diff --git a/server/render.html b/server/render.html
index fed9d3a..16e773f 100644
--- a/server/render.html
+++ b/server/render.html
@@ -86,6 +86,33 @@
   var canvas=document.getElementById('c');
   var ctx=canvas.getContext('2d');
   var scene=null, images={};
+  function drawMaskShape(ctxm, layer, am, time){
+    var asset=am[layer.assetId];
+    if(!asset) return;
+    if(asset.kind!=='rect' && asset.kind!=='circle' && asset.kind!=='text') return;
+    var p=sample(layer,time);
+    ctxm.save();
+    ctxm.translate(p.x,p.y);
+    ctxm.rotate(p.rotation*Math.PI/180);
+    ctxm.scale(p.scaleX,p.scaleY);
+    ctxm.fillStyle='#000000';
+    if(asset.kind==='rect'){
+      var mrX=layer.offsetX!=null?layer.offsetX:asset.width/2;
+      var mrY=layer.offsetY!=null?layer.offsetY:asset.height/2;
+      ctxm.fillRect(-mrX,-mrY,asset.width,asset.height);
+    } else if(asset.kind==='circle'){
+      var mcX=layer.offsetX!=null?layer.offsetX:0;
+      var mcY=layer.offsetY!=null?layer.offsetY:0;
+      ctxm.beginPath(); ctxm.arc(-mcX,-mcY,asset.radius,0,Math.PI*2); ctxm.fill();
+    } else {
+      var mtX=layer.offsetX!=null?layer.offsetX:0;
+      var mtY=layer.offsetY!=null?layer.offsetY:0;
+      ctxm.font=asset.fontSize+'px sans-serif';
+      ctxm.textBaseline='top';
+      ctxm.fillText(asset.text,-mtX,-mtY);
+    }
+    ctxm.restore();
+  }
   function drawLayerNode(layer, am, time){
     var asset=am[layer.assetId];
     if(!asset || !layer.visible) return;
@@ -164,31 +191,60 @@
     var rawOrder=scene.trackOrder||[];
     var groupById2={}; var groups2=scene.groups||[];
     for(var gi2=0;gi2<groups2.length;gi2++) groupById2[groups2[gi2].id]=groups2[gi2];
-    var order=[];
-    for(var roi=0;roi<rawOrder.length;roi++){
-      var rid=rawOrder[roi];
-      var grp=groupById2[rid];
-      if(grp){ for(var gc=0;gc<grp.childIds.length;gc++) order.push(grp.childIds[gc]); }
-      else order.push(rid);
-    }
-    if(!order.length){ for(var oj=0;oj<scene.layers.length;oj++) order.push(scene.layers[oj].id); }
-    for(var oi=order.length-1;oi>=0;oi--){
-      var id=order[oi];
+    if(!rawOrder.length){ rawOrder=[]; for(var oj=0;oj<scene.layers.length;oj++) rawOrder.push(scene.layers[oj].id); }
+    for(var oi=rawOrder.length-1;oi>=0;oi--){
+      var id=rawOrder[oi];
       var lyr=layerById[id];
       if(lyr){ drawLayerNode(lyr, am, time); continue; }
       var tInsts=instByTrack[id]||[];
-      for(var ti=0;ti<tInsts.length;ti++){
-        var inst=tInsts[ti];
-        var clip=clipById[inst.clipId];
-        if(!clip) continue;
-        var tS=inst.trimStart||0;
-        var tE=inst.trimEnd||0;
-        var visible=Math.max(0, clip.duration - tS - tE);
-        var offset=time - inst.startTime;
-        if(offset<0 || offset>visible) continue;
-        var localT = tS + offset;
-        for(var cl=clip.layers.length-1; cl>=0; cl--){
-          drawLayerNode(clip.layers[cl], am, localT);
+      if(tInsts.length){
+        for(var ti=0;ti<tInsts.length;ti++){
+          var inst=tInsts[ti];
+          var clipRef=clipById[inst.clipId];
+          if(!clipRef) continue;
+          var tS=inst.trimStart||0;
+          var tE=inst.trimEnd||0;
+          var visibleClip=Math.max(0, clipRef.duration - tS - tE);
+          var offsetIntoBlock=time - inst.startTime;
+          if(offsetIntoBlock<0 || offsetIntoBlock>visibleClip) continue;
+          var localT = tS + offsetIntoBlock;
+          for(var cll=clipRef.layers.length-1; cll>=0; cll--){
+            drawLayerNode(clipRef.layers[cll], am, localT);
+          }
+        }
+        continue;
+      }
+      var grp=groupById2[id];
+      if(grp){
+        var gContent=[], gMasks=[];
+        for(var ch=grp.childIds.length-1;ch>=0;ch--){
+          var cl0=layerById[grp.childIds[ch]];
+          if(!cl0) continue;
+          if(cl0.isMask) gMasks.push(cl0); else gContent.push(cl0);
+        }
+        if(!gMasks.length){
+          for(var gi3=0;gi3<gContent.length;gi3++) drawLayerNode(gContent[gi3], am, time);
+        } else {
+          var mcanvas=document.createElement('canvas');
+          mcanvas.width=canvas.width; mcanvas.height=canvas.height;
+          var mctx=mcanvas.getContext('2d');
+          var tm=ctx.getTransform();
+          mctx.setTransform(tm.a,tm.b,tm.c,tm.d,tm.e,tm.f);
+          var savedCtx=ctx;
+          ctx=mctx;
+          for(var gi4=0;gi4<gContent.length;gi4++) drawLayerNode(gContent[gi4], am, time);
+          for(var gm=0;gm<gMasks.length;gm++){
+            var mL=gMasks[gm];
+            mctx.save();
+            mctx.globalCompositeOperation=(mL.maskMode==='negative'?'destination-out':'destination-in');
+            drawMaskShape(mctx, mL, am, time);
+            mctx.restore();
+          }
+          ctx=savedCtx;
+          ctx.save();
+          ctx.setTransform(1,0,0,1,0,0);
+          ctx.drawImage(mcanvas,0,0);
+          ctx.restore();
         }
       }
     }
diff --git a/src/components/CanvasStage.vue b/src/components/CanvasStage.vue
index 59834d0..e7f8e87 100644
--- a/src/components/CanvasStage.vue
+++ b/src/components/CanvasStage.vue
@@ -1,10 +1,11 @@
 <script setup lang="ts">
 import { computed, nextTick, onBeforeUnmount, onMounted, ref, watch } from 'vue'
 import type Konva from 'konva'
-import { PhCornersIn, PhCrop } from '@phosphor-icons/vue'
+import { PhCornersIn, PhCrop, PhMaskHappy, PhPencilSimple } from '@phosphor-icons/vue'
 import { useSceneStore } from '@/stores/scene'
 import { sampleLayer } from '@/lib/interpolate'
 import { loadImage } from '@/lib/fileLoader'
+import { drawSceneFrame, loadSceneImages } from '@/lib/render'
 import type { AnimatableProps, Asset, Layer } from '@/types/scene'
 import { t } from '@/i18n'
 import { useKfSelection } from '@/composables/useKfSelection'
@@ -22,6 +23,9 @@ const zoom = ref(1)
 const viewX = ref(0)
 const viewY = ref(0)
 const cropObjects = ref(false)
+const maskPreview = ref(false)
+const previewCanvasEl = ref<HTMLCanvasElement | null>(null)
+const previewImages = ref<Record<string, HTMLImageElement>>({})
 let didInitialFit = false
 
