need to run the filtered values backwards
not the original and average

@@ -389,0 +438,4 @@
+}
+/**
+ * \param samples are expected to be centered around the segment with a buffer of size filter_order
+ * at the left.

@@ -389,0 +463,4 @@
+  const float fwd_offset = samples[0];
+
+  for (int i = 0; i < sample_count; i++) {
+    double x = (double)(samples[i] - fwd_offset);

@@ -389,0 +469,4 @@
+  }
+
+  for (int i = 0; i < filter_order; i++) {
+    w0[i] = 0.0;

@@ -389,0 +478,4 @@
+
+  /* Run the filter backwards as well to remove phase offset. */
+  for (int i = sample_count - 1; i >= 0; i--) {
+    double x = (double)(filtered_values[i] - bwd_offset);

@@ -389,0 +488,4 @@
+  const float blend_in_y = fcu->bezt[segment->start_index].vec[1][1];
+  const float blend_out_y = fcu->bezt[segment_end_index - 1].vec[1][1];
+
+  for (int i = segment->start_index; i < segment_end_index; i++) {

@@ -389,0 +506,4 @@
+      blend_in_out_factor = min_ff((float)(segment_end_index - i - 1) / blend_in_out, 1.0f);
+      blend_value = blend_out_y;
+    }
+    key_y_value = interpf(key_y_value, blend_value, blend_in_out_factor);

@@ -1210,0 +1251,4 @@
+    FCurve *fcu = (FCurve *)ale->key_data;
+    ListBase fcu_segments = find_fcurve_segments(fcu);
+
+    LISTBASE_FOREACH (FCurveSegment *, segment, &fcu_segments) {

@@ -389,0 +503,4 @@
+      blend_value = blend_out_y;
+    }
+
+    const float x_delta = fcu->bezt[i].vec[1][0] - left_bezt.vec[1][0] + filter_order;