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

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