... BIG ERROR...(but simple fix)... I used a "Distance from Point to
Line" function, when I needed to use a "Distance from Point to Line SEGMENT"
function.... I believe that same website had an algorithm for this function,
too. I don't have one coded.

 

Maksim,
Yes, my reply was attempting to do the same thing that you are, "Add a New
Vertex" functionality. Maybe my explanation paragraph was not all that
great. (Also, see my other reply for an error I just saw). As you
suggested, if the OSNAP NEAREST was used, you know that the picked point
would lie on the PLINE. All I was trying to add was a good way to figure
out which segment was clicked on. The answer was that the segment that was
clicked should have near-zero distance between the point and the segment,
measured perpendicular.

From here, the options are endless... if you don't want to use OSNAP, you
can just iterate all the segments and use the minimum distance as the picked
segment... very user-friendly and forgiving in case the user changes the
OSNAP manually or doesn't pick close enough to the PLINE to make the OSNAP
happen. You can also have a tolerance similar to the PICK WINDOW size -- if
the closest segment is further away than this distance, then the user didn't
get close enough to any segments.... so return nothing. Also, not forcing
OSNAPS allows you to add the vertex away from the PLINE and add shape.


James