- start with asgd, finish with l-bfgs

src/register.rs

@@ -62,8 +62,12 @@ impl RegistrationStep {
 
     /// Default registration steps matching elastix `FixedSmoothingImagePyramid`.
     ///
-    /// Uses a multi-resolution approach with L-BFGS optimizer and all pixels on all levels
-    /// for deterministic, precise convergence.
+    /// Uses a hybrid multi-fidelity approach:
+    /// - Coarse levels (sigma=4,2): ASGD optimizer with all pixels for speed
+    /// - Fine levels (sigma=1,0.5): L-BFGS optimizer with all pixels for accuracy
+    ///
+    /// Note: Samples are cached per level (not randomized per iteration like SimpleElastix),
+    /// but ASGD's sigmoid momentum handles the stochastic gradients effectively.
     pub fn default_steps(ndim: usize, n: usize) -> Vec<Self> {
         if ndim == 1 {
             return vec![Self {
@@ -89,8 +93,16 @@ impl RegistrationStep {
             .map(|(level, (&s, &d))| {
                 let n_pixels = (n / (d * d)).max(4);
                 let is_finest = level == n_levels - 1;
-                // Use all pixels on all levels for deterministic results
-                let (max_iter, tol) = if is_finest { (2048, 1e-8) } else { (512, 1e-6) };
+                let is_coarse = level < 2;
+                // Hybrid: ASGD on coarse levels for speed, L-BFGS on fine levels for accuracy
+                // Use all pixels on all levels for consistent gradient estimates
+                let (optimizer, max_iter, tol) = if is_coarse {
+                    (Optimizer::ASGD, 512, 1e-4)
+                } else if is_finest {
+                    (Optimizer::LBFGS, 2048, 1e-8)
+                } else {
+                    (Optimizer::LBFGS, 512, 1e-6)
+                };
                 Self {
                     sigma: Sigma::Absolute(vec![s; ndim]),
                     samples: SamplingArg::Random(n_pixels),
@@ -100,7 +112,7 @@ impl RegistrationStep {
                     max_iterations: max_iter,
                     learning_rate: 1.0,
                     downsample: d,
-                    optimizer: Optimizer::LBFGS,
+                    optimizer,
                 }
             })
             .collect()