 const ZOOM_MIN = 0.05
@@ -251,6 +255,15 @@ function shadowCfg(props: AnimatableProps): Record<string, unknown> {
 }
 
 function baseNodeConfig(layer: Layer, props: AnimatableProps) {
+  const maskGhost = layer.isMask
+    ? {
+        opacity: props.opacity * 0.45,
+        stroke: layer.maskMode === 'negative' ? '#ff6b6b' : '#6ea8ff',
+        strokeWidth: 1.5,
+        dash: [6, 4],
+        strokeScaleEnabled: false,
+      }
+    : {}
   return {
     id: layer.id,
     x: props.x,
@@ -262,6 +275,7 @@ function baseNodeConfig(layer: Layer, props: AnimatableProps) {
     visible: layer.visible,
     draggable: true,
     ...shadowCfg(props),
+    ...maskGhost,
   }
 }
 
@@ -825,6 +839,62 @@ function onContainerDrop(e: DragEvent) {
   }
 }
 
+function resizePreviewCanvas() {
+  const canvas = previewCanvasEl.value
+  if (!canvas) return
+  const dpr = window.devicePixelRatio || 1
+  const cw = containerSize.value.width
+  const ch = containerSize.value.height
+  canvas.width = Math.max(1, Math.round(cw * dpr))
+  canvas.height = Math.max(1, Math.round(ch * dpr))
+  canvas.style.width = `${cw}px`
+  canvas.style.height = `${ch}px`
+}
+
+function drawPreview() {
+  const canvas = previewCanvasEl.value
+  if (!canvas) return
+  const ctx = canvas.getContext('2d')
+  if (!ctx) return
+  const dpr = window.devicePixelRatio || 1
+  const cw = containerSize.value.width
+  const ch = containerSize.value.height
+  if (!cw || !ch) return
+  ctx.setTransform(dpr, 0, 0, dpr, 0, 0)
+  ctx.clearRect(0, 0, cw, ch)
+  ctx.save()
+  ctx.translate(viewX.value, viewY.value)
+  ctx.scale(zoom.value, zoom.value)
+  drawSceneFrame(ctx, store.scene, store.playhead, previewImages.value)
+  ctx.restore()
+}
+
+watch(
+  [
+    () => store.playhead,
+    () => store.scene,
+    () => containerSize.value,
+    zoom,
+    viewX,
+    viewY,
+  ],
+  async () => {
+    if (!maskPreview.value) return
+    await nextTick()
+    resizePreviewCanvas()
+    drawPreview()
+  },
+  { deep: true },
+)
+
+watch(maskPreview, async (on) => {
+  if (!on) return
+  previewImages.value = await loadSceneImages(store.scene)
+  await nextTick()
+  resizePreviewCanvas()
+  drawPreview()
+})
+
 function zoomBy(factor: number) {
   const cw = containerSize.value.width
   const ch = containerSize.value.height
@@ -966,6 +1036,11 @@ onBeforeUnmount(() => {
         <v-transformer ref="transformerRef" :config="transformerConfig" />
       </v-layer>
     </v-stage>
+    <canvas
+      v-show="maskPreview"
+      ref="previewCanvasEl"
+      class="mask-preview-canvas"
+    ></canvas>
 
     <div class="zoom-controls" @pointerdown.stop>
       <button class="zc-btn" :title="t('canvas.zoomOut')" @click="zoomBy(1 / 1.25)">−</button>
@@ -987,6 +1062,15 @@ onBeforeUnmount(() => {
       >
         <PhCrop :size="13" />
       </button>
+      <button
+        class="zc-btn"
+        :class="{ active: maskPreview }"
+        :title="maskPreview ? t('canvas.maskPreviewOff') : t('canvas.maskPreviewOn')"
+        @click="maskPreview = !maskPreview"
+      >
+        <PhMaskHappy v-if="!maskPreview" :size="13" />
+        <PhPencilSimple v-else :size="13" />
+      </button>
     </div>
 
     <div class="dims">{{ store.scene.canvas.width }} × {{ store.scene.canvas.height }}</div>
@@ -1001,6 +1085,13 @@ onBeforeUnmount(() => {
   overflow: hidden;
 }
 
+.mask-preview-canvas {
+  position: absolute;
+  inset: 0;
+  pointer-events: none;
+  background: var(--bg);
+}
+
 .dims {
   position: absolute;
   bottom: 10px;
diff --git a/src/components/PropertiesPanel.vue b/src/components/PropertiesPanel.vue
index 863b604..1d00c6c 100644
--- a/src/components/PropertiesPanel.vue
+++ b/src/components/PropertiesPanel.vue
@@ -39,6 +39,17 @@ const blurAsset = computed(() => {
   return a && (a.kind === 'rect' || a.kind === 'circle') ? a : null
 })
 
+const canBeMask = computed(() => {
+  const a = selectedAsset.value
+  return !!a && (a.kind === 'rect' || a.kind === 'circle' || a.kind === 'text')
+})
+
+const maskInGroup = computed(() => {
+  if (!selected.value) return false
+  const id = selected.value.id
+  return (store.scene.groups ?? []).some((g) => g.childIds.includes(id))
+})
+
 const blurValue = computed<number>({
   get: () => {
     const a = blurAsset.value
@@ -255,6 +266,43 @@ function removeKeyframeHere() {
         <RangeSlider v-model="opacity" :min="0" :max="1" :step="0.01" :format="formatPercent" />
       </label>
 
