diff --git a/frontend/src/capture/MeasurePanel.svelte b/frontend/src/capture/MeasurePanel.svelte
index a07c605..2befc41 100644
--- a/frontend/src/capture/MeasurePanel.svelte
+++ b/frontend/src/capture/MeasurePanel.svelte
@@ -8,6 +8,7 @@
   import PhotoStage from './PhotoStage.svelte';
   import { getCropFill, setCropFill } from '../settings/cropFill';
   import { cropMarginFraction } from '../png/crop';
+  import { createMaskedPreviewBlob } from '../png/maskedPreview';
   import { getTaxonomy } from '../api/slabs';
   import { speciesLeavesFromTaxonomy } from '../create/settings/speciesLeaves';
   import {
@@ -41,6 +42,40 @@
   $: tape = state.axis.tape;
   $: tiltShown = Math.round(tape.tiltDeg * 10) / 10;
 
+  let maskedPreview: Blob | null = null;
+  let previewError = '';
+  let previewGeneration = 0;
+  let previewSource: Blob | null = null;
+  let previewMask: Uint8Array | null = null;
+  let previewW = 0;
+  let previewH = 0;
+
+  // Build the tape photo once from the oriented source plus the approved mask.
+  // Never fall back to the raw source here: tape alignment must show the same
+  // background-removed slab footprint that was approved in Mask.
+  $: {
+    const sourceBlob = state.source.blob;
+    const bitset = state.approvedMask.bitset;
+    const width = state.source.widthPx;
+    const height = state.source.heightPx;
+    if (sourceBlob !== previewSource || bitset !== previewMask || width !== previewW || height !== previewH) {
+      previewSource = sourceBlob;
+      previewMask = bitset;
+      previewW = width;
+      previewH = height;
+      maskedPreview = null;
+      previewError = '';
+      const generation = ++previewGeneration;
+      void createMaskedPreviewBlob(sourceBlob, bitset)
+        .then((blob) => {
+          if (generation === previewGeneration) maskedPreview = blob;
+        })
+        .catch(() => {
+          if (generation === previewGeneration) previewError = 'Could not build the background-removed preview.';
+        });
+    }
+  }
+
   let lineupUrls: string[] = [];
   let lineupItems: PhotosReadyLineupItem[] = [];
 
@@ -84,6 +119,7 @@
   });
 
   onDestroy(() => {
+    previewGeneration += 1;
     for (const u of lineupUrls) URL.revokeObjectURL(u);
   });
 
@@ -154,16 +190,19 @@
     <p class="hint">Orient the slab on the fixed tape line, then enter length, thickness, and species.</p>
     <p class="nudge">The black line is <b>fixed</b>: tilting, dragging and zooming the slab never move it. It stays in the middle of the rectangle.</p>
 
-    <PhotoStage
-      mode="tape"
-      blob={state.source.blob}
-      maskBitset={state.approvedMask.bitset}
-      {maskW}
-      {maskH}
-      {tape}
-      {cropFill}
-      onTapeChange={onTapeChange}
-    />
+    {#if maskedPreview}
+      <PhotoStage
+        mode="tape"
+        blob={maskedPreview}
+        {tape}
+        {cropFill}
+        onTapeChange={onTapeChange}
+      />
+    {:else}
+      <div class="preview-loading" role="status">
+        {previewError || 'Preparing background-removed preview…'}
+      </div>
+    {/if}
 
