Turns out there's a built in way to get line segments out of shapes in OpenRNDR.

Using this I get less points and about the same quality as what I originally intended. Plotting seems to be working. I still need to finish setting up the coordinate system and scaling.

src/main/kotlin/TemplateProgram.kt

@@ -1,13 +1,11 @@
-import com.martianwabbit.HPGL
+
-import com.martianwabbit.logger
+import com.martianwabbit.saveToHPGL
+import com.martianwabbit.selectPen
 import org.openrndr.application
 import org.openrndr.color.ColorRGBa
 import org.openrndr.math.Vector2
 import org.openrndr.shape.CompositionDrawer
 import java.io.File
-import com.martianwabbit.saveToHPGL
-import org.openrndr.Configuration
-import org.openrndr.Program
 
 fun main() = application {
     configure {
@@ -18,11 +16,14 @@ fun main() = application {
     program {
         extend {
             val c = CompositionDrawer()
-            c.circle(Vector2(20.0, 20.0), 12.0)
+            c.selectPen(1)
+            c.circle(Vector2(40.0, 40.0), 30.0)
 
+            c.rectangle(75.0, 60.0, 80.0, 200.0)
+
+            c.lineSegment(Vector2(0.0, 0.0), Vector2(200.0, 200.0))
             c.composition.saveToHPGL(File("output.plt"))
 
-
             this.application.exit()
             drawer.background(ColorRGBa.WHITE)
             drawer.composition(c.composition)

src/main/kotlin/com/martianwabbit/hpglWriter.kt

@@ -3,37 +3,74 @@ package com.martianwabbit
 import org.openrndr.math.Vector2
 import org.openrndr.shape.*
 import java.io.File
-import java.util.logging.Logger
-import kotlin.math.pow
-import kotlin.math.sqrt
-
-val logger = Logger.getLogger("")
 
 fun Composition.saveToHPGL(file: File) {
-    HPGL().generateFromComposition(this)
+    val plot = HPGL().generateFromComposition(this)
+    file.writeText(plot)
 }
 
 class HPGL(pageSize: PageSize = PageSize.A4) {
-    fun generateFromComposition(composition: Composition) {
+    private var p1: Vector2
-        composition.root.traverse { shape ->
+    private var p2: Vector2
-            if (shape == Stage.Before) {
+    private val resolution = 35
-                if (this is ShapeNode) {
+
-                    this.shape.contours.forEach { shape ->
+    init {
-                        val i = shape.sampleEquidistant(200)
+        // TODO: Set these according to plotter
-                        val rr = StringBuilder()
+        when (pageSize) {
-                        for (v in i.segments) {
+            PageSize.A3 -> {
-                            rr.appendln("${v.start.x}, ${v.start.y}")
+                p1 = Vector2(0.0, 0.0)
+                p2 = Vector2(0.0, 0.0)
+            }
+
+            PageSize.A4 -> {
+                p1 = Vector2(603.0, 521.0)
+                p2 = Vector2(10603.0, 7721.0)
+            }
+        }
+    }
+
+    fun generateFromComposition(composition: Composition): String {
+        val result = StringBuilder()
+
+        result.appendln("IN;") // Initialize the plotter
+        result.appendln("IP${p1.getCoords()},${p2.getCoords()};") // Setup coordinates
+        result.appendln("SC0,${p2.div(10.0).x},0,${p2.div(10.0).y};") // Setup coordinates
+        result.appendln("PA0,0;") // Plot absolutely
+
+        composition.root.traverse { stage ->
+            if (stage == Stage.Before) {
+                when(this) {
+                    is ShapeNode -> {
+                        this.shape.contours.forEach { shape ->
+                            val segments = shape.sampleLinear().segments
+                            segments.forEachIndexed { i, segment ->
+                                if (i == 0) {
+                                    result.appendln("PU${segment.start.getCoords()};") // Go to shape start
+                                    result.appendln("PD${segment.end.getCoords()};")
+                                } else {
+                                    result.appendln("PD${segment.start.getCoords()};") // PD coordinates for all other segments
+                                    result.appendln("PD${segment.end.getCoords()};") // PD coordinates for all other segments
+                                }
+                            }
+
+                            result.appendln("PU;") // Done plotting
                         }
-                        calculateSpline(shape.segments)
+                    }
+
+                    is SelectPen -> {
+                        result.appendln("SP${this.penNumber};")
                     }
                 }
             }
         }
+
+        result.appendln("PA0,0")
+        return result.toString()
     }
 }
 
-private fun Vector2.distanceTo(point: Vector2): Double {
+private fun Vector2.getCoords(): String {
-    return sqrt((point.x - this.x).pow(2.0) + (point.y - this.y).pow(2.0))
+    return "$x,$y"
 }
 
 enum class PageSize {
@@ -49,51 +86,21 @@ private enum class Stage {
 private fun CompositionNode.traverse(cb: CompositionNode.(stage: Stage) -> Unit) {
     this.cb(Stage.Before)
 
-    if (this is GroupNode) {
+    if (this is GroupNode && this.children.size > 0) {
         this.children.forEach { it.traverse(cb) }
     }
+
+    this.cb(Stage.After)
 }
 
-// These default values correspond to a centripetal Catmull-Rom spline
+class SelectPen(val penNumber: Int) : GroupNode()
-private fun calculateSpline(segments: List<Segment>, resolution: Int = 100, alpha: Double = .5, tension: Double = .0) {
+class VelocitySelect(val velocity: Int) : GroupNode()
-    val result = mutableListOf<Vector2>()
-    segments.forEach {
-        val points = mutableListOf<Vector2>()
-        points.add(it.start)
-        points.addAll(it.control)
-        points.add(it.end)
 
-        when(points.size) {
+fun CompositionDrawer.selectPen(pen: Int) {
-            4 -> { // Catmull-Rom
+    this.root.children.add(SelectPen(pen))
-                val t0 = 0.0
-                val t1 = t0 + points[0].distanceTo(points[1]).pow(alpha)
-                val t2 = t1 + points[1].distanceTo(points[2]).pow(alpha)
-                val t3 = t2 + points[2].distanceTo(points[3]).pow(alpha)
-
-                val p0 = points[0]
-                val p1 = points[1]
-                val p2 = points[2]
-                val p3 = points[3]
-
-                val m1 = ((p1 - p0) / (t1 - t0) - (p2 - p0) / (t2 - t0) + (p2 - p1) / (t2 - t1)) * (1.0 - tension) * (t2 - t1)
-                val m2 = ((p2 - p1) / (t2 - t1) - (p3 - p1) / (t3 - t1) + (p3 - p2) / (t3 - t2)) * (1.0 - tension) * (t2 - t1)
-
-                val a = (p1 - p2) + m1 + m2 * 2.0
-                val b = ((p1 - p2) * -3.0) - m1 - m1 - m2
-
-                for (i in 0..100 step 10) {
-                    val t = (i / 100).toDouble()
-                    val r = a * t.pow(3) + b * t.pow(2) + m1 * t.pow(1) + p1
-                    result.add(r)
-                }
-            }
-        }
-    }
-
-    val rr = StringBuilder()
-    for (v in result) {
-        rr.appendln("${v.x}, ${v.y}")
-    }
-    val a = Vector2(0.0, 10.0).distanceTo(Vector2(13.0, 12.0))
-    logger.info("done")
 }
+
+fun CompositionDrawer.velocitySelect(velocity: Int) {
+    this.root.children.add(VelocitySelect(velocity))
+}
+