+      <section v-if="canBeMask" class="mask-group">
+        <h4>{{ t('properties.mask') }}</h4>
+        <label class="mask-toggle">
+          <input
+            type="checkbox"
+            :checked="!!selected.isMask"
+            @change="store.setLayerMask(selected.id, ($event.target as HTMLInputElement).checked)"
+          />
+          <span>{{ t('properties.maskUse') }}</span>
+        </label>
+        <div v-if="selected.isMask" class="mask-mode-row">
+          <label class="mask-radio">
+            <input
+              type="radio"
+              :checked="(selected.maskMode ?? 'positive') === 'positive'"
+              @change="store.setLayerMaskMode(selected.id, 'positive')"
+            />
+            <span>{{ t('properties.maskPositive') }}</span>
+          </label>
+          <label class="mask-radio">
+            <input
+              type="radio"
+              :checked="selected.maskMode === 'negative'"
+              @change="store.setLayerMaskMode(selected.id, 'negative')"
+            />
+            <span>{{ t('properties.maskNegative') }}</span>
+          </label>
+        </div>
+        <p v-if="selected.isMask && maskInGroup" class="status-active">
+          {{ t('properties.maskActive') }}
+        </p>
+        <p v-if="selected.isMask && !maskInGroup" class="status-warn">
+          {{ t('properties.maskNeedsGroup') }}
+        </p>
+        <p v-if="selected.isMask" class="hint">{{ t('properties.maskHint') }}</p>
+      </section>
+
       <button class="danger-btn" @click="store.removeLayer(selected.id)">
         <PhTrash :size="13" />
         {{ t('properties.deleteLayer') }}
@@ -332,6 +380,65 @@ input {
   font-weight: 500;
 }
 
+.mask-group {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  padding-top: 8px;
+  border-top: 1px solid var(--border);
+}
+
+.mask-group h4 {
+  margin: 0;
+  font-size: 10px;
+  text-transform: uppercase;
+  letter-spacing: 0.08em;
+  color: var(--text-dim);
+  font-weight: 500;
+}
+
+.mask-toggle,
+.mask-radio {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  font-size: 12px;
+  color: var(--text);
+  text-transform: none;
+  letter-spacing: 0;
+  cursor: pointer;
+  flex: 0 0 auto;
+}
+
+.mask-mode-row {
+  display: flex;
+  gap: 16px;
+}
+
+.mask-toggle input,
+.mask-radio input {
+  width: auto;
+  margin: 0;
+}
+
+.status-active,
+.status-warn {
+  margin: 0;
+  font-size: 11px;
+  padding: 4px 8px;
+  border-radius: var(--radius-sm);
+}
+
+.status-active {
+  color: #6ea8ff;
+  background: color-mix(in srgb, #6ea8ff 14%, transparent);
+}
+
+.status-warn {
+  color: #ffb84d;
+  background: color-mix(in srgb, #ffb84d 16%, transparent);
+}
+
 .kf-row {
   display: flex;
   align-items: center;
diff --git a/src/components/Timeline.vue b/src/components/Timeline.vue
index b5d83eb..85f2b9c 100644
--- a/src/components/Timeline.vue
+++ b/src/components/Timeline.vue
@@ -12,6 +12,7 @@ import {
   PhLock,
   PhLockOpen,
   PhMagnet,
+  PhMaskHappy,
   PhPauseCircle,
   PhTrash,
 } from '@phosphor-icons/vue'
@@ -41,6 +42,8 @@ interface TimelineLayerTrack {
   isParent: boolean
   orderIndex: number
   inGroup: boolean
+  parentGroupId?: string
+  groupChildIndex?: number
 }
 
 interface TimelineClipTrack {
@@ -49,6 +52,8 @@ interface TimelineClipTrack {
   name: string
   orderIndex: number
   inGroup: boolean
+  parentGroupId?: string
+  groupChildIndex?: number
 }
 
 interface TimelineGroupTrack {
@@ -240,6 +245,30 @@ function pxFromTime(t: number) {
   return t * effectivePxPerSec.value
 }
 
+function groupKeyframeTimes(groupId: string): number[] {
+  const group = groupById.value.get(groupId)
+  if (!group) return []
+  const times: number[] = []
+  const layerById = new Map<string, Layer>()
+  for (const l of store.scene.layers) layerById.set(l.id, l)
+  for (const childId of group.childIds) {
+    const layer = layerById.get(childId)
+    if (layer) {
+      for (const k of layer.keyframes) times.push(k.time)
+      for (const k of layer.positionKeyframes ?? []) times.push(k.time)
+      for (const k of layer.rotationKeyframes ?? []) times.push(k.time)
+      continue
+    }
+    const ct = clipTrackById.value.get(childId)
+    if (ct) {
+      for (const ci of clipInstancesByTrack.value[ct.id] ?? []) {
+        times.push(ci.startTime)
+      }
+    }
+  }
+  return times
+}
+
 function formatTick(t: number) {
   if (displayMode.value === 'frames') {
     return `${Math.round(t * fps.value)}`
@@ -340,13 +369,21 @@ function onKeyframeDblClick(e: MouseEvent, layerId: string, keyframeId: string)
   store.removeKeyframe(layerId, keyframeId)
 }
 
-function emitLayerTrack(layer: Layer, orderIndex: number, inGroup: boolean, out: TimelineTrack[]) {
+function emitLayerTrack(
+  layer: Layer,
+  orderIndex: number,
+  inGroup: boolean,
+  out: TimelineTrack[],
+  parentGroupId?: string,
+  groupChildIndex?: number,
+) {
+  const base = { type: 'layer' as const, layerId: layer.id, layer, orderIndex, inGroup, parentGroupId, groupChildIndex }
   if (!layer.separated) {
-    out.push({ type: 'layer', layerId: layer.id, layer, kfs: layer.keyframes, kind: 'main', label: layer.name, isParent: true, orderIndex, inGroup })
+    out.push({ ...base, kfs: layer.keyframes, kind: 'main', label: layer.name, isParent: true })
   } else {
-    out.push({ type: 'layer', layerId: layer.id, layer, kfs: layer.keyframes, kind: 'main', label: layer.name, isParent: true, orderIndex, inGroup })
-    out.push({ type: 'layer', layerId: layer.id, layer, kfs: layer.positionKeyframes ?? [], kind: 'position', label: t('timeline.position'), isParent: false, orderIndex, inGroup })
-    out.push({ type: 'layer', layerId: layer.id, layer, kfs: layer.rotationKeyframes ?? [], kind: 'rotation', label: t('timeline.rotation'), isParent: false, orderIndex, inGroup })
+    out.push({ ...base, kfs: layer.keyframes, kind: 'main', label: layer.name, isParent: true })
+    out.push({ ...base, kfs: layer.positionKeyframes ?? [], kind: 'position', label: t('timeline.position'), isParent: false })
+    out.push({ ...base, kfs: layer.rotationKeyframes ?? [], kind: 'rotation', label: t('timeline.rotation'), isParent: false })
   }
 }
 
