Curves: Bezier and general interpolate to evaluated utility

This commit implements generic evaluation for Bezier curves (which is
really just linear interpolation, since attributes are not stored on
Bezier handles). For complete parity with the old curve type, we would
have to add options for this (RNA: `Spline.radius_interpolation`),
but it's not clear that we want to do that.

This also adds a generic `interpolate_to_evaluate` utility on curves
that hides the implementation details. Though there is theoretically
a performance cost to that, without some abstraction calling code
would usually be too complex.

Differential Revision: https://developer.blender.org/D14447

1 files changed, 40 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/curve_bezier.cc b/source/blender/blenkernel/intern/curve_bezier.cc
index c02555dcf6a..8efe7a17a35 100644
--- a/source/blender/blenkernel/intern/curve_bezier.cc
+++ b/source/blender/blenkernel/intern/curve_bezier.cc
@@ -134,6 +134,46 @@ void calculate_evaluated_positions(const Span<float3> positions,
   }
 }
 
+template<typename T>
+static inline void linear_interpolation(const T &a, const T &b, MutableSpan<T> dst)
+{
+  dst.first() = a;
+  const float step = 1.0f / dst.size();
+  for (const int i : dst.index_range().drop_front(1)) {
+    dst[i] = attribute_math::mix2(i * step, a, b);
+  }
+}
+
+template<typename T>
+static void interpolate_to_evaluated(const Span<T> src,
+                                     const Span<int> evaluated_offsets,
+                                     MutableSpan<T> dst)
+{
+  linear_interpolation(src.first(), src[1], dst.take_front(evaluated_offsets.first()));
+
+  threading::parallel_for(
+      src.index_range().drop_back(1).drop_front(1), 512, [&](IndexRange range) {
+        for (const int i : range) {
+          const IndexRange segment_points = offsets_to_range(evaluated_offsets, i - 1);
+          linear_interpolation(src[i], src[i + 1], dst.slice(segment_points));
+        }
+      });
+
+  const IndexRange last_segment_points(evaluated_offsets.last(1),
+                                       evaluated_offsets.last() - evaluated_offsets.last(1));
+  linear_interpolation(src.last(), src.first(), dst.slice(last_segment_points));
+}
+
+void interpolate_to_evaluated(const GSpan src, const Span<int> evaluated_offsets, GMutableSpan dst)
+{
+  attribute_math::convert_to_static_type(src.type(), [&](auto dummy) {
+    using T = decltype(dummy);
+    if constexpr (!std::is_void_v<attribute_math::DefaultMixer<T>>) {
+      interpolate_to_evaluated(src.typed<T>(), evaluated_offsets, dst.typed<T>());
+    }
+  });
+}
+
 /** \} */
 
 }  // namespace blender::bke::curves::bezier