test_image_transforms.py

# coding=utf-8
# Copyright 2022 HuggingFace Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import unittest

import numpy as np
from parameterized import parameterized

from transformers.testing_utils import require_flax, require_tf, require_torch, require_vision
from transformers.utils.import_utils import is_flax_available, is_tf_available, is_torch_available, is_vision_available


if is_torch_available():
    import torch

if is_tf_available():
    import tensorflow as tf

if is_flax_available():
    import jax

if is_vision_available():
    import PIL.Image

    from transformers.image_transforms import (
        center_crop,
        center_to_corners_format,
        convert_to_rgb,
        corners_to_center_format,
        flip_channel_order,
        get_resize_output_image_size,
        id_to_rgb,
        normalize,
        pad,
        resize,
        rgb_to_id,
        to_channel_dimension_format,
        to_pil_image,
    )


def get_random_image(height, width, num_channels=3, channels_first=True):
    shape = (num_channels, height, width) if channels_first else (height, width, num_channels)
    random_array = np.random.randint(0, 256, shape, dtype=np.uint8)
    return random_array


@require_vision
class ImageTransformsTester(unittest.TestCase):
    @parameterized.expand(
        [
            ("numpy_float_channels_first", (3, 4, 5), np.float32),
            ("numpy_float_channels_last", (4, 5, 3), np.float32),
            ("numpy_float_channels_first", (3, 4, 5), np.float64),
            ("numpy_float_channels_last", (4, 5, 3), np.float64),
            ("numpy_int_channels_first", (3, 4, 5), np.int32),
            ("numpy_uint_channels_first", (3, 4, 5), np.uint8),
        ]
    )
    @require_vision
    def test_to_pil_image(self, name, image_shape, dtype):
        image = np.random.randint(0, 256, image_shape).astype(dtype)
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

        # make sure image is correctly rescaled
        self.assertTrue(np.abs(np.asarray(pil_image)).sum() > 0)

    @parameterized.expand(
        [
            ("numpy_float_channels_first", (3, 4, 5), np.float32),
            ("numpy_float_channels_first", (3, 4, 5), np.float64),
            ("numpy_float_channels_last", (4, 5, 3), np.float32),
            ("numpy_float_channels_last", (4, 5, 3), np.float64),
        ]
    )
    @require_vision
    def test_to_pil_image_from_float(self, name, image_shape, dtype):
        image = np.random.rand(*image_shape).astype(dtype)
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

        # make sure image is correctly rescaled
        self.assertTrue(np.abs(np.asarray(pil_image)).sum() > 0)

        # Make sure that an exception is raised if image is not in [0, 1]
        image = np.random.randn(*image_shape).astype(dtype)
        with self.assertRaises(ValueError):
            to_pil_image(image)

    @require_vision
    def test_to_pil_image_from_mask(self):
        # Make sure binary mask remains a binary mask
        image = np.random.randint(0, 2, (3, 4, 5)).astype(np.uint8)
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

        np_img = np.asarray(pil_image)
        self.assertTrue(np_img.min() == 0)
        self.assertTrue(np_img.max() == 1)

        image = np.random.randint(0, 2, (3, 4, 5)).astype(np.float32)
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

        np_img = np.asarray(pil_image)
        self.assertTrue(np_img.min() == 0)
        self.assertTrue(np_img.max() == 1)

    @require_tf
    def test_to_pil_image_from_tensorflow(self):
        # channels_first
        image = tf.random.uniform((3, 4, 5))
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

        # channels_last
        image = tf.random.uniform((4, 5, 3))
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

    @require_torch
    def test_to_pil_image_from_torch(self):
        # channels first
        image = torch.rand((3, 4, 5))
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

        # channels last
        image = torch.rand((4, 5, 3))
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

    @require_flax
    def test_to_pil_image_from_jax(self):
        key = jax.random.PRNGKey(0)
        # channel first
        image = jax.random.uniform(key, (3, 4, 5))
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