@@ -384,17 +421,25 @@ const tracks = computed<TimelineTrack[]>(() => {
         childCount: grp.childIds.length,
       })
       if (!grp.collapsed) {
-        for (const cid of grp.childIds) {
+        grp.childIds.forEach((cid, childIndex) => {
           const cl = layerById.get(cid)
           if (cl) {
-            emitLayerTrack(cl, -1, true, out)
-            continue
+            emitLayerTrack(cl, -1, true, out, grp.id, childIndex)
+            return
           }
           const cct = clipTrackById.value.get(cid)
           if (cct) {
-            out.push({ type: 'clip', clipTrackId: cct.id, name: cct.name ?? t('timeline.clipTrackName'), orderIndex: -1, inGroup: true })
+            out.push({
+              type: 'clip',
+              clipTrackId: cct.id,
+              name: cct.name ?? t('timeline.clipTrackName'),
+              orderIndex: -1,
+              inGroup: true,
+              parentGroupId: grp.id,
+              groupChildIndex: childIndex,
+            })
           }
-        }
+        })
       }
     }
   })
@@ -412,16 +457,44 @@ function isTrackHead(track: TimelineTrack): boolean {
   return true
 }
 
+function trackChildIndex(track: TimelineTrack): number | null {
+  if (track.type === 'layer' || track.type === 'clip') {
+    return track.groupChildIndex ?? null
+  }
+  return null
+}
+
+function trackParentGroupId(track: TimelineTrack): string | null {
+  if (track.type === 'layer' || track.type === 'clip') {
+    return track.parentGroupId ?? null
+  }
+  return null
+}
+
 function isDropBefore(track: TimelineTrack): boolean {
   if (dropIndex.value === null) return false
   if (!isTrackHead(track)) return false
   if (dragTrackId.value === trackDndId(track)) return false
+  if (dropWithinGroupId.value) {
+    if (trackParentGroupId(track) !== dropWithinGroupId.value) return false
+    return dropIndex.value === trackChildIndex(track)
+  }
+  if (track.type !== 'group' && track.inGroup) return false
   return dropIndex.value === track.orderIndex
 }
 
 function isDropAfter(track: TimelineTrack, nextTrack: TimelineTrack | undefined): boolean {
   if (dropIndex.value === null) return false
   if (dragTrackId.value === trackDndId(track)) return false
+  if (dropWithinGroupId.value) {
+    if (trackParentGroupId(track) !== dropWithinGroupId.value) return false
+    const ci = trackChildIndex(track)
+    if (ci === null) return false
+    if (dropIndex.value !== ci + 1) return false
+    if (nextTrack && trackChildIndex(nextTrack) === ci) return false
+    return true
+  }
+  if (track.type !== 'group' && track.inGroup) return false
   if (dropIndex.value !== track.orderIndex + 1) return false
   if (nextTrack && nextTrack.orderIndex === track.orderIndex) return false
   return true
@@ -727,6 +800,7 @@ const TRACK_COLORS: Array<{ value: string | null; label: string }> = [
 
 const dragTrackId = ref<string | null>(null)
 const dropIndex = ref<number | null>(null)
+const dropWithinGroupId = ref<string | null>(null)
 
 function onTrackDragStart(e: DragEvent, trackId: string) {
   if (!e.dataTransfer) return
@@ -748,6 +822,45 @@ function onTrackDragOver(e: DragEvent, orderIndex: number) {
   const rect = el.getBoundingClientRect()
   const midY = rect.top + rect.height / 2
   dropIndex.value = e.clientY < midY ? orderIndex : orderIndex + 1
+  dropWithinGroupId.value = null
+}
+
+function onGroupChildDragOver(e: DragEvent, groupId: string, childIndex: number) {
+  if (!dragTrackId.value) return
+  if (dragTrackId.value === groupId) return
+  // Groups themselves cannot be nested inside another group
+  if (groupById.value.has(dragTrackId.value)) return
+  if (!e.dataTransfer) return
+  e.preventDefault()
+  e.dataTransfer.dropEffect = 'move'
+  const el = e.currentTarget as HTMLElement
+  const rect = el.getBoundingClientRect()
+  const midY = rect.top + rect.height / 2
+  dropIndex.value = e.clientY < midY ? childIndex : childIndex + 1
+  dropWithinGroupId.value = groupId
+  dropGroupId.value = null
+}
+
+function onGroupChildDrop(e: DragEvent) {
+  e.preventDefault()
+  const srcId = dragTrackId.value
+  const target = dropIndex.value
+  const groupId = dropWithinGroupId.value
+  dragTrackId.value = null
+  dropIndex.value = null
+  dropWithinGroupId.value = null
+  dropGroupId.value = null
+  if (!srcId || target === null || !groupId) return
+  if (srcId === groupId) return
+  if (groupById.value.has(srcId)) return
+  const group = groupById.value.get(groupId)
+  if (!group) return
+  const from = group.childIds.indexOf(srcId)
+  let to = target
+  // If source is already inside this group and before the target index,
+  // removal will shift subsequent positions down by one.
+  if (from >= 0 && from < to) to -= 1
+  store.moveTrackIntoGroup(srcId, groupId, to)
 }
 
 function onTrackDragLeave() {
@@ -773,6 +886,7 @@ function onTrackDragEnd() {
   dragTrackId.value = null
   dropIndex.value = null
   dropGroupId.value = null
+  dropWithinGroupId.value = null
 }
 
