fix: refine rmbg alpha for scale drafts

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

@@ -1721,6 +1721,28 @@ function thresholdScaleAdjustAlphaMask(params: {
   return output;
 }
 
+async function applyScaleAdjustAlphaMaskToRgba(params: {
+  alphaMask: Uint8Array;
+  height: number;
+  rawRgba: Buffer;
+  width: number;
+}) {
+  const { alphaMask, height, rawRgba, width } = params;
+  const blur = Math.max(0.45, Math.min(width, height) * 0.0035);
+  const softenedAlpha = await sharp(Buffer.from(alphaMask), {
+    raw: { channels: 1, height, width },
+  })
+    .blur(blur)
+    .raw()
+    .toBuffer();
+  const output = Buffer.from(rawRgba);
+  const pixelCount = Math.min(alphaMask.length, Math.floor(output.length / 4));
+  for (let index = 0; index < pixelCount; index += 1) {
+    output[index * 4 + 3] = softenedAlpha[index] || 0;
+  }
+  return output;
+}
+
 function getScaleAdjustAlphaBandingStats(params: {
   height: number;
   maskBuffer: Buffer | Uint8Array;
@@ -1927,14 +1949,30 @@ async function buildScaleAdjustRmbgStructureLockedComposite(params: {
       rawRgba: cutoutRaw,
       width: cutoutWidth,
     });
-    const cleanedCutout = await sharp(cleanedRaw, {
+    const highConfidenceCutoutMask = thresholdScaleAdjustAlphaMask({
+      maskBuffer: cleanedAlpha,
+      threshold: 96,
+    });
+    const highConfidenceCutoutBbox = getScaleAdjustMaskBoundingBox({
+      height: cutoutHeight,
+      mask: highConfidenceCutoutMask,
+      width: cutoutWidth,
+    });
+    if (!highConfidenceCutoutBbox) return null;
+    const refinedCutoutRaw = await applyScaleAdjustAlphaMaskToRgba({
+      alphaMask: highConfidenceCutoutMask,
+      height: cutoutHeight,
+      rawRgba: cleanedRaw,
+      width: cutoutWidth,
+    });
+    const cleanedCutout = await sharp(refinedCutoutRaw, {
       raw: { channels: 4, height: cutoutHeight, width: cutoutWidth },
     })
       .png({ compressionLevel: 6 })
       .toBuffer();
     const alphaBbox = getScaleAdjustAlphaBoundingBox({
       height: cutoutHeight,
-      rawRgba: cleanedRaw,
+      rawRgba: refinedCutoutRaw,
       width: cutoutWidth,
     });
     if (!alphaBbox) return null;
@@ -1950,7 +1988,7 @@ async function buildScaleAdjustRmbgStructureLockedComposite(params: {
       return null;
     }
 
-    const fullAlphaMask = cleanedAlpha;
+    const fullAlphaMask = Buffer.from(highConfidenceCutoutMask);
     const sourceMaskBuffer = sameSizeCutout
       ? fullAlphaMask
       : await resizeScaleAdjustAlphaMask({