Fix issues with default 'img_prefix' value set to None when training (#1497)

* Still work if no img_prefix specified

* Update default param to be empty string

mmdet/datasets/custom.py

@@ -37,7 +37,7 @@ class CustomDataset(Dataset):
                  ann_file,
                  pipeline,
                  data_root=None,
-                 img_prefix=None,
+                 img_prefix='',
                  seg_prefix=None,
                  proposal_file=None,
                  test_mode=False):

mmdet/datasets/pipelines/loading.py

@@ -15,8 +15,11 @@ class LoadImageFromFile(object):
         self.to_float32 = to_float32
 
     def __call__(self, results):
+        if results['img_prefix'] is not None:
             filename = osp.join(results['img_prefix'],
                                 results['img_info']['filename'])
+        else:
+            filename = results['img_info']['filename']
         img = mmcv.imread(filename)
         if self.to_float32:
             img = img.astype(np.float32)
@@ -52,8 +55,11 @@ class LoadAnnotations(object):
         ann_info = results['ann_info']
         results['gt_bboxes'] = ann_info['bboxes']
         if len(results['gt_bboxes']) == 0 and self.skip_img_without_anno:
+            if results['img_prefix'] is not None:
                 file_path = osp.join(results['img_prefix'],
                                      results['img_info']['filename'])
+            else:
+                file_path = results['img_info']['filename']
             warnings.warn(
                 'Skip the image "{}" that has no valid gt bbox'.format(
                     file_path))