send an image if no map available

feeder/server.py

@@ -2,8 +2,11 @@ import datetime
 import io
 
 import redis
+from PIL import Image, ImageDraw, ImageFont
 from flask import Flask, send_file, make_response
 
+NO_MAP_STR = 'NO GLOBAL MAP AVAILABLE'
+
 app = Flask(__name__)
 redis_client = redis.Redis(host='localhost', port=6379)
 
@@ -12,7 +15,15 @@ redis_client = redis.Redis(host='localhost', port=6379)
 def serve_global_map():
     global_map_data = redis_client.get('global_map')
     if global_map_data is None:
-        return "No global map available", 400
+        img = Image.new('RGB', (500, 300), color=(255, 255, 255))
+        d = ImageDraw.Draw(img)
+        fnt = ImageFont.load_default(size=30)
+        w, h = fnt.getbbox(NO_MAP_STR)[2:4]
+        d.text(((500 - w) / 2, (300 - h) / 2), NO_MAP_STR, font=fnt, fill=(0, 0, 0))
+        buf = io.BytesIO()
+        img.save(buf, format='PNG')
+        buf.seek(0)
+        return send_file(buf, mimetype='image/png')
 
     buf = io.BytesIO(global_map_data)
     buf.seek(0)