Feat/fixed step ref

include/ncorr.h

@@ -451,7 +451,15 @@ struct DIC_analysis_parallel_input final {
     // Seed-based parallelization parameters
     double cutoff_max_diffnorm;          // Diffnorm threshold for failure prediction
     double cutoff_max_corrcoef;          // Corrcoef threshold for failure prediction
-
+
+    // Fixed-step reference updates (MATLAB ncorr step analysis semantics):
+    // when > 0, every seed segment is capped at this many frames, forcing a
+    // reference change (with seed propagation and — in the exact_matlab_*
+    // path — chain composition back to the global reference) every N frames
+    // regardless of seed quality. 0 (default) = segments end only on
+    // seed-quality failure (long-standing behavior).
+    difference_type fixed_step_ref = 0;
+
     // Constructors
     DIC_analysis_parallel_input() : seeds_are_optimized(false), cutoff_max_diffnorm(0.1), cutoff_max_corrcoef(0.5) { }
 

src/ncorr.cpp

@@ -4904,8 +4904,17 @@ matlab_seed_segment matlab_compute_seed_segment(const DIC_analysis_parallel_inpu
 
     segment.seeds_by_frame.reserve(DIC_input.imgs.size() - ref_idx - 1);
 
+    // Fixed-step reference updates: cap the segment at fixed_step_ref frames so
+    // the caller's segment loop performs a reference change (seed propagation +
+    // chain composition) every N frames — MATLAB ncorr step analysis semantics.
+    const difference_type last_idx_excl =
+        input.fixed_step_ref > 0
+            ? std::min(difference_type(DIC_input.imgs.size()),
+                       ref_idx + 1 + input.fixed_step_ref)
+            : difference_type(DIC_input.imgs.size());
+
     const auto A_ref = DIC_input.imgs[ref_idx].get_gs();
-    for (difference_type cur_idx = ref_idx + 1; cur_idx < difference_type(DIC_input.imgs.size()); ++cur_idx) {
+    for (difference_type cur_idx = ref_idx + 1; cur_idx < last_idx_excl; ++cur_idx) {
         const auto A_cur = DIC_input.imgs[cur_idx].get_gs();
         auto sr_nloptimizer = details::subregion_nloptimizer(
             A_ref,