# Specific method for parametric drawing programs

The hard part of coding parametric drawing program whether in lisp or VBA is managing the large number of points. The program turns into many lines of hard to read data apparently randomly named. A sketch has to be made with points labeled and equations or formulas entered. It all might make sense during the coding, but probably won’t a few weeks later when a change has to be made even if the sketch(s) is found. It won’t be obvious how the points are calculated or why lines are drawn from pt7 to pt21 to pt3. There is no one right way but I have recently worked on both lisp and vba programs and have some specific but not comprehensive suggestions. This is a special theory for creating the xy data but not a general theory for the entire program.

There are two basic ways to manage your drawing subroutine. It can accept points or xy data. Try both ways. Both methods need xy data.

In Lisp I use Visual Lisp objects rather than the “command” method. The object method can draw directly in to a block definition, and it can directly change the layer property. It requires a point object, but that can be created and passed as a parameter or the xy data can be passed and the point created in the subroutine.

For lisp I made a point creation routine and passed points to the subroutine which runs the Addline method.

(defun pt ( x y ) (vlax-3d-point x y 0))

In a very simple box example this gets called as

(setq pt1 (pt 0 0) pt2 (pt L 0) pt3 (pt L W) pt4 (pt 0 W))

Then the line routine would be

(defun linep (pt1 pt2 obj lyr / lineobj)
(setq lineobj (vla-AddLine obj pt1 pt2))
(vla-put-layer lineobj lyr) )

And be called as

(linep pt1 pt2 ms "hidden")

Or you could pass xy data

(defun line (x1 y1 x2 y2 obj lyr / pt1 pt2 lineobj)
(setq pt1 (vlax-3d-point x1 y1 0)
pt2 (vlax-3d-point x2 y2 0))
(setq lineobj (vla-AddLine obj pt1 pt2))
(vla-put-layer lineobj lyr) )

In VBA every variable has to be declared previous to use, so you might lean towards passing xy data. Assume you want to draw a notched rectangle and make it a polyline. You make a sub specifically for this purpose. After setting the xy data coordinates, any six vertex closed polyline can be drawn with

Call p6_box(x1, y1, x2, y1, x2, y2, x3, y2, x3, y3, x1, y3)

Sub p6_box(p1 As Double, p2 As Double, p3 As Double, p4 As Double, p5 As Double, p6 As Double, _
p7 As Double, p8 As Double, p9 As Double, p10 As Double, p11 As Double, p12 As Double)

Dim objent As AcadLWPolyline
Dim pt(0 To 11) As Double
pt(0) = p1: pt(1) = p2
pt(2) = p3: pt(3) = p4
pt(4) = p5: pt(5) = p6
pt(6) = p7: pt(7) = p8
pt(8) = p9: pt(9) = p10
pt(10) = p11: pt(11) = p12
Set objent = acadDoc.ModelSpace.AddLightWeightPolyline(pt)
objent.Closed = True
Set obj_Acad_Entity = objent
End Sub

This makes no sense without a sketch but the sub p6_box can draw any closed polyline with 6 points configured any way you need it.

Our notched box is L X W with an A X B notch, drawn with the lower left corner at 0,0. There are 3 X coordinates and 3 Y coordinates.
X1=0 , X2=L-B , X3=L
Y1=0 , Y2=A , Y3=W

You can turn this box around any way you wish, move the notch to the middle, put a hole in the middle. Just label xy coordinates as needed in order from the origin. This is how you organize your xy data without duplication in a straightforward way. Sometimes its convenient to also label points, sometimes its not required, but the xy data must always be figured from the parameters as the first step.

In VBA we would probably draw in counterclockwise order.

Call p6_box(x1, y1, x2, y1, x2, y2, x3, y2, x3, y3, x1, y3)

Now it should make sense. The xydata starts at the origin. Subroutines can be written so declared point variables are not required, or required. If you have a lot of sub-routines, just declare your x1, x2, etc as public to avoid re-declaring.

In programming 101 they strongly suggest that your subroutines be simple and single purpose. Just about every autocad parametric program I have seen or written has been a mess at the actual geometry creation level. For instance in this example, the parameters A and B, L and W may need to have complicated formulas behind them. Put those upstream of the actual sub-routine that draws the geometry. Make the geometry creation as simple as possible. Pass the actual parameters if possible, do not develop them. Interface is top down thinking, but geometry is bottom up.
Such as

Sub draw_notch_box(W As Double, L As Double, A As Double, B As Double)
x1 = 0
x2 = L - B
x3 = L
y1 = 0
y2 = A
y3 = W
Call p6_box(x1, y1, x2, y1, x2, y2, x3, y2, x3, y3, x1, y3)
End Sub

You will be able to read that next year if you remember that xy data starts at the origin.

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