Fail fast on missing terrain assets and recover empty renders

renderer/image-validation.mjs

@@ -0,0 +1,28 @@
+import sharp from "sharp";
+
+// Empty sky faces are valid when looking up. A surface map or the view straight
+// down from a surface capture must contain terrain, even if other faces are sky.
+export async function assertTerrainImage(bytes, label) {
+  const { data, info } = await sharp(bytes)
+    .resize(64, 64, { fit: "fill" })
+    .toColourspace("srgb")
+    .removeAlpha()
+    .raw()
+    .toBuffer({ resolveWithObject: true });
+  const sky = [0x95, 0xc4, 0xeb];
+  let terrainPixels = 0;
+  for (let i = 0; i < data.length; i += info.channels) {
+    if (
+      Math.abs(data[i] - sky[0]) > 12 ||
+      Math.abs(data[i + 1] - sky[1]) > 12 ||
+      Math.abs(data[i + 2] - sky[2]) > 12
+    )
+      terrainPixels++;
+  }
+  if (terrainPixels < 4) {
+    throw new Error(
+      `${label} contains only sky. Check Minecraft asset loading, capture coordinates and graphics errors; this render will not be submitted as a playable map.`,
+    );
+  }
+  return terrainPixels;
+}