From 57b9de50085aa008c11b49d69eaa4a970a1f7522 Mon Sep 17 00:00:00 2001
From: ww-6 <114972930+ww-6@users.noreply.github.com>
Date: Thu, 22 Oct 2026 12:14:04 +0100
Subject: [PATCH] :bug: Fix `imresize` interpolation choice when only
 `output_size` is given

`imresize` documents `scale_factor` as output / input (e.g. 0.5 halves
the image), and the "optimize" interpolation picks `INTER_AREA` below
1.0 and `INTER_CUBIC` otherwise. When only `output_size` was passed, the
scale factor was derived as input / output, so every downscale was
treated as an upscale and got bicubic interpolation.

Derive the scale factor as output / input and add a regression test
checking that `output_size` and the equivalent `scale_factor` choose
the same interpolation.
---
 tests/test_utils.py            | 12 ++++++++++++
 tiatoolbox/utils/transforms.py |  2 +-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/tests/test_utils.py b/tests/test_utils.py
index f27115de5..7c8a822dc 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -163,6 +163,18 @@ def test_imresize_no_scale_factor() -> None:
     assert resized_img.shape == (100, 100, 3)
 
 
+def test_imresize_output_size_interpolation() -> None:
+    """Test imresize selects interpolation from output_size like scale_factor."""
+    img = np.zeros((448, 448, 3), dtype=np.uint8)
+    for output_size, scale_factor in [((224, 224), 0.5), ((896, 896), 2.0)]:
+        interpolations = []
+        for kwargs in ({"output_size": output_size}, {"scale_factor": scale_factor}):
+            with patch.object(cv2, "resize", wraps=cv2.resize) as mock_resize:
+                utils.transforms.imresize(img, **kwargs)
+            interpolations.append(mock_resize.call_args.kwargs["interpolation"])
+        assert interpolations[0] == interpolations[1]
+
+
 def test_imresize_no_scale_factor_or_output_size() -> None:
     """Test imresize with no scale_factor or output_size."""
     img = np.zeros((2000, 1000, 3))
diff --git a/tiatoolbox/utils/transforms.py b/tiatoolbox/utils/transforms.py
index f926df451..13ab3ad78 100644
--- a/tiatoolbox/utils/transforms.py
+++ b/tiatoolbox/utils/transforms.py
@@ -140,7 +140,7 @@ def imresize(
     )
 
     if scale_factor_array is None:
-        scale_factor_array = img.shape[:2][::-1] / np.array(output_size_array)
+        scale_factor_array = np.array(output_size_array) / img.shape[:2][::-1]
 
     # Return original if scale factor is 1
     if np.all(scale_factor_array == 1.0):