diff --git a/frontend/js/labs/stereo.js b/frontend/js/labs/stereo.js
index eba8485..20d9585 100644
--- a/frontend/js/labs/stereo.js
+++ b/frontend/js/labs/stereo.js
@@ -53,6 +53,8 @@ class StereoSim {
       else if (this._connectMode) { el.style.cursor = 'pointer'; if (!wasDrag) this._onConnectClick(e); }
       else if (this._measureMode) { el.style.cursor = 'crosshair'; if (!wasDrag) this._onMeasureClick(e); }
       else if (this._angleMode) { el.style.cursor = 'crosshair'; if (!wasDrag) this._onAngleClick(e); }
+      else if (this._markMode) { el.style.cursor = 'pointer'; if (!wasDrag) this._onMarkClick(e); }
+      else if (this._deriveMode) { el.style.cursor = 'crosshair'; if (!wasDrag) this._onDeriveClick(e); }
       else el.style.cursor = 'grab';
     });
     window.addEventListener('pointermove', e => {
@@ -115,11 +117,15 @@ class StereoSim {
     this._sphereGroup  = new THREE.Group();
     this._measureGroup = new THREE.Group();
     this._gridGroup    = new THREE.Group();
+    this._markGroup    = new THREE.Group();
+    this._derivedGroup = new THREE.Group();
     this.scene.add(this._gridGroup);
     this.scene.add(this._figGroup);
     this.scene.add(this._sectionGroup);
     this.scene.add(this._sphereGroup);
     this.scene.add(this._measureGroup);
+    this.scene.add(this._markGroup);
+    this.scene.add(this._derivedGroup);
     this.scene.add(this._labelGroup);
 
     /* state */
@@ -176,6 +182,18 @@ class StereoSim {
     this._edges    = []; // [{from: Vector3, to: Vector3}]
     this._faces    = []; // [[Vector3, ...]]
 
+    /* edge marks (tick / parallel) — аналог _drawTickMark() из планиметрии */
+    this._edgeMarks = {};  // { edgeIdx: { ticks: 0-3, parallel: 0-3 } }
+    this._markMode  = null; // 'ticks' | 'parallel' | null
+
+    /* derived 3D constructions — аналог midpoint/altitude_foot/centroid из планиметрии */
+    this._derived3D  = [];   // [{type, ...args}]
+    this._deriveMode = null; // 'midpoint'|'face_centroid'|'alt_foot'|'solid_centroid'|null
+    this._derivePicks = [];
+
+    /* edge length labels */
+    this.showEdgeLengths = false;
+
     this.onUpdate = null;
 
     this._buildGrid();
@@ -204,6 +222,13 @@ class StereoSim {
     this._clearGroup(this._pointGroup);
     this._clearGroup(this._angleGroup);
     this._clearGroup(this._measureGroup);
+    this._edgeMarks = {};
+    this._markMode = null;
+    this._clearGroup(this._markGroup);
+    this._derived3D = [];
+    this._deriveMode = null;
+    this._derivePicks = [];
+    this._clearGroup(this._derivedGroup);
     this._buildFigure();
     this._notify();
   }
@@ -347,6 +372,56 @@ class StereoSim {
   getCustomPoints() { return this._customPoints; }
   getConnections()  { return this._connections; }
 
+  /* ── Edge mark mode ── */
+  setMarkMode(mode) {
+    // mode: 'ticks' | 'parallel' | null
+    this._markMode   = mode;
+    this._deriveMode = null;
+    this._derivePicks = [];
+    this._measureMode = false;
+    this._pointMode   = false;
+    this._connectMode = false;
+    this._angleMode   = null;
+    this.renderer.domElement.style.cursor = mode ? 'pointer' : 'grab';
+  }
+
+  clearMarks() {
+    this._edgeMarks = {};
+    this._renderEdgeMarks();
+  }
+
+  /* ── Derived 3D constructions mode ── */
+  setDeriveMode(mode) {
+    // mode: 'midpoint' | 'face_centroid' | 'alt_foot' | 'solid_centroid' | null
+    this._deriveMode  = mode;
+    this._derivePicks = [];
+    this._markMode    = null;
+    this._measureMode = false;
+    this._pointMode   = false;
+    this._connectMode = false;
+    this._angleMode   = null;
+    this.renderer.domElement.style.cursor = mode ? 'crosshair' : 'grab';
+    if (mode === 'solid_centroid') this._addSolidCentroid();
+  }
+
+  clearDerived() {
+    this._derived3D   = [];
+    this._derivePicks = [];
+    this._clearGroup(this._derivedGroup);
+  }
+
+  removeLastDerived() {
+    if (!this._derived3D.length) return;
+    this._derived3D.pop();
+    this._buildDerived3D();
+  }
+
+  /* ── Edge length labels ── */
+  toggleEdgeLengths(on) {
+    this.showEdgeLengths = on;
+    this._buildFigure();
+  }
+
   getFormulas() {
     const p = this.params;
     const PI = Math.PI;
@@ -567,6 +642,8 @@ class StereoSim {
     this._drawApothemLine();
     this._drawDiagonals();
     this._drawMidpoints();
+    this._renderEdgeMarks();
+    this._buildDerived3D();
   }
 
