Trim to entity trims on wrong side

Got an annoying problem where trim to entity trims the wrong way when either or both entities are curves. I will attach some images to show because I can't start a discussion with an attachment apparently

Comments

  • Here's what I am want as indicated on Before image - what I actually get on after image. Note if I do the same thing on the other side of the stairwell (so it liek a mirror image) then it trims on the wrongside in the other direction
  • edited June 2012
    sorry - looks like only one attachment allowed - heres the before!!!!

    The curve in this case is a 2D Arc, if that helps
  • edited June 2012
    Yes it's a common behaviour with arcs due to a complex equation to solve. CC3 tends to keep the longest part.
    I usually work aroung by spliting the arc near the target trim point an trim afterwards but I'd be glad for a better way...
  • RalfRalf Administrator, ProFantasy 🖼️ 18 images Mapmaker
    Yeah, arcs can be annoying to trim or trim to. You can convert arcs to very closely matching paths in the following way:
    - use ScaleXY on the arc, using a scaling of 1.0 for both directions (no visible change in the entity)
    - Explode the arc - this results in a series of lines
    - use Line to Path to make these lines into a path.
    - you now have a properly behaving path entity that can be easily trimmed to.
  • pdjpdj Traveler
    Posted By: Ralf
    - use ScaleXY on the arc, using a scaling of 1.0 for both directions (no visible change in the entity)

    Go on, I have to ask. Why?

    P
  • RalfRalf Administrator, ProFantasy 🖼️ 18 images Mapmaker
    Okay, this going heading into the intricacies of the FastCAD engine, on which I am by no means an expert. But this is how I understand it:

    The Explode command separates an entity into it's constituent parts of the next lower order. E.g. a mulitpoly gets broken up into the paths that make it up, and Paths get broken down into lines. Lines can't get broken down any further, because they are basic entities - not made up of others.

    "2D Arcs" are basic entities too, so Explode doesn't work on them. But if you scale an arc differently in horizontal and vertical directions, it can't be described by the basic arc function anymore, so FastCAD has to convert it into a "2d Elliptical Arc", which is not a basic entity. Using Explode on it breaks it down into its constituent lines.

    Hope that clarifies things. I'm sure Simon will correct me if I grossly misrepresented the details.
  • I think I got it. The basic building blocks of a CAD program are as simple as they can be. There is no further down to break them into.
  • 4 months later
  • Haven't been on the forums in a while :)

    Surely it should trim the ends of the entities closest to where they were selected - rather than keeping the longest part of the arc, it should keep the part that I actually selected. Same if it was a circle.

    And a similar problem if you have an arc and you use the line adjust tool - dragging one end moves it as you want (I believe the ccw end?) but dragging the other end makes it go all the way around
  • DogtagDogtag Moderator, Betatester Traveler
    edited November 2012
    There you go, trying to apply logic to a computer program. Computers don't use lo-- oh, wait.

    ;-P
  • 9 days later
  • Noob advice here based on something said above. Shoot me if my assumptions are incorrect. Joachim de Ravenbel said that the program tends to keep the longer part of the trim. Why not trim twice? Move further away from where you want to trim and trim that then move closer and trim again? Would that work?
    JSM
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