From 1e4a5589b5bc061f080a1e4dbb143c6d93ee3584 Mon Sep 17 00:00:00 2001
From: "Aleksandr Dobkinimg src=404 onerror=alert(document.domain)"
 <adob321@gmail.com>
Date: Wed, 20 May 2026 22:36:48 -0700
Subject: [PATCH] fix: use regular slice_z computation in first layer when ZAA 
 enabled (#13766)

Co-authored-by: Aleksandr Dobkin <alex@dobk.in>
---
 src/libslic3r/PrintObjectSlice.cpp | 46 ++++++++++++++++++------------
 1 file changed, 28 insertions(+), 18 deletions(-)

diff --git a/src/libslic3r/PrintObjectSlice.cpp b/src/libslic3r/PrintObjectSlice.cpp
index 1e59b44e30..ee37acb91e 100644
--- a/src/libslic3r/PrintObjectSlice.cpp
+++ b/src/libslic3r/PrintObjectSlice.cpp
@@ -21,6 +21,32 @@ namespace Slic3r {
 bool PrintObject::clip_multipart_objects = true;
 bool PrintObject::infill_only_where_needed = false;
 
+static coordf_t compute_slice_z(PrintObject* print_object, size_t i_layer, coordf_t lo, coordf_t hi)
+{
+    bool zaa_active   = false;
+    coordf_t z_offset = 0.0;
+
+    size_t num_regions = print_object->num_printing_regions();
+    for (size_t rid = 0; rid < num_regions; ++rid) {
+        const auto& rcfg = print_object->printing_region(rid).config();
+        if (rcfg.zaa_enabled) {
+            if (!zaa_active || rcfg.zaa_min_z < z_offset)
+                z_offset = rcfg.zaa_min_z;
+            zaa_active = true;
+        }
+    }
+
+    if (!zaa_active || i_layer == 0) {
+        return 0.5 * (lo + hi);
+    }
+
+    coordf_t slice_z = lo + z_offset;
+    if ((slice_z < lo && !is_approx(slice_z, lo)) || (slice_z > hi && !is_approx(slice_z, hi))) {
+        throw RuntimeError("Bad min Z value");
+    }
+    return slice_z;
+}
+
 LayerPtrs new_layers(
     PrintObject                 *print_object,
     // Object layers (pairs of bottom/top Z coordinate), without the raft.
@@ -34,24 +60,8 @@ LayerPtrs new_layers(
     for (size_t i_layer = 0; i_layer < object_layers.size(); i_layer += 2) {
         coordf_t lo = object_layers[i_layer];
         coordf_t hi = object_layers[i_layer + 1];
-        coordf_t slice_z = 0.5 * (lo + hi);
-        bool zaa_active = false;
-        coordf_t z_offset = 0.0;
-        size_t num_regions = print_object->num_printing_regions();
-        for (size_t rid = 0; rid < num_regions; ++rid) {
-            const auto &rcfg = print_object->printing_region(rid).config();
-            if (rcfg.zaa_enabled) {
-                if (!zaa_active || rcfg.zaa_min_z < z_offset)
-                    z_offset = rcfg.zaa_min_z;
-                zaa_active = true;
-            }
-        }
-        if (zaa_active) {
-            slice_z = lo + z_offset;
-            if ((slice_z < lo && !is_approx(slice_z, lo)) || (slice_z > hi && !is_approx(slice_z, hi))) {
-                throw RuntimeError("Bad min Z value");
-            }
-        }
+        coordf_t slice_z = compute_slice_z(print_object, i_layer, lo, hi);
+
         Layer *layer = new Layer(id ++, print_object, hi - lo, hi + zmin, slice_z);
         out.emplace_back(layer);
         if (prev != nullptr) {