Tangent Extension to an Arc LISP
@JA5.15 wrote:
.... a LISP routine that allowed me to extend a line tangent from the endpoint of an arc. ....
A related routine, if you want to:
A) draw an Arc as a tangent continuation from either another Arc or a Line or the open end of a Polyline;
B) draw a Line as a tangent continuation from [yes, an Arc as in this thread] or another Line or the open end of a Polyline.
Try out the attached [in-progress -- see below] PolylineContinue .lsp with its PLC command. It finds the direction that the nearer end of any of those things is headed, and applies that if you take advantage of it to the subsequent segment of a Polyline. If what you pick on isn't already a Polyline, it will make it one, which you can Explode if you don't want it that way afterwards.
If what you want to do is a Line, since it will be in a Polyline command, you would use the Length option to get a line segment that's a tangent continuation from whatever you picked on. That's admittedly a little more cumbersome than what the other routines here do, when what you pick is an Arc, but it works in ways that the others don't, i.e. as a tangent continuation of another Line or an open Polyline. If what you want to do is an Arc, just go into PLINE's Arc option and it will do a tangent continuation [unless you then call for some other option such as to specify a Second point -- all options are still open to you].
As mentioned, it's in progress, with some issues I need to work on. I took out most of my notes to myself and trial approaches to try to overcome the issues mentioned at the top of the file. But in most ordinary circumstances it works fine. However, one that I would mention specifically is that you should use Enter/space to end the PLINE command that it goes into, rather than ESCape, or [until I work on it some more] it won't reset certain things for you.
@alanjt_ wrote:
The method you describe of drawing an arc, then being able to draw a line from that arc is precisely the reason I wrote the routine, as to avoid this extra set of steps that previously annoyed me for years.
Your routine is just the PLine command that is a continuation of a selected line/arc/pline. It does not account for the next segment being tangent from the selected segment. I mean no offense, but I'm not sure why you are posting this here.
If what you want to do is draw a Line that's a tangent continuation from the end of an Arc immediately after drawing the Arc [as your first sentence implies], then you don't need any routine at all. Draw the Arc, start a Line command, and when it's asking for the first point, just hit Enter, and see what happens. A routine would be needed only if you want to draw a Line tangent from the end of an Arc that isn't the last thing drawn, and that The User must select.
And yes, my routine does account for tangent continuation from the end of the selected object* [did you try it?], though with the admitted "cumbersome"-ness I described in Post 7, that if it's a Line you want to continue with, you need to use PLINE's Length option and give it a numerical length [which will be drawn tangentially], rather than pick along a tangential "drag" line. The main reason it determines the direction the end is headed is for the possibility of a tangent arc-segment continuation, but you can continue tangentially with a line segment. Its doing that for an arc extension, and its being able to continue three different entity types [whether or not you want to continue them tangentially], are for me of enough benefit to outweigh the cumbersomeness in that one circumstance. But for those who never need any such thing except to draw Line entities that are tangent continuations of Arc entities, when they haven't just drawn the Arc in question, your routine is certainly better.
I posted it because it draws exactly what's in the Subject line [people will find this thread by Searching for something like that], including being more universal than what's described in the first sentence of Post 1.
* I use the word "object" rather than your word "segment," because segment apply only in Polylines, and if in using PLC you should pick on a segment of an open Polyline that is not the end segment, it will find the nearer end and go from there, rather than from the segment you pick on [which of course can't be "continued" from], and if you should pick on a segment of a closed Polyline, it will tell you it can't work with one of those.
@alanjt_ wrote:
Immediately after drawing the arc, being the catalyst. Hence the reason I wrote the routine, as I stated.
I tried your routine, and when I selected an arc, it just started the pline command as normal, not drawing it tangent to anything.
Immediately after drawing the Arc, if you're continuing off the end rather than the start of the Arc, you don't need any routine to draw a tangent-continuation Line, as described in Post 9. [TLA even imposes an unnecessary extra step, since it requires selecting the Arc you just drew, which the method in Post 9 does not. But that method won't do what you're after if you want to draw the Line from the start of that Arc. However, it does "know" in which direction you drew it, even though an Arc's entity data stores information always with the "start" at the clockwise end, always proceeding counterclockwise.]
Yes, PLC starts a normal Pline command, and even though the rubber band doesn't lock into the tangent direction, if [as described in both Posts 7 & 9 -- did you try it?] you choose the Length option and give it a length, it does draw it as a tangent continuation from the end of the Arc [or Polyline or Line, if that's what you selected -- choices that TLA doesn't allow].
It could be made to lock into that direction with a < entry as in earlier posts here. But PLC is designed to continue a Polyline in whatever way you want to continue it, not only tangentially. Also, a locked-in direction at the start will be lost if the User calls for any other Pline-prompt options [Width, etc.] before picking a next-vertex point [not an issue with a Line, obviously].
And if you choose the Arc option, PLC does draw it tangent to the end [at least it "drags" that way, though you can overrule that with several of the Pline-prompt options], even though the selected object is not part of the Polyline it's drawing yet -- it's not truly "continuing" -- but will be joined to it when you're done.
PLC won't be all things to all people, and isn't preferable to TLA in all situations, but is just another way of achieving tangent continuations, in more situations than TLA can do.
@alanjt_ wrote:
I didn't realize you had to specify "length" first to get a tangent continuation in your command.
I have ... set cmdecho to 0, so I didn't see any prompts after selecting the initial arc.....
It's not part of my command, except as a result of being part of the PLINE command that it uses [when in line-segment mode]. PLC does assume command echoing being on to begin with. When I've worked out those kinks, it will turn it off for parts of what it does, and could be made to turn it on [rather than just reset whatever it was set to] for relevant parts, for anyone who may generally have it off.