        # channel last
        image = jax.random.uniform(key, (4, 5, 3))
        pil_image = to_pil_image(image)
        self.assertIsInstance(pil_image, PIL.Image.Image)
        self.assertEqual(pil_image.size, (5, 4))

    def test_to_channel_dimension_format(self):
        # Test that function doesn't reorder if channel dim matches the input.
        image = np.random.rand(3, 4, 5)
        image = to_channel_dimension_format(image, "channels_first")
        self.assertEqual(image.shape, (3, 4, 5))

        image = np.random.rand(4, 5, 3)
        image = to_channel_dimension_format(image, "channels_last")
        self.assertEqual(image.shape, (4, 5, 3))

        # Test that function reorders if channel dim doesn't match the input.
        image = np.random.rand(3, 4, 5)
        image = to_channel_dimension_format(image, "channels_last")
        self.assertEqual(image.shape, (4, 5, 3))

        image = np.random.rand(4, 5, 3)
        image = to_channel_dimension_format(image, "channels_first")
        self.assertEqual(image.shape, (3, 4, 5))

        # Can pass in input_data_format and works if data format is ambiguous or unknown.
        image = np.random.rand(4, 5, 6)
        image = to_channel_dimension_format(image, "channels_first", input_channel_dim="channels_last")
        self.assertEqual(image.shape, (6, 4, 5))

    def test_get_resize_output_image_size(self):
        image = np.random.randint(0, 256, (3, 224, 224))

        # Test the output size defaults to (x, x) if an int is given.
        self.assertEqual(get_resize_output_image_size(image, 10), (10, 10))
        self.assertEqual(get_resize_output_image_size(image, [10]), (10, 10))
        self.assertEqual(get_resize_output_image_size(image, (10,)), (10, 10))

        # Test the output size is the same as the input if a two element tuple/list is given.
        self.assertEqual(get_resize_output_image_size(image, (10, 20)), (10, 20))
        self.assertEqual(get_resize_output_image_size(image, [10, 20]), (10, 20))
        self.assertEqual(get_resize_output_image_size(image, (10, 20), default_to_square=True), (10, 20))
        # To match pytorch behaviour, max_size is only relevant if size is an int
        self.assertEqual(get_resize_output_image_size(image, (10, 20), max_size=5), (10, 20))

        # Test output size = (int(size * height / width), size) if size is an int and height > width
        image = np.random.randint(0, 256, (3, 50, 40))
        self.assertEqual(get_resize_output_image_size(image, 20, default_to_square=False), (25, 20))

        # Test output size = (size, int(size * width / height)) if size is an int and width <= height
        image = np.random.randint(0, 256, (3, 40, 50))
        self.assertEqual(get_resize_output_image_size(image, 20, default_to_square=False), (20, 25))

        # Test size is resized if longer size > max_size
        image = np.random.randint(0, 256, (3, 50, 40))
        self.assertEqual(get_resize_output_image_size(image, 20, default_to_square=False, max_size=22), (22, 17))

        # Test output size = (int(size * height / width), size) if size is an int and height > width and
        # input has 4 channels
        image = np.random.randint(0, 256, (4, 50, 40))
        self.assertEqual(
            get_resize_output_image_size(image, 20, default_to_square=False, input_data_format="channels_first"),
            (25, 20),
        )

        # Test correct channel dimension is returned if output size if height == 3
        # Defaults to input format - channels first
        image = np.random.randint(0, 256, (3, 18, 97))
        resized_image = resize(image, (3, 20))
        self.assertEqual(resized_image.shape, (3, 3, 20))

        # Defaults to input format - channels last
        image = np.random.randint(0, 256, (18, 97, 3))
        resized_image = resize(image, (3, 20))
        self.assertEqual(resized_image.shape, (3, 20, 3))

        image = np.random.randint(0, 256, (3, 18, 97))
        resized_image = resize(image, (3, 20), data_format="channels_last")
        self.assertEqual(resized_image.shape, (3, 20, 3))

        image = np.random.randint(0, 256, (18, 97, 3))
        resized_image = resize(image, (3, 20), data_format="channels_first")
        self.assertEqual(resized_image.shape, (3, 3, 20))

    def test_resize(self):
        image = np.random.randint(0, 256, (3, 224, 224))

