[Fix] Fix potential interger overflow in imequalize. (#1198)

* Fix potential interger overflow in `imequalize`.

* Modify imequalize unit test image size to generate potential integer
overflow.

mmcv/image/photometric.py

@@ -157,6 +157,8 @@ def _scale_channel(im, c):
             lut = (np.cumsum(histo) + (step // 2)) // step
             # Shift lut, prepending with 0.
             lut = np.concatenate([[0], lut[:-1]], 0)
+            # handle potential integer overflow
+            lut[lut > 255] = 255
         # If step is zero, return the original image.
         # Otherwise, index from lut.
         return np.where(np.equal(step, 0), im, lut[im])

tests/test_image/test_photometric.py

@@ -131,9 +131,8 @@ def _imequalize(img):
 
         # test equalize with randomly sampled image.
         for _ in range(nb_rand_test):
-            img = np.clip(
-                np.random.normal(0, 1, (1000, 1200, 3)) * 260, 0,
-                255).astype(np.uint8)
+            img = np.clip(np.random.normal(0, 1, (256, 256, 3)) * 260, 0,
+                          255).astype(np.uint8)
             equalized_img = mmcv.imequalize(img)
             assert_array_equal(equalized_img, _imequalize(img))