     <div class="controls">
       <div>
@@ -344,6 +383,7 @@
   h1 { font-size: 1.3rem; margin: 0 0 8px; }
   .hint { color: #aaa; font-size: 0.9rem; margin: 0 0 8px; }
   .nudge { color: #e8a; font-size: 0.74rem; margin: 6px 0 0; line-height: 1.45; }
+  .preview-loading { min-height: 420px; border: 1px solid #333; border-radius: 8px; display: flex; align-items: center; justify-content: center; color: #999; text-align: center; padding: 16px; box-sizing: border-box; }
   .controls { margin-top: 16px; display: flex; flex-direction: column; gap: 14px; }
   .ctl-label { font-size: 0.78rem; color: #aaa; margin: 0 0 6px; }
   .ctl-label .sub { font-size: 0.7rem; color: #777; }
diff --git a/frontend/src/png/maskedPreview.test.ts b/frontend/src/png/maskedPreview.test.ts
new file mode 100644
index 0000000..6315f60
--- /dev/null
+++ b/frontend/src/png/maskedPreview.test.ts
@@ -0,0 +1,14 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { applyMaskAlpha } from './maskedPreview';
+
+test('applyMaskAlpha keeps slab pixels and clears the background', () => {
+  const rgba = new Uint8ClampedArray([
+    1, 2, 3, 255,
+    4, 5, 6, 255,
+    7, 8, 9, 255,
+  ]);
+  const out = applyMaskAlpha(rgba, new Uint8Array([1, 0]));
+  assert.deepEqual([...out], [1, 2, 3, 255, 4, 5, 6, 0, 7, 8, 9, 0]);
+  assert.deepEqual([...rgba], [1, 2, 3, 255, 4, 5, 6, 255, 7, 8, 9, 255]);
+});
diff --git a/frontend/src/png/maskedPreview.ts b/frontend/src/png/maskedPreview.ts
new file mode 100644
index 0000000..07b872a
--- /dev/null
+++ b/frontend/src/png/maskedPreview.ts
@@ -0,0 +1,52 @@
+import { decodeOrientedBitmap } from '../decodeOrientedBitmap';
+
+/**
+ * Apply a source-grid mask to decoded RGBA pixels without changing their
+ * dimensions. The transparent pixels remain in the full source frame so the
+ * tape transform and all measurement geometry keep using source coordinates.
+ */
+export function applyMaskAlpha(
+  rgba: Uint8ClampedArray,
+  bitset: Uint8Array,
+): Uint8ClampedArray {
+  const out = new Uint8ClampedArray(rgba);
+  const pixels = out.length / 4;
+  for (let p = 0; p < pixels; p++) {
+    if (p >= bitset.length || !bitset[p]) out[p * 4 + 3] = 0;
+  }
+  return out;
+}
+
+/**
+ * Encode an EXIF-oriented source photo with the approved mask as alpha.
+ * This is display-only; the original source blob and mask stay in session
+ * state for geometry, measurement, and inventory encoding.
+ */
+export async function createMaskedPreviewBlob(
+  source: Blob,
+  bitset: Uint8Array,
+): Promise<Blob> {
+  const bitmap = await decodeOrientedBitmap(source);
+  try {
+    const width = bitmap.width;
+    const height = bitmap.height;
+    const hasOffscreenCanvas = typeof OffscreenCanvas !== 'undefined';
+    const canvas: OffscreenCanvas | HTMLCanvasElement = hasOffscreenCanvas
+      ? new OffscreenCanvas(width, height)
+      : Object.assign(document.createElement('canvas'), { width, height });
+    const ctx = canvas.getContext('2d') as CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D | null;
+    if (!ctx) throw new Error('Could not create preview canvas');
+    ctx.drawImage(bitmap, 0, 0);
+    const image = ctx.getImageData(0, 0, width, height);
+    image.data.set(applyMaskAlpha(image.data, bitset));
+    ctx.putImageData(image, 0, 0);
+    if (hasOffscreenCanvas) {
+      return await (canvas as OffscreenCanvas).convertToBlob({ type: 'image/png' });
+    }
+    return await new Promise<Blob>((resolve, reject) => {
+      (canvas as HTMLCanvasElement).toBlob((blob: Blob | null) => (blob ? resolve(blob) : reject(new Error('Could not encode preview'))), 'image/png');
+    });
+  } finally {
+    bitmap.close();
+  }
+}