        # Check the channel order is the same by default
        resized_image = resize(image, (30, 40))
        self.assertIsInstance(resized_image, np.ndarray)
        self.assertEqual(resized_image.shape, (3, 30, 40))

        # Check channel order is changed if specified
        resized_image = resize(image, (30, 40), data_format="channels_last")
        self.assertIsInstance(resized_image, np.ndarray)
        self.assertEqual(resized_image.shape, (30, 40, 3))

        # Check PIL.Image.Image is returned if return_numpy=False
        resized_image = resize(image, (30, 40), return_numpy=False)
        self.assertIsInstance(resized_image, PIL.Image.Image)
        # PIL size is in (width, height) order
        self.assertEqual(resized_image.size, (40, 30))

        # Check an image with float values between 0-1 is returned with values in this range
        image = np.random.rand(3, 224, 224)
        resized_image = resize(image, (30, 40))
        self.assertIsInstance(resized_image, np.ndarray)
        self.assertEqual(resized_image.shape, (3, 30, 40))
        self.assertTrue(np.all(resized_image >= 0))
        self.assertTrue(np.all(resized_image <= 1))

        # Check that an image with 4 channels is resized correctly
        image = np.random.randint(0, 256, (4, 224, 224))
        resized_image = resize(image, (30, 40), input_data_format="channels_first")
        self.assertIsInstance(resized_image, np.ndarray)
        self.assertEqual(resized_image.shape, (4, 30, 40))

    def test_normalize(self):
        image = np.random.randint(0, 256, (224, 224, 3)) / 255

        # Test that exception is raised if inputs are incorrect
        # Not a numpy array image
        with self.assertRaises(ValueError):
            normalize(5, 5, 5)

        # Number of mean values != number of channels
        with self.assertRaises(ValueError):
            normalize(image, mean=(0.5, 0.6), std=1)

        # Number of std values != number of channels
        with self.assertRaises(ValueError):
            normalize(image, mean=1, std=(0.5, 0.6))

        # Test result is correct - output data format is channels_first and normalization
        # correctly computed
        mean = (0.5, 0.6, 0.7)
        std = (0.1, 0.2, 0.3)
        expected_image = ((image - mean) / std).transpose((2, 0, 1))

        normalized_image = normalize(image, mean=mean, std=std, data_format="channels_first")
        self.assertIsInstance(normalized_image, np.ndarray)
        self.assertEqual(normalized_image.shape, (3, 224, 224))
        self.assertTrue(np.allclose(normalized_image, expected_image, atol=1e-6))

        # Test image with 4 channels is normalized correctly
        image = np.random.randint(0, 256, (224, 224, 4)) / 255
        mean = (0.5, 0.6, 0.7, 0.8)
        std = (0.1, 0.2, 0.3, 0.4)
        expected_image = (image - mean) / std
        self.assertTrue(
            np.allclose(
                normalize(image, mean=mean, std=std, input_data_format="channels_last"), expected_image, atol=1e-6
            )
        )

        # Test float32 image input keeps float32 dtype
        image = np.random.randint(0, 256, (224, 224, 3)).astype(np.float32) / 255
        mean = (0.5, 0.6, 0.7)
        std = (0.1, 0.2, 0.3)
        expected_image = ((image - mean) / std).astype(np.float32)
        normalized_image = normalize(image, mean=mean, std=std)
        self.assertEqual(normalized_image.dtype, np.float32)
        self.assertTrue(np.allclose(normalized_image, expected_image, atol=1e-6))

        # Test float16 image input keeps float16 dtype
        image = np.random.randint(0, 256, (224, 224, 3)).astype(np.float16) / 255
        mean = (0.5, 0.6, 0.7)
        std = (0.1, 0.2, 0.3)

        # The mean and std are cast to match the dtype of the input image
        cast_mean = np.array(mean, dtype=np.float16)
        cast_std = np.array(std, dtype=np.float16)
        expected_image = (image - cast_mean) / cast_std
        normalized_image = normalize(image, mean=mean, std=std)
        self.assertEqual(normalized_image.dtype, np.float16)
        self.assertTrue(np.allclose(normalized_image, expected_image, atol=1e-6))