 const dropGroupId = ref<string | null>(null)
@@ -1470,16 +1584,26 @@ onBeforeUnmount(() => {
               :class="{
                 'sub-track': !track.isParent,
                 inactive: store.selectedLayerId !== null && track.layerId !== store.selectedLayerId,
-                'drop-before': !track.inGroup && isDropBefore(track),
-                'drop-after': !track.inGroup && isDropAfter(track, tracks[idx + 1]),
+                'drop-before': isDropBefore(track),
+                'drop-after': isDropAfter(track, tracks[idx + 1]),
                 dragging: track.isParent && dragTrackId === track.layerId,
                 colored: !!track.layer.color,
                 locked: !!track.layer.locked,
                 'in-group': track.inGroup,
               }"
               :style="trackColorStyle(track.layer.color)"
-              @dragover="track.isParent && !track.inGroup ? onTrackDragOver($event, track.orderIndex) : null"
-              @drop="track.isParent && !track.inGroup ? onTrackDrop($event) : null"
+              @dragover="
+                track.isParent
+                  ? (track.inGroup && track.parentGroupId !== undefined && track.groupChildIndex !== undefined
+                      ? onGroupChildDragOver($event, track.parentGroupId, track.groupChildIndex)
+                      : onTrackDragOver($event, track.orderIndex))
+                  : null
+              "
+              @drop="
+                track.isParent
+                  ? (track.inGroup ? onGroupChildDrop($event) : onTrackDrop($event))
+                  : null
+              "
               @dragleave="onTrackDragLeave"
             >
               <span
@@ -1506,6 +1630,12 @@ onBeforeUnmount(() => {
                   :size="11"
                   class="name-badge"
                 />
+                <PhMaskHappy
+                  v-if="track.isParent && track.layer.isMask"
+                  :size="11"
+                  class="name-badge"
+                  :class="{ 'mask-negative': track.layer.maskMode === 'negative' }"
+                />
                 <span v-if="!track.isParent" class="sub-indent">↳</span>
                 {{ track.label }}
               </span>
@@ -1556,16 +1686,20 @@ onBeforeUnmount(() => {
               v-else-if="track.type === 'clip'"
               class="track clip-track"
               :class="{
-                'drop-before': !track.inGroup && isDropBefore(track),
-                'drop-after': !track.inGroup && isDropAfter(track, tracks[idx + 1]),
+                'drop-before': isDropBefore(track),
+                'drop-after': isDropAfter(track, tracks[idx + 1]),
                 dragging: dragTrackId === track.clipTrackId,
                 colored: !!clipTrackById.get(track.clipTrackId)?.color,
                 locked: !!clipTrackById.get(track.clipTrackId)?.locked,
                 'in-group': track.inGroup,
               }"
               :style="trackColorStyle(clipTrackById.get(track.clipTrackId)?.color)"
-              @dragover="!track.inGroup ? onTrackDragOver($event, track.orderIndex) : null"
-              @drop="!track.inGroup ? onTrackDrop($event) : null"
+              @dragover="
+                track.inGroup && track.parentGroupId !== undefined && track.groupChildIndex !== undefined
+                  ? onGroupChildDragOver($event, track.parentGroupId, track.groupChildIndex)
+                  : onTrackDragOver($event, track.orderIndex)
+              "
+              @drop="track.inGroup ? onGroupChildDrop($event) : onTrackDrop($event)"
               @dragleave="onTrackDragLeave"
             >
               <span
@@ -1683,9 +1817,18 @@ onBeforeUnmount(() => {
                 @drop="onGroupBodyDrop($event, track.groupId)"
                 @dragleave="onGroupBodyDragLeave"
               >
+                <span class="playhead" :style="{ left: pxFromTime(store.playhead) + 'px' }" />
                 <span v-if="track.collapsed && track.childCount === 0" class="group-empty-hint">
                   {{ t('timeline.groupEmpty') }}
                 </span>
+                <template v-if="track.collapsed">
+                  <span
+                    v-for="(time, kfIdx) in groupKeyframeTimes(track.groupId)"
+                    :key="`gkf-${kfIdx}`"
+                    class="group-kf-dot"
+                    :style="{ left: pxFromTime(time) + 'px' }"
+                  />
+                </template>
               </div>
             </div>
           </template>
@@ -2289,6 +2432,10 @@ onBeforeUnmount(() => {
   flex-shrink: 0;
 }
 
+.name-badge.mask-negative {
+  color: #ff6b6b;
+}
+
 .drag-handle {
   color: var(--text-dim);
   margin-right: 4px;
@@ -2649,12 +2796,25 @@ onBeforeUnmount(() => {
   font-style: italic;
 }
 
+.group-kf-dot {
+  position: absolute;
+  top: 50%;
+  width: 5px;
+  height: 5px;
+  margin-left: -2.5px;
+  margin-top: -2.5px;
+  border-radius: 50%;
+  background: var(--track-color, var(--accent));
+  opacity: 0.75;
+  pointer-events: none;
+}
+
 .track.in-group .track-name {
-  padding-left: 22px;
+  padding-left: 38px;
 }
 
 .track.in-group.sub-track .track-name {
-  padding-left: 34px;
+  padding-left: 50px;
 }
 
 .clip-track-dropzone {
diff --git a/src/devtools/tests.ts b/src/devtools/tests.ts
index 7e2ea57..045f0af 100644
--- a/src/devtools/tests.ts
+++ b/src/devtools/tests.ts
@@ -33,6 +33,7 @@ interface MountedHandle {
 interface RecordingCtx {
   ctx: CanvasRenderingContext2D
   filterSets: string[]
+  compositeOps: string[]
 }
 
 function makeRecordingCtx(): RecordingCtx {
@@ -41,9 +42,11 @@ function makeRecordingCtx(): RecordingCtx {
   canvas.height = 200
   const real = canvas.getContext('2d')!
   const filterSets: string[] = []
+  const compositeOps: string[] = []
   const proxy = new Proxy(real, {
     set(target, prop, value) {
       if (prop === 'filter') filterSets.push(value as string)
+      if (prop === 'globalCompositeOperation') compositeOps.push(value as string)
       ;(target as unknown as Record<string | symbol, unknown>)[prop as string | symbol] = value
       return true
     },
@@ -55,7 +58,7 @@ function makeRecordingCtx(): RecordingCtx {
       return v
     },
   })
-  return { ctx: proxy as CanvasRenderingContext2D, filterSets }
+  return { ctx: proxy as CanvasRenderingContext2D, filterSets, compositeOps }
 }
 
 function mountComponent(
@@ -1571,6 +1574,136 @@ export function registerAllTests(): void {
     })
   })
 
