[transforms] Crop an image without any box in crop_detection

doctr/transforms/functional/base.py

@@ -26,6 +26,8 @@ def crop_boxes(
         the cropped boxes
     """
     boxes = boxes.copy()
+    if boxes.shape[0] == 0:  # nothing to crop (e.g. an image without any text)
+        return boxes
     is_box_rel = boxes.max() <= 1
     is_crop_rel = max(crop_box) <= 1
 

doctr/transforms/functional/pytorch.py

@@ -117,7 +117,7 @@ def crop_detection(
     xmax, ymax = int(round(crop_box[2] * w)), int(round(crop_box[3] * h))
     cropped_img = F.crop(img, ymin, xmin, ymax - ymin, xmax - xmin)
     # Crop the box
-    boxes = crop_boxes(boxes, crop_box if boxes.max() <= 1 else (xmin, ymin, xmax, ymax))
+    boxes = crop_boxes(boxes, crop_box if boxes.size == 0 or boxes.max() <= 1 else (xmin, ymin, xmax, ymax))
 
     return cropped_img, boxes
 

tests/pytorch/test_transforms_pt.py

@@ -408,6 +408,12 @@ def test_crop_detection():
     with pytest.raises(AssertionError):
         crop_detection(img, abs_boxes, (2, 6, 24, 56))
 
+    # An image without any box (e.g. no text) is still cropped, and keeps no box
+    img = torch.ones((3, 50, 50), dtype=torch.float32)
+    c_img, c_boxes = crop_detection(img, np.zeros((0, 4), dtype=np.float32), (0.24, 0.46, 1.0, 1.0))
+    assert c_img.shape == (3, 27, 38)
+    assert c_boxes.shape == (0, 4)
+
 
 @pytest.mark.parametrize(
     "target",