Workletize the logistic map.

examples/logistic_map/LogisticMap.cxx

@@ -13,51 +13,90 @@
 #include <vector>
 #include <vtkm/cont/DataSetBuilderUniform.h>
 #include <vtkm/io/ImageWriterPNG.h>
+#include <vtkm/worklet/WorkletMapField.h>
+
+
+// The logistic map is xᵢ₊₁ = rxᵢ(1-xᵢ).
+// If we start this iteration out at (say) x₀ = 0.5, the map has "transients",
+// which we must iterate away to produce the final image.
+struct LogisticBurnIn : public vtkm::worklet::WorkletMapField
+{
+  using ControlSignature = void(WholeArrayOut);
+  using ExecutionSignature = void(_1, WorkIndex);
+
+  template <typename OutputArrayPortalType>
+  VTKM_EXEC void operator()(OutputArrayPortalType& outputArrayPortal, vtkm::Id workIndex) const
+  {
+    vtkm::Id width = outputArrayPortal.GetNumberOfValues();
+    double rmin = 2.9;
+    double r = rmin + (4.0 - rmin) * workIndex / (width - 1);
+    double x = 0.5;
+    // 2048 should be enough iterations to get rid of the transients:
+    int n = 0;
+    while (n++ < 2048)
+    {
+      x = r * x * (1 - x);
+    }
+    outputArrayPortal.Set(workIndex, x);
+  }
+};
+
+// After burn-in, the iteration is periodic but in general not convergent,
+// i.e., for large enough i, there exists an integer p > 0 such that
+// xᵢ₊ₚ = xᵢ for all i.
+// So color the pixels corresponding to xᵢ, xᵢ₊₁, .. xᵢ₊ₚ.
+struct LogisticLimitPoints : public vtkm::worklet::WorkletMapField
+{
+  using ControlSignature = void(WholeArrayIn, WholeArrayOut);
+  using ExecutionSignature = void(_1, _2, WorkIndex);
+
+  template <typename InputArrayPortalType, typename OutputArrayPortalType>
+  VTKM_EXEC void operator()(const InputArrayPortalType& inputArrayPortal,
+                            OutputArrayPortalType& outputArrayPortal,
+                            vtkm::Id workIndex) const
+  {
+    vtkm::Id width = inputArrayPortal.GetNumberOfValues();
+    double x = inputArrayPortal.Get(workIndex);
+    double rmin = 2.9;
+    double r = rmin + (4.0 - rmin) * workIndex / (width - 1);
+    vtkm::Vec4f orange(1.0, 0.5, 0.0, 0.0);
+
+    // We can't display need more limit points than pixels of height:
+    vtkm::Id limit_points = 0;
+    vtkm::Id height = 1800;
+    while (limit_points++ < height)
+    {
+      vtkm::Id j = vtkm::Round(x * (height - 1));
+      outputArrayPortal.Set(j * width + workIndex, orange);
+      x = r * x * (1 - x);
+    }
+  }
+};
 
 
 int main()
 {
-  size_t height = 1800;
-  size_t width = height * 1.618;
+  vtkm::Id height = 1800;
+  vtkm::Id width = height * 1.618;
   vtkm::cont::DataSetBuilderUniform dsb;
   vtkm::cont::DataSet ds = dsb.Create(vtkm::Id2(width, height));
 
-  std::vector<double> x(width, 0.5);
+  vtkm::cont::ArrayHandle<double> x;
+  x.Allocate(width);
+  vtkm::cont::Invoker invoke;
+  invoke(LogisticBurnIn{}, x);
 
-  double rmin = 2.9;
-  for (size_t i = 0; i < width; ++i)
+  vtkm::cont::ArrayHandle<vtkm::Vec4f> pixels;
+  pixels.Allocate(width * height);
+  auto wp = pixels.WritePortal();
+  for (vtkm::Id i = 0; i < pixels.GetNumberOfValues(); ++i)
   {
-    double r = rmin + (4.0 - rmin) * i / (width - 1);
-    int n = 0;
-    // 2048 should be enough iterations to be "converged";
-    // though of course the iterations actually don't all converge but cycle or are chaotic.
-    while (n++ < 2048)
-    {
-      x[i] = r * x[i] * (1 - x[i]);
-    }
-  }
-
-
-  vtkm::Vec4f v(1.0, 0.5, 0.0, 0.0);
-  std::vector<vtkm::Vec4f> pixelValues(width * height, vtkm::Vec4f(0, 0, 0, 0));
-  size_t iterates = 0;
-  // We don't need more iterates than pixels of height,
-  // by the pigeonhole principle.
-  while (iterates++ < height)
-  {
-    for (size_t i = 0; i < width; ++i)
-    {
-      double r = rmin + (4.0 - rmin) * i / (width - 1);
-      double y = x[i];
-      assert(y >= 0 && y <= 1);
-      size_t j = std::round(y * (height - 1));
-      pixelValues[j * width + i] = v;
-      x[i] = r * x[i] * (1 - x[i]);
-    }
+    wp.Set(i, vtkm::Vec4f(0, 0, 0, 0));
   }
 
+  invoke(LogisticLimitPoints{}, x, pixels);
   std::string colorFieldName = "pixels";
-  ds.AddPointField(colorFieldName, pixelValues);
+  ds.AddPointField(colorFieldName, pixels);
   std::string filename = "logistic.png";
   vtkm::io::ImageWriterPNG writer(filename);
   writer.WriteDataSet(ds, colorFieldName);