        # Test int image input is converted to float32
        image = np.random.randint(0, 2, (224, 224, 3), dtype=np.uint8)
        mean = (0.5, 0.6, 0.7)
        std = (0.1, 0.2, 0.3)
        expected_image = (image.astype(np.float32) - mean) / std
        normalized_image = normalize(image, mean=mean, std=std)
        self.assertEqual(normalized_image.dtype, np.float32)
        self.assertTrue(np.allclose(normalized_image, expected_image, atol=1e-6))

    def test_center_crop(self):
        image = np.random.randint(0, 256, (3, 224, 224))

        # Test that exception is raised if inputs are incorrect
        with self.assertRaises(ValueError):
            center_crop(image, 10)

        # Test result is correct - output data format is channels_first and center crop
        # correctly computed
        expected_image = image[:, 52:172, 82:142].transpose(1, 2, 0)
        cropped_image = center_crop(image, (120, 60), data_format="channels_last")
        self.assertIsInstance(cropped_image, np.ndarray)
        self.assertEqual(cropped_image.shape, (120, 60, 3))
        self.assertTrue(np.allclose(cropped_image, expected_image))

        # Test that image is padded with zeros if crop size is larger than image size
        expected_image = np.zeros((300, 260, 3))
        expected_image[38:262, 18:242, :] = image.transpose((1, 2, 0))
        cropped_image = center_crop(image, (300, 260), data_format="channels_last")
        self.assertIsInstance(cropped_image, np.ndarray)
        self.assertEqual(cropped_image.shape, (300, 260, 3))
        self.assertTrue(np.allclose(cropped_image, expected_image))

        # Test that odd numbered padding requirement still leads to correct output dimensions
        cropped_image = center_crop(image, (300, 259), data_format="channels_last")
        self.assertEqual(cropped_image.shape, (300, 259, 3))

        # Test image with 4 channels is cropped correctly
        image = np.random.randint(0, 256, (224, 224, 4))
        expected_image = image[52:172, 82:142, :]
        self.assertTrue(np.allclose(center_crop(image, (120, 60), input_data_format="channels_last"), expected_image))

    def test_center_to_corners_format(self):
        bbox_center = np.array([[10, 20, 4, 8], [15, 16, 3, 4]])
        expected = np.array([[8, 16, 12, 24], [13.5, 14, 16.5, 18]])
        self.assertTrue(np.allclose(center_to_corners_format(bbox_center), expected))

        # Check that the function and inverse function are inverse of each other
        self.assertTrue(np.allclose(corners_to_center_format(center_to_corners_format(bbox_center)), bbox_center))

    def test_corners_to_center_format(self):
        bbox_corners = np.array([[8, 16, 12, 24], [13.5, 14, 16.5, 18]])
        expected = np.array([[10, 20, 4, 8], [15, 16, 3, 4]])
        self.assertTrue(np.allclose(corners_to_center_format(bbox_corners), expected))

        # Check that the function and inverse function are inverse of each other
        self.assertTrue(np.allclose(center_to_corners_format(corners_to_center_format(bbox_corners)), bbox_corners))

    def test_rgb_to_id(self):
        # test list input
        rgb = [125, 4, 255]
        self.assertEqual(rgb_to_id(rgb), 16712829)

        # test numpy array input
        color = np.array(
            [
                [
                    [213, 54, 165],
                    [88, 207, 39],
                    [156, 108, 128],
                ],
                [
                    [183, 194, 46],
                    [137, 58, 88],
                    [114, 131, 233],
                ],
            ]
        )
        expected = np.array([[10827477, 2608984, 8416412], [3064503, 5782153, 15303538]])
        self.assertTrue(np.allclose(rgb_to_id(color), expected))

    def test_id_to_rgb(self):
        # test int input
        self.assertEqual(id_to_rgb(16712829), [125, 4, 255])

        # test array input
        id_array = np.array([[10827477, 2608984, 8416412], [3064503, 5782153, 15303538]])
        color = np.array(
            [
                [
                    [213, 54, 165],
                    [88, 207, 39],
                    [156, 108, 128],
                ],
                [
                    [183, 194, 46],
                    [137, 58, 88],
                    [114, 131, 233],
                ],
            ]
        )
        self.assertTrue(np.allclose(id_to_rgb(id_array), color))

    def test_pad(self):
        # fmt: off
        image = np.array([[
            [0, 1],
            [2, 3],
        ]])
        # fmt: on

