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h_slices = (
slice(0, -self.window_size),
slice(-self.window_size, -self.shift_size),
slice(-self.shift_size, None),
)
w_slices = (
slice(0, -self.window_size),
slice(-self.window_size, -self.shift_size),
slice(-self.shift_size, None),
)
mask_array = np.zeros((1, height, width, 1))
count = 0
for h in h_slices:
for w in w_slices:
mask_array[:, h, w, :] = count
count += 1
mask_array = tf.convert_to_tensor(mask_array)
# mask array to windows
mask_windows = window_partition(mask_array, self.window_size)
mask_windows = tf.reshape(
mask_windows, shape=[-1, self.window_size * self.window_size]
)
attn_mask = tf.expand_dims(mask_windows, axis=1) - tf.expand_dims(
mask_windows, axis=2
)
attn_mask = tf.where(attn_mask != 0, -100.0, attn_mask)
attn_mask = tf.where(attn_mask == 0, 0.0, attn_mask)
self.attn_mask = tf.Variable(initial_value=attn_mask, trainable=False)
def call(self, x):
height, width = self.num_patch
_, num_patches_before, channels = x.shape
x_skip = x
x = self.norm1(x)
x = tf.reshape(x, shape=(-1, height, width, channels))
if self.shift_size > 0:
shifted_x = tf.roll(
x, shift=[-self.shift_size, -self.shift_size], axis=[1, 2]
)
else:
shifted_x = x
x_windows = window_partition(shifted_x, self.window_size)
x_windows = tf.reshape(
x_windows, shape=(-1, self.window_size * self.window_size, channels)
)
attn_windows = self.attn(x_windows, mask=self.attn_mask)
attn_windows = tf.reshape(
attn_windows, shape=(-1, self.window_size, self.window_size, channels)
)
shifted_x = window_reverse(
attn_windows, self.window_size, height, width, channels
)
if self.shift_size > 0:
x = tf.roll(
shifted_x, shift=[self.shift_size, self.shift_size], axis=[1, 2]
)
else:
x = shifted_x
x = tf.reshape(x, shape=(-1, height * width, channels))
x = self.drop_path(x)
x = x_skip + x
x_skip = x
x = self.norm2(x)
x = self.mlp(x)
x = self.drop_path(x)
x = x_skip + x
return x
Model training and evaluation
Extract and embed patches
We first create 3 layers to help us extract, embed and merge patches from the images on top of which we will later use the Swin Transformer class we built.
class PatchExtract(layers.Layer):
def __init__(self, patch_size, **kwargs):
super(PatchExtract, self).__init__(**kwargs)
self.patch_size_x = patch_size[0]
self.patch_size_y = patch_size[0]
def call(self, images):
batch_size = tf.shape(images)[0]
patches = tf.image.extract_patches(
images=images,
sizes=(1, self.patch_size_x, self.patch_size_y, 1),
strides=(1, self.patch_size_x, self.patch_size_y, 1),
rates=(1, 1, 1, 1),
padding=\"VALID\",
)
patch_dim = patches.shape[-1]
patch_num = patches.shape[1]
return tf.reshape(patches, (batch_size, patch_num * patch_num, patch_dim))
class PatchEmbedding(layers.Layer):
def __init__(self, num_patch, embed_dim, **kwargs):
super(PatchEmbedding, self).__init__(**kwargs)
self.num_patch = num_patch
self.proj = layers.Dense(embed_dim)