ifcviewer: move volume readout helpers into ViewportCore (#84-j)

Tiny followup to #84-i — move the const-lookup volume helpers used by
bonsai's measurement HUD:

  double volumeOfObjects(const std::vector<uint32_t>&) const
  vector<pair<uint32_t, double>> volumesPerObject(
                              const std::vector<uint32_t>&) const

The det3OfPlacement static helper moves with them into ViewportCore.cpp's
anonymous namespace (the original kept its mirror in
ViewportWindow.cpp; ViewportWindow's own internal callers are gone now
since these methods moved).

Pure read of models_gpu_ + mesh_local_volumes — all in core already.
Trivial transplant.

Builds: desktop / bonsai / web all green. Tests 100/100.

src/ifcviewer/ViewportCore.cpp

@@ -489,3 +489,57 @@ bool ViewportCore::computeObjectAabb(uint32_t object_id,
     mx = Eigen::Vector3f(fmax[0], fmax[1], fmax[2]);
     return true;
 }
+
+// |det| of the 3×3 linear part of a column-major double[16] placement
+// matrix. Picks up uniform scale + mirror so a 2× clone of a 1m³ mesh
+// reports 8m³. Used by the volume readout below.
+namespace {
+double det3OfPlacement(const double M[16]) {
+    const double m00 = M[0],  m10 = M[1],  m20 = M[2];
+    const double m01 = M[4],  m11 = M[5],  m21 = M[6];
+    const double m02 = M[8],  m12 = M[9],  m22 = M[10];
+    return m00 * (m11 * m22 - m12 * m21)
+         - m01 * (m10 * m22 - m12 * m20)
+         + m02 * (m10 * m21 - m11 * m20);
+}
+} // namespace
+
+double ViewportCore::volumeOfObjects(
+        const std::vector<uint32_t>& object_ids) const {
+    if (object_ids.empty()) return 0.0;
+    double total = 0.0;
+    for (uint32_t oid : object_ids) {
+        for (const auto& [mid, m] : models_gpu_) {
+            auto it = m.object_id_to_instance.find(oid);
+            if (it == m.object_id_to_instance.end()) continue;
+            const InstanceCpu& inst = m.instances[it->second];
+            if (inst.mesh_id >= m.mesh_local_volumes.size()) break;
+            const double v_local = m.mesh_local_volumes[inst.mesh_id];
+            const double det = std::abs(det3OfPlacement(inst.placement_transformation));
+            total += v_local * det;
+            break;  // object_id is globally unique → at most one hit
+        }
+    }
+    return total;
+}
+
+std::vector<std::pair<uint32_t, double>>
+ViewportCore::volumesPerObject(
+        const std::vector<uint32_t>& object_ids) const {
+    std::vector<std::pair<uint32_t, double>> out;
+    if (object_ids.empty()) return out;
+    out.reserve(object_ids.size());
+    for (uint32_t oid : object_ids) {
+        for (const auto& [mid, m] : models_gpu_) {
+            auto it = m.object_id_to_instance.find(oid);
+            if (it == m.object_id_to_instance.end()) continue;
+            const InstanceCpu& inst = m.instances[it->second];
+            if (inst.mesh_id >= m.mesh_local_volumes.size()) break;
+            const double v_local = m.mesh_local_volumes[inst.mesh_id];
+            const double det = std::abs(det3OfPlacement(inst.placement_transformation));
+            out.emplace_back(oid, v_local * det);
+            break;
+        }
+    }
+    return out;
+}

src/ifcviewer/ViewportCore.h

@@ -40,6 +40,8 @@
 #include <cstdint>
 #include <string>
 #include <unordered_map>
+#include <utility>
+#include <vector>
 
 #include "BufferPool.h"
 #include "InstanceCompose.h"
@@ -158,6 +160,17 @@ public:
     bool computeObjectAabb(uint32_t object_id,
                            Eigen::Vector3f& mn, Eigen::Vector3f& mx) const;
 
+    // Sum of mesh-local volumes (m³) of every instance whose object_id
+    // is in `object_ids`. Each instance is scaled by |det(placement_3x3)|
+    // to pick up mapped-item scale/mirror; signed-tetrahedra absolute
+    // value means winding is ignored. Volumes are precomputed at
+    // applyCachedModel — this call is just lookups + multiplies.
+    double volumeOfObjects(const std::vector<uint32_t>& object_ids) const;
+    // Per-object variant. Used by the Volume tool to drive both the
+    // total HUD and the per-object overlay labels at AABB centres.
+    std::vector<std::pair<uint32_t, double>>
+        volumesPerObject(const std::vector<uint32_t>& object_ids) const;
+
     // Friend access for ViewportWindow's reference proxies. As each
     // render method moves into ViewportCore it stops needing these
     // (it touches the fields directly); once everything has migrated

src/ifcviewer/ViewportWindow.cpp

@@ -3547,44 +3547,15 @@ void ViewportWindow::setToolMode(ToolMode m) {
     if (isExposed()) requestUpdate();
 }
 
+// volumeOfObjects / volumesPerObject moved to ViewportCore (#84-j).
 double ViewportWindow::volumeOfObjects(
         const std::vector<uint32_t>& object_ids) const {
-    if (object_ids.empty()) return 0.0;
+    return core_.volumeOfObjects(object_ids);
-    double total = 0.0;
-    for (uint32_t oid : object_ids) {
-        for (const auto& [mid, m] : models_gpu_) {
-            auto it = m.object_id_to_instance.find(oid);
-            if (it == m.object_id_to_instance.end()) continue;
-            const InstanceCpu& inst = m.instances[it->second];
-            if (inst.mesh_id >= m.mesh_local_volumes.size()) break;
-            const double v_local = m.mesh_local_volumes[inst.mesh_id];
-            const double det = std::abs(det3OfPlacement(inst.placement_transformation));
-            total += v_local * det;
-            break;  // object_id is globally unique → at most one hit
-        }
-    }
-    return total;
 }
-
 std::vector<std::pair<uint32_t, double>>
 ViewportWindow::volumesPerObject(
         const std::vector<uint32_t>& object_ids) const {
-    std::vector<std::pair<uint32_t, double>> out;
+    return core_.volumesPerObject(object_ids);
-    if (object_ids.empty()) return out;
-    out.reserve(object_ids.size());
-    for (uint32_t oid : object_ids) {
-        for (const auto& [mid, m] : models_gpu_) {
-            auto it = m.object_id_to_instance.find(oid);
-            if (it == m.object_id_to_instance.end()) continue;
-            const InstanceCpu& inst = m.instances[it->second];
-            if (inst.mesh_id >= m.mesh_local_volumes.size()) break;
-            const double v_local = m.mesh_local_volumes[inst.mesh_id];
-            const double det = std::abs(det3OfPlacement(inst.placement_transformation));
-            out.emplace_back(oid, v_local * det);
-            break;
-        }
-    }
-    return out;
 }
 
 void ViewportWindow::updateVolumeReadout() {