サムネイルを予め生成するように

src/drive/gen-thumbnail.ts

@@ -0,0 +1,25 @@
+import * as stream from 'stream';
+import * as Gm from 'gm';
+import { IDriveFile, getDriveFileBucket } from '../models/drive-file';
+
+const gm = Gm.subClass({
+	imageMagick: true
+});
+
+export default async function(file: IDriveFile): Promise<stream.Readable> {
+	if (!/^image\/.*$/.test(file.contentType)) return null;
+
+	const bucket = await getDriveFileBucket();
+	const readable = bucket.openDownloadStream(file._id);
+
+	const g = gm(readable);
+
+	const stream = g
+		.resize(256, 256)
+		.compress('jpeg')
+		.quality(70)
+		.interlace('line')
+		.stream();
+
+	return stream;
+}