+  suite('mask layers — store', () => {
+    test('setLayerMask(true) enables positive mask by default', () => {
+      const s = makeFreshStore()
+      const a = s.addAsset({ kind: 'rect', name: 'M', width: 40, height: 40, fill: '#fff' })
+      const l = s.addLayer(a.id)
+      s.setLayerMask(l.id, true)
+      const layer = s.scene.layers.find((x) => x.id === l.id)!
+      expect(layer.isMask).toBe(true)
+      expect(layer.maskMode).toBe('positive')
+    })
+
+    test('setLayerMask(true, "negative") stores negative mode', () => {
+      const s = makeFreshStore()
+      const a = s.addAsset({ kind: 'circle', name: 'N', radius: 20, fill: '#fff' })
+      const l = s.addLayer(a.id)
+      s.setLayerMask(l.id, true, 'negative')
+      const layer = s.scene.layers.find((x) => x.id === l.id)!
+      expect(layer.maskMode).toBe('negative')
+    })
+
+    test('setLayerMask(false) clears mask flags', () => {
+      const s = makeFreshStore()
+      const a = s.addAsset({ kind: 'rect', name: 'M', width: 40, height: 40, fill: '#fff' })
+      const l = s.addLayer(a.id)
+      s.setLayerMask(l.id, true, 'negative')
+      s.setLayerMask(l.id, false)
+      const layer = s.scene.layers.find((x) => x.id === l.id)!
+      expect(layer.isMask).toBe(undefined)
+      expect(layer.maskMode).toBe(undefined)
+    })
+
+    test('setLayerMaskMode flips mode without changing isMask', () => {
+      const s = makeFreshStore()
+      const a = s.addAsset({ kind: 'rect', name: 'M', width: 40, height: 40, fill: '#fff' })
+      const l = s.addLayer(a.id)
+      s.setLayerMask(l.id, true)
+      s.setLayerMaskMode(l.id, 'negative')
+      const layer = s.scene.layers.find((x) => x.id === l.id)!
+      expect(layer.isMask).toBe(true)
+      expect(layer.maskMode).toBe('negative')
+    })
+  })
+
+  suite('mask layers — rendering', () => {
+    function setupMaskScene(mode: 'positive' | 'negative') {
+      const s = makeFreshStore()
+      s.scene.canvas.width = 100
+      s.scene.canvas.height = 100
+      // Content: full-canvas red rect
+      const content = s.addAsset({
+        kind: 'rect',
+        name: 'BG',
+        width: 100,
+        height: 100,
+        fill: '#ff0000',
+      })
+      const contentLayer = s.addLayer(content.id)
+      s.upsertKeyframe(contentLayer.id, 0, { x: 50, y: 50 })
+      // Mask: small circle centered at (50,50)
+      const mask = s.addAsset({
+        kind: 'circle',
+        name: 'M',
+        radius: 20,
+        fill: '#ffffff',
+      })
+      const maskLayer = s.addLayer(mask.id)
+      s.upsertKeyframe(maskLayer.id, 0, { x: 50, y: 50 })
+      s.setLayerMask(maskLayer.id, true, mode)
+      const group = s.addTrackGroup('G')
+      s.moveTrackIntoGroup(maskLayer.id, group.id)
+      s.moveTrackIntoGroup(contentLayer.id, group.id)
+      return { store: s }
+    }
+
+    test('positive mask keeps content inside the mask shape and clears outside', () => {
+      const { store } = setupMaskScene('positive')
+      const canvas = document.createElement('canvas')
+      canvas.width = 100
+      canvas.height = 100
+      const ctx = canvas.getContext('2d')!
+      drawSceneFrame(ctx, store.scene, 0, {})
+      const inside = ctx.getImageData(50, 50, 1, 1).data
+      const outside = ctx.getImageData(5, 5, 1, 1).data
+      // Inside mask: red content visible
+      expect(inside[0] > 200).toBe(true)
+      expect(inside[3] > 200).toBe(true)
+      // Outside mask: cleared (alpha 0)
+      expect(outside[3]).toBe(0)
+    })
+
+    test('negative mask punches a hole in the content where the mask overlaps', () => {
+      const { store } = setupMaskScene('negative')
+      const canvas = document.createElement('canvas')
+      canvas.width = 100
+      canvas.height = 100
+      const ctx = canvas.getContext('2d')!
+      drawSceneFrame(ctx, store.scene, 0, {})
+      const inside = ctx.getImageData(50, 50, 1, 1).data
+      const outside = ctx.getImageData(5, 5, 1, 1).data
+      // Inside mask: hole (alpha 0)
+      expect(inside[3]).toBe(0)
+      // Outside mask: red content still there
+      expect(outside[0] > 200).toBe(true)
+      expect(outside[3] > 200).toBe(true)
+    })
+
+    test('layer outside a masked group is not affected by the mask', () => {
+      const { store } = setupMaskScene('positive')
+      // Add a green rect layer OUTSIDE the group
+      const outer = store.addAsset({
+        kind: 'rect',
+        name: 'Outer',
+        width: 20,
+        height: 20,
+        fill: '#00ff00',
+      })
+      const outerLayer = store.addLayer(outer.id)
+      store.upsertKeyframe(outerLayer.id, 0, { x: 90, y: 90 })
+      const canvas = document.createElement('canvas')
+      canvas.width = 100
+      canvas.height = 100
+      const ctx = canvas.getContext('2d')!
+      drawSceneFrame(ctx, store.scene, 0, {})
+      // The outer layer renders fully regardless of mask
+      const outerPx = ctx.getImageData(90, 90, 1, 1).data
+      expect(outerPx[1] > 200).toBe(true)
+      expect(outerPx[3] > 200).toBe(true)
+    })
+  })
+
   suite('scene store — shiftKeyframesPosition', () => {
     test('shifts x/y of a single keyframe by delta', async () => {
       const s = makeFreshStore()
diff --git a/src/i18n/messages.ts b/src/i18n/messages.ts
index 047b9a7..9c646f2 100644
--- a/src/i18n/messages.ts
+++ b/src/i18n/messages.ts
@@ -102,6 +102,14 @@ export const en = {
     shadowAngle: 'Shadow angle',
     shadowDistance: 'Shadow distance',
     shadowColor: 'Shadow color',
+    mask: 'Mask',
+    maskUse: 'Use as mask',
+    maskMode: 'Mask mode',
+    maskPositive: 'Positive',
+    maskNegative: 'Negative',
+    maskHint: 'Masks everything below it in the same group.',
+    maskActive: 'Mask is active — affects layers below it in its group.',
+    maskNeedsGroup: 'Add this layer to a track group to activate masking. Right-click a track and choose "Add group".',
   },
 
   fullscreen: {
@@ -182,6 +190,8 @@ export const en = {
     fitToView: 'Fit to view',
     cropOn: 'Show stage only (crop)',
     cropOff: 'Show everything',
+    maskPreviewOn: 'Preview masks (render view)',
+    maskPreviewOff: 'Back to edit view',
   },
 
