fix: reject banded rmbg scale drafts

app/api/canvas/local-edit/route.ts

@@ -1709,6 +1709,83 @@ function getScaleAdjustAlphaBoundingBox(params: {
   };
 }
 
+function thresholdScaleAdjustAlphaMask(params: {
+  maskBuffer: Buffer | Uint8Array;
+  threshold?: number;
+}) {
+  const { maskBuffer, threshold = 96 } = params;
+  const output = new Uint8Array(maskBuffer.length);
+  for (let index = 0; index < maskBuffer.length; index += 1) {
+    output[index] = (maskBuffer[index] || 0) >= threshold ? 255 : 0;
+  }
+  return output;
+}
+
+function getScaleAdjustAlphaBandingStats(params: {
+  height: number;
+  maskBuffer: Buffer | Uint8Array;
+  threshold?: number;
+  width: number;
+}) {
+  const { height, maskBuffer, threshold = 24, width } = params;
+  const mask = thresholdScaleAdjustAlphaMask({ maskBuffer, threshold });
+  const bbox = getScaleAdjustMaskBoundingBox({ height, mask, width });
+  if (!bbox) {
+    return {
+      activeRowRatio: 0,
+      bboxHeightRatio: 0,
+      bboxWidthRatio: 0,
+      detected: false,
+      transitionRatio: 0,
+      wideRowRatio: 0,
+    };
+  }
+
+  let activeRows = 0;
+  let transitions = 0;
+  let wideRows = 0;
+  let previousCoverage: number | null = null;
+  for (let y = bbox.top; y < bbox.top + bbox.height; y += 1) {
+    let rowCount = 0;
+    for (let x = bbox.left; x < bbox.left + bbox.width; x += 1) {
+      if (mask[y * width + x] !== 0) rowCount += 1;
+    }
+    const coverage = rowCount / Math.max(1, bbox.width);
+    if (coverage >= 0.08) activeRows += 1;
+    if (coverage >= 0.45) wideRows += 1;
+    if (
+      previousCoverage !== null &&
+      Math.abs(coverage - previousCoverage) >= 0.28
+    ) {
+      transitions += 1;
+    }
+    previousCoverage = coverage;
+  }
+
+  const bboxHeightRatio = bbox.height / Math.max(1, height);
+  const bboxWidthRatio = bbox.width / Math.max(1, width);
+  const activeRowRatio = activeRows / Math.max(1, bbox.height);
+  const transitionRatio = transitions / Math.max(1, bbox.height - 1);
+  const wideRowRatio = wideRows / Math.max(1, bbox.height);
+  const detected =
+    (bboxWidthRatio >= 0.38 &&
+      bboxHeightRatio >= 0.22 &&
+      wideRowRatio >= 0.18 &&
+      transitionRatio >= 0.12) ||
+    (bboxWidthRatio >= 0.55 &&
+      activeRowRatio >= 0.45 &&
+      transitionRatio >= 0.09);
+
+  return {
+    activeRowRatio: Number(activeRowRatio.toFixed(3)),
+    bboxHeightRatio: Number(bboxHeightRatio.toFixed(3)),
+    bboxWidthRatio: Number(bboxWidthRatio.toFixed(3)),
+    detected,
+    transitionRatio: Number(transitionRatio.toFixed(3)),
+    wideRowRatio: Number(wideRowRatio.toFixed(3)),
+  };
+}
+
 async function cleanScaleAdjustAlphaChannel(params: {
   height: number;
   referenceRawRgb?: Buffer | null;
@@ -1883,14 +1960,45 @@ async function buildScaleAdjustRmbgStructureLockedComposite(params: {
           targetWidth: placement.sourceBox.width,
           width: cutoutWidth,
         });
+    const alphaBandingStats = getScaleAdjustAlphaBandingStats({
+      height: placement.sourceBox.height,
+      maskBuffer: sourceMaskBuffer,
+      width: placement.sourceBox.width,
+    });
+    if (alphaBandingStats.detected) {
+      console.warn(
+        "[Canvas Local Edit] Scale adjust RMBG alpha banding detected; fallback to pixel mask:",
+        alphaBandingStats
+      );
+      return null;
+    }
     const background = estimateScaleAdjustBoxBackground({
       box: placement.sourceBox,
       raw,
       width: sourceWidth,
     });
+    const highConfidenceSourceMask = thresholdScaleAdjustAlphaMask({
+      maskBuffer: sourceMaskBuffer,
+      threshold: 96,
+    });
+    const filteredSourceMask = filterScaleAdjustStructureComponents({
+      box: placement.sourceBox,
+      mask: highConfidenceSourceMask,
+    });
+    const filteredSourceBbox = getScaleAdjustMaskBoundingBox({
+      height: placement.sourceBox.height,
+      mask: filteredSourceMask,
+      width: placement.sourceBox.width,
+    });
+    const sourceWashBaseMask = filteredSourceBbox
+      ? solidifyScaleAdjustStructureMask({
+          box: placement.sourceBox,
+          mask: filteredSourceMask,
+        })
+      : highConfidenceSourceMask;
     const sourceWashMask = dilateScaleAdjustStructureMask({
       height: placement.sourceBox.height,
-      mask: new Uint8Array(sourceMaskBuffer),
+      mask: sourceWashBaseMask,
       radius: Math.max(
         2,
         Math.round(
@@ -1977,6 +2085,7 @@ async function buildScaleAdjustRmbgStructureLockedComposite(params: {
       placedHeight,
       placedWidth,
       sameSizeCutout,
+      alphaBandingStats,
       sourceHeight: placement.sourceBox.height,
       sourceWidth: placement.sourceBox.width,
     });