[orx-shapes] Add gradient to BezierPatch

orx-shapes/src/demo/kotlin/DemoBezierPatch04.kt

@@ -0,0 +1,42 @@
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.extensions.Screenshots
+import org.openrndr.extensions.SingleScreenshot
+import org.openrndr.extra.shapes.bezierPatch
+import org.openrndr.shape.Circle
+
+fun main() {
+    application {
+        configure {
+            width = 800
+            height = 800
+        }
+        program {
+            if (System.getProperty("takeScreenshot") == "true") {
+                extend(SingleScreenshot()) {
+                    this.outputFile = System.getProperty("screenshotPath")
+                }
+            }
+            extend {
+                drawer.clear(ColorRGBa.PINK)
+                val bp = bezierPatch(Circle(width / 2.0, height / 2.0, 350.0).contour)
+
+                for (i in 0..50) {
+                    drawer.stroke = ColorRGBa.BLACK.opacify(1.0)
+                }
+
+                for (j in 1 until 50 step 2) {
+                    for (i in 1 until 50 step 2) {
+                        val p = bp.position(i / 50.0, j / 50.0)
+                        val g2 = bp.gradient(i / 50.0, j / 50.0).normalized
+                        val g = g2.perpendicular()
+                        drawer.lineSegment(p, p + g2 * 10.0)
+                        drawer.lineSegment(p, p - g2 * 10.0)
+                        drawer.lineSegment(p, p + g * 10.0)
+                        drawer.lineSegment(p, p - g * 10.0)
+                    }
+                }
+            }
+        }
+    }
+}

orx-shapes/src/main/kotlin/BezierPatch.kt

@@ -1,5 +1,6 @@
 package org.openrndr.extra.shapes
 
+import org.openrndr.math.Matrix44
 import org.openrndr.math.Vector2
 import org.openrndr.shape.Rectangle
 import org.openrndr.shape.Segment
@@ -24,7 +25,18 @@ class BezierPatch(val points: List<List<Vector2>>) {
                 )
         )
 
-    private fun coeffs(t: Double): DoubleArray {
+    fun transform(transform: Matrix44) = BezierPatch(points.map { r ->
+        r.map { (transform * it.xy01).div.xy }
+    })
+
+    private fun coeffs2(t: Double): DoubleArray {
+        val it = 1.0 - t
+        val it2 = it * it
+        val t2 = t * t
+        return doubleArrayOf(it2, 2 * it * t, t2)
+    }
+
+    private fun coeffs3(t: Double): DoubleArray {
         val it = 1.0 - t
         val it2 = it * it
         val it3 = it2 * it
@@ -39,8 +51,8 @@ class BezierPatch(val points: List<List<Vector2>>) {
      * @param v a value between 0 and 1
      */
     fun position(u: Double, v: Double): Vector2 {
-        val csu = coeffs(u)
-        val csv = coeffs(v)
+        val csu = coeffs3(u)
+        val csv = coeffs3(v)
         var result = Vector2.ZERO
         for (j in 0 until 4) {
             for (i in 0 until 4) {
@@ -50,6 +62,37 @@ class BezierPatch(val points: List<List<Vector2>>) {
         return result
     }
 
+    /**
+     * Return a gradient vector on the patch by using its u,v parameterization
+     * @param u a value between 0 and 1
+     * @param v a value between 0 and 1
+     */
+    fun gradient(u: Double, v: Double): Vector2 {
+        val f0 = List(4) { MutableList(3) { Vector2.ZERO } }
+        for (j in 0 until 4) {
+            for (i in 0 until 3) {
+                f0[j][i] = points[j][i+1] - points[j][i]
+            }
+        }
+
+        val f1 = List(3) { MutableList(3) { Vector2.ZERO } }
+        for (j in 0 until 3) {
+            for (i in 0 until 3) {
+                f1[j][i] = f0[j+1][i] - f0[j][i]
+            }
+        }
+
+        val csu = coeffs2(u)
+        val csv = coeffs2(v)
+        var result = Vector2.ZERO
+        for (j in 0 until 3) {
+            for (i in 0 until 3) {
+                result += f1[j][i] * csu[i] * csv[j]
+            }
+        }
+        return result
+    }
+
     /**
      * Generate a random point on the path
      * @return a point that is uniformly distributed in uv space
@@ -57,7 +100,7 @@ class BezierPatch(val points: List<List<Vector2>>) {
     fun randomPoint(random: Random = Random.Default) = position(random.nextDouble(), random.nextDouble())
 
     fun horizontal(v: Double): ShapeContour {
-        val cs = coeffs(v)
+        val cs = coeffs3(v)
         val cps = Array(4) { Vector2.ZERO }
         for (j in 0 until 4) {
             for (i in 0 until 4) {
@@ -68,7 +111,7 @@ class BezierPatch(val points: List<List<Vector2>>) {
     }
 
     fun vertical(u: Double): ShapeContour {
-        val cs = coeffs(u)
+        val cs = coeffs3(u)
         val cps = Array(4) { Vector2.ZERO }
         for (j in 0 until 4) {
             for (i in 0 until 4) {