A basic class implementation for turtle graphics in autocad –

The basic idea of turtle graphics is that the pen has a location and a direction. The instruction to draw a line only needs distance. The ending point is the new current location. To translate distance at angle from a given point to coordinates the Sin and Cosine are used.

The public class variables are x1, y1 – the current location, the beginning of the line to be drawn, and x2, y2, the calculated end points. Heading is a private double. It is private so that the class can always keep the angle heading between 0 <= heading < 360 no matter how many cumulative turns. A boolean PEN variable allows a PENUP or PENDOWN state.

Turtle1.FD 6 draws a line 6 units (assuming PEN is DOWN). Turtle.Left 45 and Turtle.Right 45 turn heading to the left or right 45 degrees or any number. Input to the user is in degrees. The class module converts to radians in private.

some example code using the turtle class –

Sub turtle_demo_A()

connect_acad

```
```Dim turtle1 As CTurtle

Set turtle1 = New CTurtle

Debug.Print turtle1.x1

Debug.Print turtle1.y1

Debug.Print turtle1.pen

'x1 and y1 are zero

'pen is true

'these are class defaults

Debug.Print turtle1.heading

'access to private variable pheading is thru LET and GET HEADING

'class_initialize default is 90

turtle1.heading = 540

Debug.Print turtle1.heading

'property Let heading (double)

'seems like an argument but you call LET with assignment

'property let heading sets and controls value between 0 <= pheading < 360

'heading is 180

turtle1.heading = turtle1.heading + 180

Debug.Print turtle1.heading

'heading is 0

'just like A = A + 3

'right side of equation is GET and left side is LET

turtle1.heading = turtle1.heading - 45

Debug.Print turtle1.heading

'heading is 315

turtle1.left 90

Debug.Print turtle1.heading

'heading is 45

turtle1.right 90

Debug.Print turtle1.heading

'heading is 315

turtle1.heading = turtle1.heading + 180

Debug.Print turtle1.heading

'heading is 135

turtle1.fd 12

'line is 45 to left

`End Sub`

all this boils down to –

Sub turtle_demo_B()

connect_acad

```
```Dim turtle1 As CTurtle

Set turtle1 = New CTurtle

turtle1.heading = 45

turtle1.x1 = 2

turtle1.y1 = 3

`turtle1.fd 12`

End Sub

The Class Module code for the turtle to draw a line contains the trigonometry to calculate the point at dist and angle. It draws the line using the familiar AddLine method with EndPoints as an array of 3 doubles, xyz. This is a 2D implementation, Z is always zero for now but does not have to be.

Public Sub fd(dist As Double)

'assumes x1 y1 and heading

x2 = x1 + dist * Cos(ang2rad(pheading))

y2 = y1 + dist * Sin(ang2rad(pheading))

```
```If pen Then

Call drawline(x1, y1, x2, y2)

End If

'updates to new position

x1 = x2

y1 = y2

End Sub

Sub drawline(x1 As Double, y1 As Double, x2 As Double, y2 As Double)

'internal sub to draw line

Dim acadline As acadline

Dim pt1(0 To 2) As Double

Dim pt2(0 To 2) As Double

pt1(0) = x1: pt1(1) = y1: pt1(2) = 0

pt2(0) = x2: pt2(1) = y2: pt2(2) = 0

`Set acadline = acadDoc.ModelSpace.AddLine(pt1, pt2)`

Update

End Sub

Drawing the same line in code with no class implementation –

Sub turtle_demo_C()

connect_acad

```
```Dim acadline As acadline

Dim pt1(0 To 2) As Double

Dim pt2(0 To 2) As Double

Dim x1 As Double, y1 As Double

Dim x2 As Double, y2 As Double

Dim ang As Double

Dim dist As Double

x1 = 2

y1 = 3

ang = 45

dist = 12

x2 = x1 + dist * Cos(ang2rad(ang))

y2 = y1 + dist * Sin(ang2rad(ang))

pt1(0) = x1: pt1(1) = y1: pt1(2) = 0

pt2(0) = x2: pt2(1) = y2: pt2(2) = 0

`Set acadline = acadDoc.ModelSpace.AddLine(pt1, pt2)`

acadApp.Update

End Sub

Instead of doing the calculations, Autocad provides the Utility PolarPoint to do the trig. Polarpoint returns an array. Autodesk help uses a Variant to capture it, but a dynamic array works fine (see pt2).

