diff --git a/docker/frontend/src/engine/superRes.ts b/docker/frontend/src/engine/superRes.ts
index 72cc7af..2a79e2a 100644
--- a/docker/frontend/src/engine/superRes.ts
+++ b/docker/frontend/src/engine/superRes.ts
@@ -78,7 +78,9 @@ export async function upscaleJpeg(
     const srcCanvas = new OffscreenCanvas(w, h);
     const srcCtx = srcCanvas.getContext('2d', { willReadFrequently: true });
     const dstCanvas = new OffscreenCanvas(Math.max(1, Math.round(w * scale)), Math.max(1, Math.round(h * scale)));
-    const dstCtx = dstCanvas.getContext('2d');
+    // Opaque: a partly covered edge pixel would otherwise survive as transparency
+    // and the JPEG export flattens that onto black — a dark line down every seam.
+    const dstCtx = dstCanvas.getContext('2d', { alpha: false });
     if (!srcCtx || !dstCtx) return bytes;
     srcCtx.drawImage(bitmap, 0, 0);
     const src = srcCtx.getImageData(0, 0, w, h);
@@ -138,16 +140,22 @@ export async function upscaleJpeg(
           img.data[d + 3] = 255;
         }
         tileCtx.putImageData(img, 0, 0);
+        // Destination edges snapped to whole pixels: neighbouring tiles then share
+        // the exact same boundary, so nothing is left half-covered (and half-black)
+        // where they meet. `scale` is rarely an integer, so `x0 * scale` alone
+        // lands on a fraction and antialiases the seam.
+        const dx0 = Math.round(x0 * scale);
+        const dy0 = Math.round(y0 * scale);
         dstCtx.drawImage(
           tile,
           left * MODEL_SCALE,
           top * MODEL_SCALE,
           tw * MODEL_SCALE,
           th * MODEL_SCALE,
-          x0 * scale,
-          y0 * scale,
-          tw * scale,
-          th * scale
+          dx0,
+          dy0,
+          Math.round((x0 + tw) * scale) - dx0,
+          Math.round((y0 + th) * scale) - dy0
         );
 
         done++;