   /* ── BOX helpers ── */
@@ -1027,6 +1104,15 @@ class StereoSim {
       const geo = new THREE.BufferGeometry().setFromPoints(pts);
       const mat = new THREE.LineBasicMaterial({ color: 0xFFFFFF, transparent: true, opacity: opac, linewidth: 2 });
       this._figGroup.add(new THREE.Line(geo, mat));
+
+      if (this.showEdgeLengths) {
+        const len = e.from.distanceTo(e.to);
+        const mid = new THREE.Vector3().addVectors(e.from, e.to).multiplyScalar(0.5);
+        const lbl = this._makeTextSprite(len.toFixed(2), '#A8E063', 44);
+        lbl.position.copy(mid).add(new THREE.Vector3(0.1, 0.12, 0.1));
+        lbl.scale.set(0.9, 0.4, 1);
+        this._labelGroup.add(lbl);
+      }
     }
   }
 
@@ -2662,6 +2748,190 @@ class StereoSim {
     }
   }
 
+  /* ════════════════ EDGE MARKS ════════════════ */
+
+  _pickNearestEdgeIdx(e) {
+    const { mx, my } = this._screenCoords(e);
+    let bestDist = 0.10;
+    let bestIdx  = -1;
+    for (let i = 0; i < this._edges.length; i++) {
+      const edge = this._edges[i];
+      const p1   = edge.from.clone().project(this.camera);
+      const p2   = edge.to.clone().project(this.camera);
+      // distance from click to edge midpoint in NDC
+      const mx2  = (p1.x + p2.x) / 2;
+      const my2  = (p1.y + p2.y) / 2;
+      const d    = Math.sqrt((mx2 - mx) ** 2 + (my2 - my) ** 2);
+      if (d < bestDist) { bestDist = d; bestIdx = i; }
+    }
+    return bestIdx;
+  }
+
+  _onMarkClick(e) {
+    const idx = this._pickNearestEdgeIdx(e);
+    if (idx < 0) return;
+    if (!this._edgeMarks[idx]) this._edgeMarks[idx] = { ticks: 0, parallel: 0 };
+    const m = this._edgeMarks[idx];
+    if (this._markMode === 'ticks')    m.ticks    = (m.ticks    + 1) % 4;  // 0→1→2→3→0
+    if (this._markMode === 'parallel') m.parallel = (m.parallel + 1) % 4;
+    this._renderEdgeMarks();
+  }
+
+  _renderEdgeMarks() {
+    this._clearGroup(this._markGroup);
+    for (const [idxStr, mark] of Object.entries(this._edgeMarks)) {
+      const idx = parseInt(idxStr);
+      if (idx < 0 || idx >= this._edges.length) continue;
+      const { from, to } = this._edges[idx];
+      if (mark.ticks    > 0) this._drawEdgeTick3D(from, to, mark.ticks, '#FFD166');
+      if (mark.parallel > 0) this._drawEdgeParallel3D(from, to, mark.parallel, '#06D6E0');
+    }
+  }
+
+  _drawEdgeTick3D(from, to, count, color) {
+    // Perpendicular ticks crossing the edge near its midpoint
+    const dir  = new THREE.Vector3().subVectors(to, from).normalize();
+    const up   = Math.abs(dir.y) < 0.9 ? new THREE.Vector3(0, 1, 0) : new THREE.Vector3(1, 0, 0);
+    const perp = new THREE.Vector3().crossVectors(dir, up).normalize();
+    const mid  = new THREE.Vector3().addVectors(from, to).multiplyScalar(0.5);
+    const step = 0.22;
+    const half = (count - 1) / 2;
+    const mat  = new THREE.LineBasicMaterial({ color: new THREE.Color(color), linewidth: 2 });
+
+    for (let i = 0; i < count; i++) {
+      const offset = (i - half) * step;
+      const center = mid.clone().add(dir.clone().multiplyScalar(offset));
+      const p1 = center.clone().sub(perp.clone().multiplyScalar(0.18));
+      const p2 = center.clone().add(perp.clone().multiplyScalar(0.18));
+      const geo = new THREE.BufferGeometry().setFromPoints([p1, p2]);
+      this._markGroup.add(new THREE.Line(geo, mat.clone()));
+    }
+  }
+
+  _drawEdgeParallel3D(from, to, count, color) {
+    // Chevron (arrow-head) marks indicating parallel edges
+    const dir  = new THREE.Vector3().subVectors(to, from).normalize();
+    const up   = Math.abs(dir.y) < 0.9 ? new THREE.Vector3(0, 1, 0) : new THREE.Vector3(1, 0, 0);
+    const perp = new THREE.Vector3().crossVectors(dir, up).normalize();
+    const mid  = new THREE.Vector3().addVectors(from, to).multiplyScalar(0.5);
+    const step = 0.22;
+    const half = (count - 1) / 2;
+    const mat  = new THREE.LineBasicMaterial({ color: new THREE.Color(color), linewidth: 2 });
+
+    for (let i = 0; i < count; i++) {
+      const offset = (i - half) * step;
+      const center = mid.clone().add(dir.clone().multiplyScalar(offset));
+      // chevron: two lines meeting at a tip (along perp), base spread along dir
+      const tip   = center.clone().add(perp.clone().multiplyScalar( 0.18));
+      const base1 = center.clone().add(perp.clone().multiplyScalar(-0.10)).sub(dir.clone().multiplyScalar(0.14));
+      const base2 = center.clone().add(perp.clone().multiplyScalar(-0.10)).add(dir.clone().multiplyScalar(0.14));
+      const geo = new THREE.BufferGeometry().setFromPoints([base1, tip, base2]);
+      this._markGroup.add(new THREE.Line(geo, mat.clone()));
+    }
+  }
+
+  /* ════════════════ DERIVED 3D CONSTRUCTIONS ════════════════ */
+
+  _addSolidCentroid() {
+    if (!this._vertices.length) return;
