[orx-color] Add deltaE76

orx-color/src/commonMain/kotlin/statistics/DeltaE.kt

@@ -0,0 +1,20 @@
+package org.openrndr.extra.color.statistics
+
+import org.openrndr.color.ColorLABa
+import org.openrndr.color.ConvertibleToColorRGBa
+import org.openrndr.math.Vector3
+
+/**
+ * Computes delta E between two colors.
+ */
+fun <T: ConvertibleToColorRGBa> T.deltaE76(other: T): Double {
+    return if (this is ColorLABa && other is ColorLABa) {
+        val tv = Vector3(l, a, b)
+        val ov = Vector3(other.l, other.a, other.b)
+        tv.distanceTo(ov)
+    } else {
+        val tLab = if (this is ColorLABa) this else this.toRGBa().toLABa()
+        val oLab = if (other is ColorLABa) other else other.toRGBa().toLABa()
+        tLab.deltaE76(oLab)
+    }
+}

orx-color/src/jvmDemo/kotlin/DemoDeltaE.kt

@@ -0,0 +1,39 @@
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.extra.color.spaces.toOKHSVa
+import org.openrndr.extra.color.statistics.deltaE76
+import org.openrndr.math.Polar
+
+fun main() {
+    application {
+        configure {
+            width = 720
+            height = 720
+        }
+        program {
+            extend {
+                drawer.clear(ColorRGBa.BLACK)
+                drawer.fill = null
+                drawer.stroke = ColorRGBa.WHITE.opacify(0.2)
+                for (i in 10 until 270 step 10) {
+                    drawer.circle(drawer.bounds.center, i.toDouble())
+                }
+
+                drawer.stroke = null
+
+                val startColor = ColorRGBa.RED.toOKHSVa().shiftHue(seconds*36.0).toRGBa()
+                drawer.circles {
+                    for (j in 99 downTo 0) {
+                        for (i in 0 until 360 step 10) {
+                            val color = startColor.toOKHSVa().shiftHue(i.toDouble()).saturate(j / 99.0).toRGBa()
+                            val distance = color.deltaE76(startColor)
+                            val p = Polar(seconds * 36.0 + i.toDouble(), distance).cartesian + drawer.bounds.center
+                            fill = color
+                            circle(p, 2.0)
+                        }
+                    }
+                }
+            }
+        }
+    }
+}