   settings: {
diff --git a/src/lib/exportPlayer.ts b/src/lib/exportPlayer.ts
index 6137868..e73b0bf 100644
--- a/src/lib/exportPlayer.ts
+++ b/src/lib/exportPlayer.ts
@@ -140,6 +140,35 @@ const ENGINE_SOURCE = `(function(){
       draw(t);
       requestAnimationFrame(frame);
     }
+    function drawMaskShape(ctxm,layer,time){
+      var asset=assetMap[layer.assetId];
+      if(!asset) return;
+      if(asset.kind!=='rect' && asset.kind!=='circle' && asset.kind!=='text') return;
+      var p=sample(layer,time);
+      ctxm.save();
+      ctxm.translate(p.x,p.y);
+      ctxm.rotate(p.rotation*Math.PI/180);
+      ctxm.scale(p.scaleX,p.scaleY);
+      ctxm.fillStyle='#000000';
+      if(asset.kind==='rect'){
+        var mrOff=layer.offsetX!=null?layer.offsetX:asset.width/2;
+        var mrOffY=layer.offsetY!=null?layer.offsetY:asset.height/2;
+        ctxm.fillRect(-mrOff,-mrOffY,asset.width,asset.height);
+      } else if(asset.kind==='circle'){
+        var mcOff=layer.offsetX!=null?layer.offsetX:0;
+        var mcOffY=layer.offsetY!=null?layer.offsetY:0;
+        ctxm.beginPath();
+        ctxm.arc(-mcOff,-mcOffY,asset.radius,0,Math.PI*2);
+        ctxm.fill();
+      } else {
+        var mtOff=layer.offsetX!=null?layer.offsetX:0;
+        var mtOffY=layer.offsetY!=null?layer.offsetY:0;
+        ctxm.font=asset.fontSize+'px sans-serif';
+        ctxm.textBaseline='top';
+        ctxm.fillText(asset.text,-mtOff,-mtOffY);
+      }
+      ctxm.restore();
+    }
     function drawLayer(layer,time){
       var asset=assetMap[layer.assetId];
       if(!asset||!layer.visible)return;
@@ -233,31 +262,60 @@ const ENGINE_SOURCE = `(function(){
       var rawOrder=scene.trackOrder||[];
       var groupById2={}; var groups2=scene.groups||[];
       for(var gi2=0;gi2<groups2.length;gi2++) groupById2[groups2[gi2].id]=groups2[gi2];
-      var order=[];
-      for(var roi=0;roi<rawOrder.length;roi++){
-        var rid=rawOrder[roi];
-        var grp=groupById2[rid];
-        if(grp){ for(var gc=0;gc<grp.childIds.length;gc++) order.push(grp.childIds[gc]); }
-        else order.push(rid);
-      }
-      if(!order.length){for(var lj=0;lj<scene.layers.length;lj++)order.push(scene.layers[lj].id);}
-      for(var oi=order.length-1;oi>=0;oi--){
-        var id=order[oi];
+      if(!rawOrder.length){ rawOrder=[]; for(var lj=0;lj<scene.layers.length;lj++) rawOrder.push(scene.layers[lj].id); }
+      for(var oi=rawOrder.length-1;oi>=0;oi--){
+        var id=rawOrder[oi];
         var lyr=layerById[id];
         if(lyr){ drawLayer(lyr,time); continue; }
         var tInsts=instByTrack[id]||[];
-        for(var ti=0;ti<tInsts.length;ti++){
-          var inst2=tInsts[ti];
-          var clip2=clipById2[inst2.clipId];
-          if(!clip2)continue;
-          var tS=inst2.trimStart||0;
-          var tE=inst2.trimEnd||0;
-          var visible=Math.max(0,clip2.duration-tS-tE);
-          var offset=time-inst2.startTime;
-          if(offset<0||offset>visible)continue;
-          var localT=tS+offset;
-          for(var cl2=clip2.layers.length-1;cl2>=0;cl2--){
-            drawLayer(clip2.layers[cl2],localT);
+        if(tInsts.length){
+          for(var ti=0;ti<tInsts.length;ti++){
+            var inst2=tInsts[ti];
+            var clip2=clipById2[inst2.clipId];
+            if(!clip2)continue;
+            var tS=inst2.trimStart||0;
+            var tE=inst2.trimEnd||0;
+            var visibleClip=Math.max(0,clip2.duration-tS-tE);
+            var offset=time-inst2.startTime;
+            if(offset<0||offset>visibleClip)continue;
+            var localT=tS+offset;
+            for(var cl2=clip2.layers.length-1;cl2>=0;cl2--){
+              drawLayer(clip2.layers[cl2],localT);
+            }
+          }
+          continue;
+        }
+        var grp=groupById2[id];
+        if(grp){
+          var gContent=[], gMasks=[];
+          for(var ch=grp.childIds.length-1;ch>=0;ch--){
+            var cl0=layerById[grp.childIds[ch]];
+            if(!cl0) continue;
+            if(cl0.isMask) gMasks.push(cl0); else gContent.push(cl0);
+          }
+          if(!gMasks.length){
+            for(var gi3=0;gi3<gContent.length;gi3++) drawLayer(gContent[gi3],time);
+          } else {
+            var mcanvas=document.createElement('canvas');
+            mcanvas.width=canvas.width; mcanvas.height=canvas.height;
+            var mctx=mcanvas.getContext('2d');
+            var tm=ctx.getTransform();
+            mctx.setTransform(tm.a,tm.b,tm.c,tm.d,tm.e,tm.f);
+            var savedCtx=ctx;
+            ctx=mctx;
+            for(var gi4=0;gi4<gContent.length;gi4++) drawLayer(gContent[gi4],time);
+            for(var gm=0;gm<gMasks.length;gm++){
+              var mL=gMasks[gm];
+              mctx.save();
+              mctx.globalCompositeOperation=(mL.maskMode==='negative'?'destination-out':'destination-in');
+              drawMaskShape(mctx,mL,time);
+              mctx.restore();
+            }
+            ctx=savedCtx;
+            ctx.save();
+            ctx.setTransform(1,0,0,1,0,0);
+            ctx.drawImage(mcanvas,0,0);
+            ctx.restore();
           }
         }
       }
diff --git a/src/lib/render.ts b/src/lib/render.ts
index b16458d..938e860 100644
--- a/src/lib/render.ts
+++ b/src/lib/render.ts
@@ -1,6 +1,5 @@
 import type { AnimatableProps, Asset, Layer, Scene } from '@/types/scene'
 import { sampleLayer } from './interpolate'
-import { expandedTrackOrder } from './sceneOrder'
 
 export async function loadSceneImages(scene: Scene): Promise<Record<string, HTMLImageElement>> {
   const result: Record<string, HTMLImageElement> = {}
@@ -145,6 +144,80 @@ function drawLayers(
   }
 }
 
