s0 can now be set manually for extrapolation to zero

glassure/calc.py

@@ -108,7 +108,11 @@ def process_input(input: Input) -> Pattern:
     sq = calculate_sq(norm, f_squared_mean, f_mean_squared)
 
     # extrapolation
-    s0 = calculate_s0(composition)
+    if config.transform.extrapolation.s0 is not None:
+        s0 = config.transform.extrapolation.s0
+    else:
+        s0 = calculate_s0(composition)
+
     extrapolation = transform.extrapolation
     match extrapolation.method:
         case ExtrapolationMethod.STEP:

glassure/configuration.py

@@ -49,6 +49,7 @@ class OptimizeConfig:
 @dataclass
 class ExtrapolationConfig:
     method: ExtrapolationMethod = ExtrapolationMethod.STEP
+    s0: Optional[float] = field(default=None)
     overlap: float = 0.2
     replace: bool = False
 

tests/test_calc.py

@@ -97,6 +97,16 @@ def test_process_input_poly_extrapolation():
 
     assert not np.array_equal(res.sq.y, res_poly.sq.y)
 
+def test_process_extrapolation_with_s0():
+    input = prepare_input()
+    res = process_input(input)
+
+    input.config.transform.extrapolation.method = "linear"
+    input.config.transform.extrapolation.s0 = 0.1
+    res_s0 = process_input(input)
+
+    assert not np.array_equal(res.sq.y, res_s0.sq.y)
+
 
 def test_process_input_kn_correction():
     input = prepare_input()