+    const c = new THREE.Vector3();
+    this._vertices.forEach(v => c.add(v.pos));
+    c.divideScalar(this._vertices.length);
+    const n = this._derived3D.filter(d => d.type === 'point').length;
+    this._derived3D.push({ type: 'point', pos: c.clone(), label: n ? 'G' + (n + 1) : 'G', color: '#9B5DE5' });
+    this._buildDerived3D();
+    this._deriveMode = null;
+    this.renderer.domElement.style.cursor = 'grab';
+  }
+
+  _onDeriveClick(e) {
+    if (this._deriveMode === 'midpoint') {
+      const idx = this._pickNearestEdgeIdx(e);
+      if (idx < 0) return;
+      const { from, to } = this._edges[idx];
+      const mid = new THREE.Vector3().addVectors(from, to).multiplyScalar(0.5);
+      const n = this._derived3D.filter(d => d.type === 'point').length;
+      this._derived3D.push({ type: 'point', pos: mid.clone(), label: n ? 'M' + (n + 1) : 'M', color: '#FFD166' });
+      this._buildDerived3D();
+
+    } else if (this._deriveMode === 'face_centroid') {
+      const face = this._pickNearestFace(e);
+      if (!face) return;
+      const c = new THREE.Vector3();
+      face.forEach(v => c.add(v));
+      c.divideScalar(face.length);
+      const n = this._derived3D.filter(d => d.type === 'point').length;
+      this._derived3D.push({ type: 'point', pos: c.clone(), label: n ? 'O' + (n + 1) : 'O', color: '#A8E063' });
+      this._buildDerived3D();
+
+    } else if (this._deriveMode === 'alt_foot') {
+      const pick = this._pickNearestPoint(e);
+      if (!pick) return;
+      this._derivePicks.push(pick);
+      // Highlight first pick
+      if (this._derivePicks.length === 1) {
+        const sGeo = new THREE.SphereGeometry(0.14, 10, 10);
+        const sMat = new THREE.MeshBasicMaterial({ color: 0xF15BB5 });
+        const s = new THREE.Mesh(sGeo, sMat);
+        s.position.copy(pick.pos);
+        this._derivedGroup.add(s);
+      } else if (this._derivePicks.length === 2) {
+        // Vertex + base point: find foot of perpendicular from V onto the edge containing E
+        const V = this._derivePicks[0].pos;
+        const E = this._derivePicks[1].pos;
+        const eps = 0.12;
+        let foot = E.clone();
+        for (const edge of this._edges) {
+          if (edge.from.distanceTo(E) < eps || edge.to.distanceTo(E) < eps) {
+            const d  = new THREE.Vector3().subVectors(edge.to, edge.from);
+            const t  = new THREE.Vector3().subVectors(V, edge.from).dot(d) / d.lengthSq();
+            foot = edge.from.clone().add(d.clone().multiplyScalar(Math.max(0, Math.min(1, t))));
+            break;
+          }
+        }
+        const n = this._derived3D.filter(d => d.type === 'point').length;
+        this._derived3D.push({ type: 'point', pos: foot.clone(), label: n ? 'H' + (n + 1) : 'H', color: '#FF9F43' });
+        this._derived3D.push({ type: 'line', from: V.clone(), to: foot.clone(), color: '#FF9F43', dashed: true });
+        this._buildDerived3D();
+        this._derivePicks = [];
+      }
+    }
+  }
+
+  _buildDerived3D() {
+    this._clearGroup(this._derivedGroup);
+    for (const d of this._derived3D) {
+      if (d.type === 'point') {
+        const sGeo = new THREE.SphereGeometry(0.12, 12, 12);
+        const sMat = new THREE.MeshBasicMaterial({ color: new THREE.Color(d.color || '#FFD166') });
+        const s    = new THREE.Mesh(sGeo, sMat);
+        s.position.copy(d.pos);
+        this._derivedGroup.add(s);
+        if (d.label) {
+          const sprite = this._makeTextSprite(d.label, d.color || '#FFD166', 52);
+          sprite.position.copy(d.pos).add(new THREE.Vector3(0.18, 0.25, 0));
+          sprite.scale.set(1.0, 0.45, 1);
+          this._derivedGroup.add(sprite);
+        }
+      } else if (d.type === 'line') {
+        const pts = [d.from, d.to];
+        const geo = new THREE.BufferGeometry().setFromPoints(pts);
+        let mat;
+        if (d.dashed) {
+          mat = new THREE.LineDashedMaterial({
+            color: new THREE.Color(d.color || '#FFD166'),
+            dashSize: 0.1, gapSize: 0.07, transparent: true, opacity: 0.85,
+          });
+        } else {
+          mat = new THREE.LineBasicMaterial({ color: new THREE.Color(d.color || '#FFD166'), transparent: true, opacity: 0.85 });
+        }
+        const line = new THREE.Line(geo, mat);
+        if (d.dashed) line.computeLineDistances();
+        this._derivedGroup.add(line);
+      }
+    }
+  }
+
   _notify() {
     if (this.onUpdate) this.onUpdate(this.info());
   }
diff --git a/frontend/lab.html b/frontend/lab.html
index 200c8ca..1fb823b 100644
--- a/frontend/lab.html
+++ b/frontend/lab.html
@@ -3648,6 +3648,24 @@
             <button class="gp-btn" id="stereo-circumscribed-btn" onclick="stereoCircumscribed(this)">Описанная</button>
           </div>
 
