Added velocitySelect, some extra checks to the custom commands. Finally settled on a scaling strategy (Stick with the default IP but scale according to a given size (this size should ideally be the screen size openrndr is using on the screen)).

src/main/kotlin/TemplateProgram.kt

@@ -18,11 +18,10 @@ fun main() = application {
             val c = CompositionDrawer()
             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"))
+
+            c.composition.saveToHPGL(File("output.plt"), this.width, this.height)
 
             this.application.exit()
             drawer.background(ColorRGBa.WHITE)

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

@@ -4,44 +4,41 @@ import org.openrndr.math.Vector2
 import org.openrndr.shape.*
 import java.io.File
 
-fun Composition.saveToHPGL(file: File) {
-    val plot = HPGL().generateFromComposition(this)
+fun Composition.saveToHPGL(file: File, width: Int = 0, height: Int = 0) {
+    val plot = HPGL(width = width, height = height).generateFromComposition(this)
     file.writeText(plot)
 }
 
-class HPGL(pageSize: PageSize = PageSize.A4) {
-    private var p1: Vector2
-    private var p2: Vector2
-    private val resolution = 35
-
-    init {
-        // TODO: Set these according to plotter
-        when (pageSize) {
-            PageSize.A3 -> {
-                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)
-            }
-        }
-    }
-
+class HPGL(
+    private val p1: Vector2 = Vector2.ZERO,
+    private val p2: Vector2 = Vector2.ZERO,
+    private val width: Int = 0,
+    private val height: Int = 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
+
+        // Setup coordinates
+        if (p1 != Vector2.ZERO || p2 != Vector2.ZERO) {
+            result.appendln("IP${p1.getCoords()},${p2.getCoords()};")
+        }
+
+        // Setup scaling
+        if (width == 0 && height == 0) {
+            result.appendln("SC;")
+        } else {
+            result.appendln("SC0,$width,0,$height;")
+        }
+
         result.appendln("PA0,0;") // Plot absolutely
 
         composition.root.traverse { stage ->
             if (stage == Stage.Before) {
-                when(this) {
+                when (this) {
                     is ShapeNode -> {
-                        this.shape.contours.forEach { shape ->
+                        for (shape in this.shape.contours) {
                             val segments = shape.sampleLinear().segments
                             segments.forEachIndexed { i, segment ->
                                 if (i == 0) {
@@ -60,11 +57,15 @@ class HPGL(pageSize: PageSize = PageSize.A4) {
                     is SelectPen -> {
                         result.appendln("SP${this.penNumber};")
                     }
+
+                    is VelocitySelect -> {
+                        result.appendln("VS${this.velocity};")
+                    }
                 }
             }
         }
 
-        result.appendln("PA0,0")
+        result.appendln("PA0,0;")
         return result.toString()
     }
 }
@@ -73,11 +74,6 @@ private fun Vector2.getCoords(): String {
     return "$x,$y"
 }
 
-enum class PageSize {
-    A4,
-    A3
-}
-
 private enum class Stage {
     Before,
     After
@@ -93,14 +89,18 @@ private fun CompositionNode.traverse(cb: CompositionNode.(stage: Stage) -> Unit)
     this.cb(Stage.After)
 }
 
+// HPGL Custom Instructions
 class SelectPen(val penNumber: Int) : GroupNode()
-class VelocitySelect(val velocity: Int) : GroupNode()
 
 fun CompositionDrawer.selectPen(pen: Int) {
+    require(pen >= 0) { "The pen can only be 0 or greater." }
     this.root.children.add(SelectPen(pen))
 }
 
-fun CompositionDrawer.velocitySelect(velocity: Int) {
+class VelocitySelect(val velocity: Double) : GroupNode()
+
+fun CompositionDrawer.velocitySelect(velocity: Double) {
+    require(velocity in 0.0..127.9999) { "Invalid velocity value. The velocity should be a value between 0 and 127.9999" }
     this.root.children.add(VelocitySelect(velocity))
 }