[orx-shadestyles] n-point gradient shadeStyle and demos (#138)

orx-shade-styles/src/demo/kotlin/DemoNPointGradient01.kt

@@ -0,0 +1,53 @@
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.color.ColorXSVa
+import org.openrndr.extensions.SingleScreenshot
+import org.openrndr.extra.shadestyles.NPointGradient
+import org.openrndr.math.Polar
+import org.openrndr.shape.ShapeContour
+import kotlin.math.PI
+import kotlin.math.cos
+
+/**
+ * Demonstrate using an n-point gradient.
+ * The gradient has 8 points in screen coordinates and 8 colors.
+ * The colors are fixed, the points of the gradient move.
+ * A contour is drawn using the same points from the gradient,
+ * but note that this is not necessary: you can animate the gradient
+ * on a static shape (a circle for example) or you can animate a shape
+ * with a static gradient.
+ */
+fun main() {
+    application {
+        program {
+            if (System.getProperty("takeScreenshot") == "true") {
+                extend(SingleScreenshot()) {
+                    this.outputFile = System.getProperty("screenshotPath")
+                }
+            }
+
+            val numPoints = 8
+            val gradient = NPointGradient(numPoints, Array(numPoints) {
+                ColorXSVa(it * 360.0 / numPoints, 1.0, 1.0).toRGBa()
+            })
+
+            extend {
+                drawer.run {
+                    clear(ColorRGBa.WHITE.shade(0.9))
+                    val t = PI * 2 * (frameCount % 300) / 300.0
+                    val points = Array(numPoints) {
+                        val lfo = cos(it * PI / 2 - t)
+                        val theta = it * 360.0 / numPoints - 22.5 * lfo
+                        val radius = 200 + 170 * lfo
+                        bounds.center + Polar(theta, radius).cartesian
+                    }
+                    gradient.points = points
+                    shadeStyle = gradient
+                    stroke = ColorRGBa.WHITE
+                    strokeWeight = 4.0
+                    contour(ShapeContour.fromPoints(points.asList(), true))
+                }
+            }
+        }
+    }
+}