+          <div class="gp-section-title" style="margin-top:8px">Метки рёбер</div>
+          <div style="display:flex;flex-wrap:wrap;gap:4px;margin-bottom:4px">
+            <button class="gp-btn" id="stereo-mark-tick-btn" onclick="stereoMarkMode('ticks',this)" title="Засечки равных рёбер — кликните на ребро (до 3 штрихов)">Засечки</button>
+            <button class="gp-btn" id="stereo-mark-par-btn" onclick="stereoMarkMode('parallel',this)" title="Метки параллельных рёбер — кликните на ребро (до 3 стрелок)">Параллельные</button>
+            <button class="gp-btn" id="stereo-edge-len-btn" onclick="stereoToggleEdgeLengths(this)" title="Показать длины всех рёбер">Длины рёбер</button>
+            <button class="gp-btn" onclick="stereoMarkClear()">Очистить</button>
+          </div>
+
+          <div class="gp-section-title" style="margin-top:8px">Производные точки</div>
+          <div style="display:flex;flex-wrap:wrap;gap:4px;margin-bottom:4px">
+            <button class="gp-btn" id="stereo-derive-mid-btn" onclick="stereoDerive('midpoint',this)" title="Середина ребра — кликните на ребро">Середина ребра</button>
+            <button class="gp-btn" id="stereo-derive-fc-btn" onclick="stereoDerive('face_centroid',this)" title="Центр грани — кликните на грань">Центр грани</button>
+            <button class="gp-btn" id="stereo-derive-alt-btn" onclick="stereoDerive('alt_foot',this)" title="Основание высоты — кликните вершину, затем точку ребра">Осн. высоты</button>
+            <button class="gp-btn" id="stereo-derive-cen-btn" onclick="stereoDerive('solid_centroid',this)" title="Центроид тела — вставляется автоматически">Центроид тела</button>
+            <button class="gp-btn" onclick="stereoDeriveUndo()">Удалить послед.</button>
+            <button class="gp-btn" onclick="stereoDeriveClear()">Очистить</button>
+          </div>
+
           <div class="gp-section-title" style="margin-top:8px">Инструменты</div>
           <div style="display:flex;flex-wrap:wrap;gap:4px;margin-bottom:4px">
             <button class="gp-btn" id="stereo-unfold-btn" onclick="stereoUnfold(this)">Развёртка</button>
@@ -3680,6 +3698,10 @@
             ∠ двугранный: 2 точки общего ребра<br>
             d(т<svg class="ic" viewBox="0 0 24 24"><line x1="5" y1="12" x2="19" y2="12"/><polyline points="12 5 19 12 12 19"/></svg>пл): точка, затем грань — перпендикуляр<br>
             ∠ скрещ.: 4 точки — P1,P2 (пр.1), P3,P4 (пр.2)<br>
+            Засечки/Парал.: кликайте рёбра (0→1→2→3→0)<br>
+            Середина ребра: клик на ребро<br>
+            Центр грани: клик на грань<br>
+            Осн. высоты: вершина → точка/ребро<br>
             Координаты: наведите на вершину
           </div>
         </div>
@@ -8177,7 +8199,9 @@
 
