ensures that alpha is a (40, 40, 3) mask matching RGB shape

app.py

@@ -193,6 +193,10 @@ def knn_path(start, targets, occ):
 # ── Robot sprite/class -──────────────────────────────────────────────────
 class Robot:
     def __init__(self, sprite, speed=2000): # Declare the robot's physical stats and routing (position, speed, movement, path)
+        img = Image.open(sprite).convert("RGBA").resize((40, 40))
+        self.png = np.array(img)
+        if self.png.shape[-1] != 4:
+            raise ValueError("Sprite image must have 4 channels (RGBA)")
         self.png = np.array(Image.open(sprite).convert("RGBA").resize((40,40)))
         self.pos = [0,0]; self.speed=speed
     def step(self, path):
@@ -464,9 +468,10 @@ def _pipeline(uid,img_path):
         # robot
         robot.step(path)
         rx,ry=robot.pos; sp=robot.png
-        a=sp[:,:,3]/255.; bgroi=frame[ry:ry+40,rx:rx+40]
-        for c in range(3): bgroi[:,:,c]=a*sp[:,:,c]+(1-a)*bgroi[:,:,c]
-        frame[ry:ry+40,rx:rx+40]=bgroi
+        a = sp[:, :, 3] / 255.0  # alpha channel
+        a = np.stack([a]*3, axis=-1)  # shape (40, 40, 3)
+        bgroi = frame[ry:ry+40, rx:rx+40]
+        frame[ry:ry+40, rx:rx+40] = (a * sp[:, :, :3] + (1 - a) * bgroi).astype(np.uint8)
         # collection check
         for o in objs:
             if not o["col"] and np.hypot(o["pos"][0]-rx,o["pos"][1]-ry)<=20: