CAD: An approach to mathematical curves for engineering, part 2, the coding.

This is a series about creating mathematical curves in BricsCAD (and AutoCAD).
Part 0, just hackish with a spreadsheet, result driven.
Part 1, the math, understanding ways to iterate.
Part 2, the coding, techniques available.

THIS ARTICLE IS UNDER CONSTRUCTION. YOUR POSITIVE INPUT IS APPRECIATED.

In part 1 the mathematics are explained. It turns out to be a solid base with a very functional algorithm in order to draw approximated curves.

Status Quo

This pages offers clues for coding. I’ve tried several concepts as described in part 1. After ending up writing a few hundred lines of code I decided to stop because a proper solution is simply a lot of work in unpaid time that lacks.

That is not the complete story, after tinkering, I also came to the conclusion that using a non uniform rational basis spline, or NURBS , or spline is a proper alternative after all and, combined with the theories in part 1, it offers best possibilities (and consumes a lot of my time).

Does it mean that this page is worthless? No, it offers additional insights that you will not find on the net in a structured way.

Making a NURBS

Ways in Lisp

In Lisp there are several ways to create a NURBS or spline:

• Very easy with (command “._spline” …), not recommended for production environments.
• The object-way with (vla-AddSpline …), not discussed further.
• The old fashioned way, using (entmake …), discussed here.

Example

“Entmake” that spline

Consider the following code:

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(setq point-list '((0 0) (1 1) (2 0) (3 -2) (4 0)))
(entmake
   (append
      (list
         '(0 . "SPLINE")
         '(100 . "AcDbEntity")
         '(100 . "AcDbSpline")
         '(70 . 4)
         '(71 . 3)
         (cons 74 (length point-list))
      )
      (mapcar '(lambda (xy) (cons 11 xy)) point-list)
   )
)

The first line is paste-able on the CAD command line as:

(setq point-list '( (0 0) (1 1) (2 0) (3 -2) (4 0)))

The second part as:

(entmake (append (list '(0 . "SPLINE") '(100 . "AcDbEntity") '(100 . "AcDbSpline") '(70 . 4) '(71 . 3) (cons 74 (length point-list))) (mapcar '(lambda (xy) (cons 11 xy)) point-list)))

Going through the code:

• “70 . 4” is DXF code 70 for spline type 4, i.e. “Rational spline”.
• “71 . 3” means degree 3.
• There are 5 points, so: “74 . 5”.
• “(mapcar “(lambda (xy) (cons 11 xy)) point-list)” returns a list with code “11” sublists: “((11 0 0) (11 1 1) (11 2 0) (11 3 -2) (11 4 0))”

That is all it takes to create a spline. Code “11” points are fit-points, going through the curve, what we want. This could also be “10” for control-points.

There is much more to be aware of, see DXF-codes for Splines. and more on NURBS at Wikipedia.

How to feed mathematical functions to the program?

We’ll start with a few examples.

Function files

Let’s say, we have a function . What we do is put it in a file with extension fun, for example: curve.fun. The filename should represent what you define in the file.

Please, do yourself a favour, use a suitable text editor like notepad++ and mark your code as Lisp during editing.

Syntax explained

What is in the file? First of all: Valid Lisp code, more on that later. What is needed is:

• A safety factor to make sure the curve is within the deviation limits: fuzzy
• A maximum deviation: dev
• A tolerance as a part of dev: tol
• A start value for x: xmin
• An end value for x: xmax
• Divide calculations: xdiv
• The function: fx

A valid file contains:

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(tol 0.2)
(fuzzy 0.000)
(dev 0.05)
(xmin 0)
(xmax 2)
(xdiv 1)
(fx (* x x))

That should be clear, setq assigns a value to a variable. What can go wrong? The number of opening parenthesis should be equal to the number of closing parenthesis. That is the number 1 error.

Defining your functions

Arithmetic functions

Here is a list of some arithmetic functions…

Examples

• Note that 6 is an integer and 6.0 is a real.
  • 
  • (!)
• Angles are in radians, 2*pi radians is 360 degrees.
• Expressions are evaluated inside out: (than-this (first-this))

The line containing a function, (fx (* x x)), is a list in a list. The examples below are like (fx (example)).

• (* x x)
• (expt x 2)
• (+ (* x x) (* 6.0 x) -2.5)
• (- (sin x) (* x (cos x)))
• (abs x)
• (int x)
• (sqrt (expt x 2))

How configuration data is handled

What curve does is very basic: It reads lines from the files an interpretes them when they are in list format. A bit more in detail: the line “(xmax 3.2)” is a string. By using the read function, the line is converted to a list. From that point, further processing in Lisp is possible with a set statement (not setq).

Additional considerations

The math part describes a way to calculate valid fit points by iterating checks based on core CAD-Lisp functions. In addition, it is possible to use a set of curve measurement Lisp functions – link broken, cache:

In BricsCAD these functions are also available on OS-X and Linux.