   function _stereoDeactivateTools() {
     ['stereo-measure-btn','stereo-point-btn','stereo-connect-btn',
-     'stereo-angle-edge-btn','stereo-angle-lp-btn','stereo-angle-dih-btn','stereo-angle-pp-btn','stereo-angle-skew-btn'].forEach(id => {
+     'stereo-angle-edge-btn','stereo-angle-lp-btn','stereo-angle-dih-btn','stereo-angle-pp-btn','stereo-angle-skew-btn',
+     'stereo-mark-tick-btn','stereo-mark-par-btn',
+     'stereo-derive-mid-btn','stereo-derive-fc-btn','stereo-derive-alt-btn','stereo-derive-cen-btn'].forEach(id => {
       document.getElementById(id)?.classList.remove('active');
     });
     if (stereoSim) {
@@ -8185,6 +8209,8 @@
       stereoSim.togglePointMode(false);
       stereoSim.toggleConnectMode(false);
       stereoSim.setAngleMode(null);
+      stereoSim.setMarkMode(null);
+      stereoSim.setDeriveMode(null);
     }
     const hint = document.getElementById('angle-hint');
     if (hint) hint.textContent = '';
@@ -8246,6 +8272,42 @@
     }
   }
 
+  /* ── Edge marks ── */
+  function stereoMarkMode(mode, btn) {
+    const on = !btn.classList.contains('active');
+    _stereoDeactivateTools();
+    btn.classList.toggle('active', on);
+    if (stereoSim) stereoSim.setMarkMode(on ? mode : null);
+  }
+
+  function stereoMarkClear() {
+    _stereoDeactivateTools();
+    if (stereoSim) stereoSim.clearMarks();
+  }
+
+  function stereoToggleEdgeLengths(btn) {
+    const on = !btn.classList.contains('active');
+    btn.classList.toggle('active', on);
+    if (stereoSim) stereoSim.toggleEdgeLengths(on);
+  }
+
+  /* ── Derived points ── */
+  function stereoDerive(mode, btn) {
+    const on = !btn.classList.contains('active');
+    _stereoDeactivateTools();
+    btn.classList.toggle('active', on);
+    if (stereoSim) stereoSim.setDeriveMode(on ? mode : null);
+  }
+
+  function stereoDeriveUndo() {
+    if (stereoSim) stereoSim.removeLastDerived();
+  }
+
+  function stereoDeriveClear() {
+    _stereoDeactivateTools();
+    if (stereoSim) stereoSim.clearDerived();
+  }
+
   function stereoPointMode(btn) {
     const on = !btn.classList.contains('active');
     _stereoDeactivateTools();