Test on a folder  got error.

```
 if os.path.isdir(args.imgs[0]):   # Maybe args.imgs.
        imgs = find_recursive(args.imgs[0])
    else:
        imgs = [args.imgs]
```
 and  "find_recursive"
```
def find_recursive(root_dir):
    files = []
    filenames = os.listdir(root_dir)
    for filename in filenames:
        files.append(os.path.join(root_dir, filename))
    return files
```