        # Test that exception is raised if unknown padding mode is specified
        with self.assertRaises(ValueError):
            pad(image, 10, mode="unknown")

        # Test that exception is raised if invalid padding is specified
        with self.assertRaises(ValueError):
            # Cannot pad on channel dimension
            pad(image, (5, 10, 10))

        # Test image is padded equally on all sides is padding is an int
        # fmt: off
        expected_image = np.array([
            [[0, 0, 0, 0],
             [0, 0, 1, 0],
             [0, 2, 3, 0],
             [0, 0, 0, 0]],
        ])
        # fmt: on
        self.assertTrue(np.allclose(expected_image, pad(image, 1)))

        # Test the left and right of each axis is padded (pad_left, pad_right)
        # fmt: off
        expected_image = np.array(
            [[0, 0, 0, 0, 0],
             [0, 0, 0, 0, 0],
             [0, 0, 0, 1, 0],
             [0, 0, 2, 3, 0],
             [0, 0, 0, 0, 0]])
        # fmt: on
        self.assertTrue(np.allclose(expected_image, pad(image, (2, 1))))

        # Test only one axis is padded (pad_left, pad_right)
        # fmt: off
        expected_image = np.array([[
            [9, 9],
            [9, 9],
            [0, 1],
            [2, 3],
            [9, 9]
        ]])
        # fmt: on
        self.assertTrue(np.allclose(expected_image, pad(image, ((2, 1), (0, 0)), constant_values=9)))

        # Test padding with a constant value
        # fmt: off
        expected_image = np.array([[
            [8, 8, 0, 1, 9],
            [8, 8, 2, 3, 9],
            [8, 8, 7, 7, 9],
            [8, 8, 7, 7, 9]
        ]])
        # fmt: on
        self.assertTrue(np.allclose(expected_image, pad(image, ((0, 2), (2, 1)), constant_values=((6, 7), (8, 9)))))

        # fmt: off
        image = np.array([[
            [0, 1, 2],
            [3, 4, 5],
            [6, 7, 8],
        ]])
        # fmt: on

        # Test padding with PaddingMode.REFLECT
        # fmt: off
        expected_image = np.array([[
            [2, 1, 0, 1, 2, 1],
            [5, 4, 3, 4, 5, 4],
            [8, 7, 6, 7, 8, 7],
            [5, 4, 3, 4, 5, 4],
            [2, 1, 0, 1, 2, 1],
        ]])
        # fmt: on
        self.assertTrue(np.allclose(expected_image, pad(image, ((0, 2), (2, 1)), mode="reflect")))

        # Test padding with PaddingMode.REPLICATE
        # fmt: off
        expected_image = np.array([[
            [0, 0, 0, 1, 2, 2],
            [3, 3, 3, 4, 5, 5],
            [6, 6, 6, 7, 8, 8],
            [6, 6, 6, 7, 8, 8],
            [6, 6, 6, 7, 8, 8],
        ]])
        # fmt: on
        self.assertTrue(np.allclose(expected_image, pad(image, ((0, 2), (2, 1)), mode="replicate")))

        # Test padding with PaddingMode.SYMMETRIC
        # fmt: off
        expected_image = np.array([[
            [1, 0, 0, 1, 2, 2],
            [4, 3, 3, 4, 5, 5],
            [7, 6, 6, 7, 8, 8],
            [7, 6, 6, 7, 8, 8],
            [4, 3, 3, 4, 5, 5],
        ]])
        # fmt: on
        self.assertTrue(np.allclose(expected_image, pad(image, ((0, 2), (2, 1)), mode="symmetric")))

        # Test we can specify the output data format
        # Test padding with PaddingMode.REFLECT
        # fmt: off
        image = np.array([[
            [0, 1],
            [2, 3],
        ]])
        expected_image = np.array([
            [[0], [1], [0], [1], [0]],
            [[2], [3], [2], [3], [2]],
            [[0], [1], [0], [1], [0]],
            [[2], [3], [2], [3], [2]]
        ])
        # fmt: on
        self.assertTrue(
            np.allclose(expected_image, pad(image, ((0, 2), (2, 1)), mode="reflect", data_format="channels_last"))
        )