Sub turtle_demo_D()

connect_acad

```
```Dim acadline As acadline

Dim pt1(0 To 2) As Double

Dim pt2() As Double

Dim x1 As Double, y1 As Double

Dim ang As Double

Dim dist As Double

x1 = 0

y1 = 0

ang = 45

dist = 12

'these are commented out

'x2 = x1 + dist * Cos(ang2rad(ang))

'y2 = y1 + dist * Sin(ang2rad(ang))

pt1(0) = x1: pt1(1) = y1: pt1(2) = 0

pt2 = acadDoc.Utility.PolarPoint(pt1, ang2rad(ang), dist)

`Set acadline = acadDoc.ModelSpace.AddLine(pt1, pt2)`

acadApp.Update

End Sub

We can further simplify this process with a function to populate point arrays. Every Point can be declared as a dynamic array.

Function Pt(x As Double, y As Double, z As Double) As Double()

Dim pnt(0 To 2) As Double

pnt(0) = x: pnt(1) = y: pnt(2) = z

Pt = pnt

End Function

```
```Sub turtle_demo_E()

connect_acad

Dim acadline As acadline

Dim pt1() As Double

Dim pt2() As Double

Dim ang As Double

Dim dist As Double

ang = 45

dist = 12

pt1 = Pt(2, 3, 0)

pt2 = acadDoc.Utility.PolarPoint(pt1, ang2rad(ang), dist)

`Set acadline = acadDoc.ModelSpace.AddLine(pt1, pt2)`

acadApp.Update

End Sub

we can streamline a little bit more with a dedicated Line sub-routine. now lets compare the turtle and more conventional autocad methods. Each will draw a line and turn before drawing the next. Using dynamic arrays, what was pt2 can become pt1 with a simple assignment. That is not possible with the conventional static array where points are declared as – Dim PT1 (0 to 2) as Double.

Sub turtle_demo_F()

connect_acad

```
```Dim turtle1 As CTurtle

Set turtle1 = New CTurtle

turtle1.heading = 30

turtle1.x1 = 1

turtle1.y1 = 2

turtle1.fd 12

turtle1.left 30

turtle1.fd 12

Dim pt1() As Double

Dim pt2() As Double

Dim ang As Double

Dim dist As Double

ang = 45

dist = 12

pt1 = Pt(1, 2, 0)

pt2 = acadDoc.Utility.PolarPoint(pt1, ang2rad(ang), dist)

line1 pt1, pt2

pt1 = pt2

pt2 = acadDoc.Utility.PolarPoint(pt1, ang2rad(ang + 30), dist)

line1 pt1, pt2

`acadApp.Update`

End Sub

Turtle graphics has geometry implications, start here, go forward, turn and repeat. Turtle graphics is local with a simple interface and limited command set. Coordinate graphics is a global grid, but its interface can also be simplified. The two approaches might be able to work together.

A random star generator –

Sub turtle_demo_6()

init_turtle

Dim dblsize As Double

Dim inc As Integer

```
```For inc = 1 To 480

dblsize = rnddbl(0, 360)

turtle1.heading = dblsize

dblsize = rnddbl(0, 1024)

turtle1.x1 = dblsize

dblsize = rnddbl(512, 1024)

turtle1.y1 = dblsize

dblsize = rnddbl(2, 17)

star_5 dblsize

Next inc

End Sub

Sub star_5(dblsize As Double)

For i = 1 To 5

turtle1.fd dblsize

turtle1.right 144

Next i

End Sub

Function rnddbl(upr As Double, lwr As Double) As Double

' Randomize

' better results without Randomize

`rnddbl = CDbl((upr - lwr + 1) * Rnd + lwr)`

End Function

I cannot get the language tag VB to stick anymore. Thats why the code doesnt stand out from the text. WordPress challenges me.