Programming

I have been working with the Civil 3D 2010 API in visual lisp recently, and I thought I would share an example of working with an alignment object. This example has plenty of comments, but basically it shows you how to find a point near the alignment based on a station and offset. Then it does the opposite and shows you how to determine the station and offset, given a point. For clarity, I have left out most of the error checking.

Ok, getting back to the last (ssget) call in part 1...

(ssget "_W" 
 (list 5.0 5.0)(list 8.0 8.0)
 '((0 . "CIRCLE")(-4 . "<")(40 . 1.0))
)

If you have written routines with Autolisp, then you have probably used the (ssget) function to select entities on the screen, either automatically or by prompting the user.

(ssget) is a powerful function that can do more than you probably realize. Let's look at a simple example.

(ssget '((0 . "TEXT")))

In the previous post about Startup Lisp Functions, the special (S::STARTUP) function was mentioned. Startup lisp code is loaded before the drawing is initialized, but you cannot call the (COMMAND) function until after the drawing is initialized. The solution is to place your (COMMAND) calls inside the (S::STARTUP) function.

If you have lisp routines that you want to make available in each drawing session, forget the "Startup Suite" and load them using the "acaddoc.lsp" file. "acaddoc.lsp" is not included with AutoCAD, you create it yourself. But it is nothing more than a plain lisp file, except that the first one found at drawing startup (just put it in your support file search path), is loaded automatically each time a drawing is opened.

Here is a simple example of how to read a TXT file into lisp and then do something with the contents. In this example, the TXT file contains coordinates, and the code will draw either points or lines.

One thing to keep in mind. Most times when you open a file for read or write, you should open the file, perform the entire operation, then close the file. Keep the code to a minimum while the file is open. This way if you run into an unhandled error, the file isn't stuck open, and in the case of shared files, you are not locking the file for an extended amount if time.

Here are some free Autolisp resources including tutorials and sample code:

If you program with autolisp, you have probably used the command function at some point, probably to construct drawing entities. There is certainly nothing wrong with that. However, if you are working on a large program that constructs a lot of drawing entities, you may have noticed that the command function runs pretty slow.

I put together some tests to compare the (command) function to two other methods of entity creation, (entmake) and (vla-add...). The test constructs 1,999 line entities using various methods.

Starting in AutoCAD 2006, you can specify a highlight color and opacity to your selection areas as illustrated by the green area shown in the example below.

Below is a description of these options and how to change them.

If you want to go through the OPTIONS dialog, open it up and switch to the Selection tab. Click on the Visual Effect Settings button. Everything you need to control is there on the right half of this dialog, shown above.

Confused about the various polyline types that you see in AutoCAD?

From the end user perspective there are 3 polyline types listed in the PROPERTIES command, even though there are only two "entity" types.

• polyline - this is an LWPOLYLINE entity, always 2D.
• 2d polyline - this is a POLYLINE entity, also 2D.
• 3d polyline - this is a POLYLINE entity, usually 3D, with each vertex at a different elevation. It is possible to have a "3d polyline" and have each vertex at the same elevation.