        # Test we can pad on an image with 2 channels
        # fmt: off
        image = np.array([
            [[0, 1], [2, 3]],
        ])
        expected_image = np.array([
            [[0, 0], [0, 1], [2, 3]],
            [[0, 0], [0, 0], [0, 0]],
        ])
        # fmt: on
        self.assertTrue(
            np.allclose(
                expected_image, pad(image, ((0, 1), (1, 0)), mode="constant", input_data_format="channels_last")
            )
        )

    @require_vision
    def test_convert_to_rgb(self):
        # Test that an RGBA image is converted to RGB
        image = np.array([[[1, 2, 3, 4], [5, 6, 7, 8]]], dtype=np.uint8)
        pil_image = PIL.Image.fromarray(image)
        self.assertEqual(pil_image.mode, "RGBA")
        self.assertEqual(pil_image.size, (2, 1))

        # For the moment, numpy images are returned as is
        rgb_image = convert_to_rgb(image)
        self.assertEqual(rgb_image.shape, (1, 2, 4))
        self.assertTrue(np.allclose(rgb_image, image))

        # And PIL images are converted
        rgb_image = convert_to_rgb(pil_image)
        self.assertEqual(rgb_image.mode, "RGB")
        self.assertEqual(rgb_image.size, (2, 1))
        self.assertTrue(np.allclose(np.array(rgb_image), np.array([[[1, 2, 3], [5, 6, 7]]], dtype=np.uint8)))

        # Test that a grayscale image is converted to RGB
        image = np.array([[0, 255]], dtype=np.uint8)
        pil_image = PIL.Image.fromarray(image)
        self.assertEqual(pil_image.mode, "L")
        self.assertEqual(pil_image.size, (2, 1))
        rgb_image = convert_to_rgb(pil_image)
        self.assertEqual(rgb_image.mode, "RGB")
        self.assertEqual(rgb_image.size, (2, 1))
        self.assertTrue(np.allclose(np.array(rgb_image), np.array([[[0, 0, 0], [255, 255, 255]]], dtype=np.uint8)))

    def test_flip_channel_order(self):
        # fmt: off
        img_channels_first = np.array([
            [[ 0,  1,  2,  3],
             [ 4,  5,  6,  7]],

            [[ 8,  9, 10, 11],
             [12, 13, 14, 15]],

            [[16, 17, 18, 19],
             [20, 21, 22, 23]],
        ])
        # fmt: on
        img_channels_last = np.moveaxis(img_channels_first, 0, -1)
        # fmt: off
        flipped_img_channels_first = np.array([
            [[16, 17, 18, 19],
             [20, 21, 22, 23]],

            [[ 8,  9, 10, 11],
             [12, 13, 14, 15]],

            [[ 0,  1,  2,  3],
             [ 4,  5,  6,  7]],
        ])
        # fmt: on
        flipped_img_channels_last = np.moveaxis(flipped_img_channels_first, 0, -1)

        self.assertTrue(np.allclose(flip_channel_order(img_channels_first), flipped_img_channels_first))
        self.assertTrue(
            np.allclose(flip_channel_order(img_channels_first, "channels_last"), flipped_img_channels_last)
        )

        self.assertTrue(np.allclose(flip_channel_order(img_channels_last), flipped_img_channels_last))
        self.assertTrue(
            np.allclose(flip_channel_order(img_channels_last, "channels_first"), flipped_img_channels_first)
        )

        # Can flip when the image has 2 channels
        # fmt: off
        img_channels_first = np.array([
            [[ 0,  1,  2,  3],
             [ 4,  5,  6,  7]],

            [[ 8,  9, 10, 11],
             [12, 13, 14, 15]],
        ])
        # fmt: on
        flipped_img_channels_first = img_channels_first[::-1, :, :]
        self.assertTrue(
            np.allclose(
                flip_channel_order(img_channels_first, input_data_format="channels_first"), flipped_img_channels_first
            )
        )