+function drawMaskShape(
+  ctx: CanvasRenderingContext2D,
+  layer: Layer,
+  assetMap: Map<string, Asset>,
+  time: number,
+) {
+  const asset = assetMap.get(layer.assetId)
+  if (!asset) return
+  if (asset.kind !== 'rect' && asset.kind !== 'circle' && asset.kind !== 'text') return
+  const p = sampleLayer(layer, time)
+  ctx.save()
+  ctx.translate(p.x, p.y)
+  ctx.rotate((p.rotation * Math.PI) / 180)
+  ctx.scale(p.scaleX, p.scaleY)
+  ctx.fillStyle = '#000000'
+  if (asset.kind === 'rect') {
+    const offX = layer.offsetX ?? asset.width / 2
+    const offY = layer.offsetY ?? asset.height / 2
+    ctx.fillRect(-offX, -offY, asset.width, asset.height)
+  } else if (asset.kind === 'circle') {
+    const offX = layer.offsetX ?? 0
+    const offY = layer.offsetY ?? 0
+    ctx.beginPath()
+    ctx.arc(-offX, -offY, asset.radius, 0, Math.PI * 2)
+    ctx.fill()
+  } else {
+    const offX = layer.offsetX ?? 0
+    const offY = layer.offsetY ?? 0
+    ctx.font = `${asset.fontSize}px sans-serif`
+    ctx.textBaseline = 'top'
+    ctx.fillText(asset.text, -offX, -offY)
+  }
+  ctx.restore()
+}
+
+function drawMaskedGroup(
+  ctx: CanvasRenderingContext2D,
+  children: Layer[],
+  assetMap: Map<string, Asset>,
+  images: Record<string, HTMLImageElement>,
+  time: number,
+) {
+  const masks = children.filter((l) => l.isMask)
+  const content = children.filter((l) => !l.isMask)
+  if (masks.length === 0) {
+    drawLayers(ctx, children, assetMap, images, time)
+    return
+  }
+
+  const w = ctx.canvas.width
+  const h = ctx.canvas.height
+  const off = document.createElement('canvas')
+  off.width = w
+  off.height = h
+  const offCtx = off.getContext('2d')
+  if (!offCtx) {
+    drawLayers(ctx, content, assetMap, images, time)
+    return
+  }
+  const m = ctx.getTransform()
+  offCtx.setTransform(m.a, m.b, m.c, m.d, m.e, m.f)
+  drawLayers(offCtx, content, assetMap, images, time)
+  for (const mask of masks) {
+    offCtx.save()
+    offCtx.globalCompositeOperation = mask.maskMode === 'negative' ? 'destination-out' : 'destination-in'
+    drawMaskShape(offCtx, mask, assetMap, time)
+    offCtx.restore()
+  }
+  ctx.save()
+  ctx.setTransform(1, 0, 0, 1, 0, 0)
+  ctx.drawImage(off, 0, 0)
+  ctx.restore()
+}
+
 export function drawSceneFrame(
   ctx: CanvasRenderingContext2D,
   scene: Scene,
@@ -174,27 +247,41 @@ export function drawSceneFrame(
   }
   const clipById = new Map<string, NonNullable<typeof scene.clips>[number]>()
   for (const c of scene.clips ?? []) clipById.set(c.id, c)
+  const groupById = new Map<string, NonNullable<typeof scene.groups>[number]>()
+  for (const g of scene.groups ?? []) groupById.set(g.id, g)
 
-  const order = expandedTrackOrder(scene).length
-    ? expandedTrackOrder(scene)
+  const rawOrder = scene.trackOrder?.length
+    ? scene.trackOrder
     : scene.layers.map((l) => l.id)
-  for (let i = order.length - 1; i >= 0; i--) {
-    const id = order[i]
+  for (let i = rawOrder.length - 1; i >= 0; i--) {
+    const id = rawOrder[i]
     const layer = layerById.get(id)
     if (layer) {
       drawLayers(ctx, [layer], assetMap, images, time)
       continue
     }
     const instances = instancesByTrack.get(id) ?? []
-    for (const inst of instances) {
-      const clip = clipById.get(inst.clipId)
-      if (!clip) continue
-      const trimStart = inst.trimStart ?? 0
-      const trimEnd = inst.trimEnd ?? 0
-      const visible = Math.max(0, clip.duration - trimStart - trimEnd)
-      const offsetIntoBlock = time - inst.startTime
-      if (offsetIntoBlock < 0 || offsetIntoBlock > visible) continue
-      drawLayers(ctx, clip.layers, assetMap, images, trimStart + offsetIntoBlock)
+    if (instances.length) {
+      for (const inst of instances) {
+        const clip = clipById.get(inst.clipId)
+        if (!clip) continue
+        const trimStart = inst.trimStart ?? 0
+        const trimEnd = inst.trimEnd ?? 0
+        const visible = Math.max(0, clip.duration - trimStart - trimEnd)
+        const offsetIntoBlock = time - inst.startTime
+        if (offsetIntoBlock < 0 || offsetIntoBlock > visible) continue
+        drawLayers(ctx, clip.layers, assetMap, images, trimStart + offsetIntoBlock)
+      }
+      continue
+    }
+    const group = groupById.get(id)
+    if (group) {
+      const childLayers: Layer[] = []
+      for (let k = group.childIds.length - 1; k >= 0; k--) {
+        const cl = layerById.get(group.childIds[k])
+        if (cl) childLayers.push(cl)
+      }
+      drawMaskedGroup(ctx, childLayers, assetMap, images, time)
     }
   }
 
diff --git a/src/stores/scene.ts b/src/stores/scene.ts
index 179b3ed..808d0c3 100644
--- a/src/stores/scene.ts
+++ b/src/stores/scene.ts
@@ -8,6 +8,7 @@ import type {
   Easing,
   Keyframe,
   Layer,
+  MaskMode,
   Scene,
   TrackGroup,
 } from '@/types/scene'
@@ -464,6 +465,23 @@ export const useSceneStore = defineStore('scene', {
         kf.props = { ...kf.props, x: curX + dx, y: curY + dy }
       }
     },
+    setLayerMask(layerId: string, isMask: boolean, mode?: MaskMode) {
+      const layer = this.activeLayers.find((l) => l.id === layerId)
+      if (!layer) return
+      if (isMask) {
+        layer.isMask = true
+        if (mode) layer.maskMode = mode
+        else if (!layer.maskMode) layer.maskMode = 'positive'
+      } else {
+        delete layer.isMask
+        delete layer.maskMode
+      }
+    },
+    setLayerMaskMode(layerId: string, mode: MaskMode) {
+      const layer = this.activeLayers.find((l) => l.id === layerId)
+      if (!layer) return
+      layer.maskMode = mode
+    },
     setTrackLocked(trackId: string, locked: boolean) {
       const layer = this.activeLayers.find((l) => l.id === trackId)
       if (layer) {
diff --git a/src/types/scene.ts b/src/types/scene.ts
index a92c52f..acd6b24 100644
--- a/src/types/scene.ts
+++ b/src/types/scene.ts
@@ -65,6 +65,8 @@ export interface TextAsset {
 
 export type Asset = ImageAsset | RectAsset | CircleAsset | TextAsset
 
+export type MaskMode = 'positive' | 'negative'
+
 export interface Layer {
   id: string
   name: string
@@ -79,6 +81,8 @@ export interface Layer {
   offsetY?: number
   color?: string | null
   locked?: boolean
+  isMask?: boolean
+  maskMode?: MaskMode
 }
 
 export interface Clip {