Add random_erasing

chainercv/transforms/__init__.py

@@ -7,6 +7,7 @@
 from chainercv.transforms.image.flip import flip  # NOQA
 from chainercv.transforms.image.pca_lighting import pca_lighting  # NOQA
 from chainercv.transforms.image.random_crop import random_crop  # NOQA
+from chainercv.transforms.image.random_erasing import random_erasing  # NOQA
 from chainercv.transforms.image.random_expand import random_expand  # NOQA
 from chainercv.transforms.image.random_flip import random_flip  # NOQA
 from chainercv.transforms.image.random_rotate import random_rotate  # NOQA

chainercv/transforms/image/random_erasing.py

@@ -0,0 +1,105 @@
+from __future__ import division
+
+import numpy as np
+import random
+
+from chainercv.transforms.image.random_sized_crop import random_sized_crop
+
+
+def random_erasing(img, prob=0.5,
+                   scale_ratio_range=(0.02, 0.4),
+                   aspect_ratio_range=(0.3, 1 / 0.3),
+                   random_value=True,
+                   fill=np.array((0.4914, 0.4822, 0.4465)),
+                   return_param=False, copy=False):
+    """Erase a rectangle region in an image with random or fixed values.
+
+    The size :math:`(H_{erase}, W_{erase})` and the left top coordinate
+    :math:`(y_{start}, x_{start})` of the region are calculated as follows:
+
+    + :math:`H_{erase} = \\lfloor{\\sqrt{s \\times H \\times W \
+        \\times a}}\\rfloor`
+    + :math:`W_{erase} = \\lfloor{\\sqrt{s \\times H \\times W \
+        \\div a}}\\rfloor`
+    + :math:`y_{start} \\sim Uniform\\{0, H - H_{erase}\\}`
+    + :math:`x_{start} \\sim Uniform\\{0, W - W_{erase}\\}`
+    + :math:`s \\sim Uniform(s_1, s_2)`
+    + :math:`b \\sim Uniform(a_1, a_2)` and \
+        :math:`a = b` or :math:`a = \\frac{1}{b}` in 50/50 probability.
+
+    Here, :math:`s_1, s_2` are the two floats in
+    :obj:`scale_ratio_range` and :math:`a_1, a_2` are the two floats
+    in :obj:`aspect_ratio_range`.
+    Also, :math:`H` and :math:`W` are the height and the width of the image.
+    Note that :math:`s \\approx \\frac{H_{erase} \\times
+    W_{erase}}{H \\times W}` and
+    :math:`a \\approx \\frac{H_{erase}}{W_{erase}}`.
+    The approximations come from flooring floats to integers.
+
+    .. note::
+
+        When it fails to sample a valid scale and aspect ratio for a hundred
+        times, it picks values in a non-uniform way.
+        If this happens, the selected scale ratio can be smaller
+        than :obj:`scale_ratio_range[0]`.
+
+    Args:
+        img (~numpy.ndarray): An image array. This is in CHW format.
+        prob (float): Erasing probability.
+        scale_ratio_range (tuple of two floats): Determines
+            the distribution from which a scale ratio is sampled.
+        aspect_ratio_range (tuple of two floats): Determines
+            the distribution from which an aspect ratio is sampled.
+        random_value (bool): Fill the rectangle region with random values.
+        fill (~numpy.ndarray): Determines pixel values
+            to fill the rectangle region.
+            The default value is the ImageNet mean value.
+        return_param (bool): Returns parameters if :obj:`True`.
+
+    Returns:
+        ~numpy.ndarray or (~numpy.ndarray, dict):
+
+        If :obj:`return_param = False`,
+        returns only the cropped image.
+
+        If :obj:`return_param = True`,
+        returns a tuple of erased image and :obj:`param`.
+        :obj:`param` is a dictionary of intermediate parameters whose
+        contents are listed below with key, value-type and the description
+        of the value.
+
+        * **y_slice** (*slice*): A slice used to erase a region in the input
+        image. The relation below holds together with :obj:`x_slice`.
+        * **x_slice** (*slice*): Similar to :obj:`y_slice`.
+        * **scale_ratio** (float): :math:`s` in the description (see above).
+        * **aspect_ratio** (float): :math:`a` in the description.
+
+    """
+
+    if random.uniform(0.0, 1.0) < prob:
+        img = img.copy()
+        crop, params = random_sized_crop(img, scale_ratio_range,
+                                         aspect_ratio_range, return_param=True)
+        if random_value:
+            crop[:] = np.random.random(crop.shape) * 255
+        else:
+            crop[:] = fill[:, None, None]
+        y_slice = params['y_slice']
+        x_slice = params['x_slice']
+        scale_ratio = params['scale_ratio']
+        aspect_ratio = params['aspect_ratio']
+
+    else:
+        y_slice = None
+        x_slice = None
+        scale_ratio = None
+        aspect_ratio = None
+
+    if copy:
+        img = img.copy()
+    if return_param:
+        params = {'y_slice': y_slice, 'x_slice': x_slice,
+                  'scale_ratio': scale_ratio, 'aspect_ratio': aspect_ratio}
+        return img, params
+    else:
+        return img

docs/source/reference/transforms.rst

@@ -23,6 +23,10 @@ random_crop
 ~~~~~~~~~~~
 .. autofunction:: random_crop
 
+random_erasing
+~~~~~~~~~~~~~
+.. autofunction:: random_erasing
+
 random_expand
 ~~~~~~~~~~~~~
 .. autofunction:: random_expand

tests/transforms_tests/image_tests/test_random_erasing.py

@@ -0,0 +1,57 @@
+from __future__ import division
+
+import unittest
+
+import numpy as np
+
+from chainer import testing
+from chainercv.transforms import random_erasing
+
+
+@testing.parameterize(*testing.product_dict(
+    [
+        {'H': 256, 'W': 256},
+        {'H': 129, 'W': 352},
+        {'H': 352, 'W': 129},
+        {'H': 35, 'W': 500},
+    ],
+    [
+        {'prob': 0.0},
+        {'prob': 0.5},
+        {'prob': 1.0},
+    ],
+    [
+        {'random_value': True},
+        {'random_value': False},
+    ]
+))
+class TestRandomErasing(unittest.TestCase):
+
+    def test_random_sized_crop(self):
+        img = np.random.uniform(size=(3, self.H, self.W))
+        prob = self.prob
+        scale_ratio_range = (0.02, 0.4)
+        aspect_ratio_range = (0.3, 1 / 0.3)
+        fill = np.array((0.4914, 0.4822, 0.4465))
+        out, params = random_erasing(img, prob, scale_ratio_range,
+                                     aspect_ratio_range, self.random_value,
+                                     fill,
+                                     return_param=True)
+
+        scale_ratio = params['scale_ratio']
+        aspect_ratio = params['aspect_ratio']
+
+        if scale_ratio is not None:
+            self.assertTrue(
+                (aspect_ratio_range[0] <= aspect_ratio) and
+                (aspect_ratio <= aspect_ratio_range[1]))
+            self.assertTrue(
+                scale_ratio <= scale_ratio_range[1])
+            scale_ratio_max = min((scale_ratio_range[1],
+                                   self.H / (self.W * aspect_ratio),
+                                   (aspect_ratio * self.W) / self.H))
+            self.assertTrue(
+                min((scale_ratio_max, scale_ratio_range[0])) <= scale_ratio)
+
+
+testing.run_module(__name__, __file__)