add note in bn and stn

tensorpack/models/batch_norm.py

@@ -15,31 +15,37 @@ __all__ = ['BatchNorm']
 @layer_register()
 def BatchNorm(x, is_training):
     """
+    Batch normalization layer as described in:
+    Batch Normalization: Accelerating Deep Network Training by Reducing Internal Covariate Shift
+    http://arxiv.org/abs/1502.03167
+    Notes:
+    Whole-population mean/variance is calculated by a running-average mean/variance, with decay rate 0.999
+    Epsilon for variance is set to 1e-5, as is torch/nn: https://github.com/torch/nn/blob/master/BatchNormalization.lua
+
     x: BHWC tensor
     is_training: bool
     """
+    EPS = 1e-5
     is_training = bool(is_training)
     shape = x.get_shape().as_list()
     assert len(shape) == 4
 
     n_out = shape[-1]  # channel
     beta = tf.get_variable('beta', [n_out])
-    gamma = tf.get_variable('gamma', [n_out],
-                        initializer=tf.constant_initializer(1.0))
+    gamma = tf.get_variable('gamma', [n_out], initializer=tf.constant_initializer(1.0))
     batch_mean, batch_var = tf.nn.moments(x, [0, 1, 2], name='moments')
 
-    ema = tf.train.ExponentialMovingAverage(decay=0.9)
+    ema = tf.train.ExponentialMovingAverage(decay=0.999)
     ema_apply_op = ema.apply([batch_mean, batch_var])
     ema_mean, ema_var = ema.average(batch_mean), ema.average(batch_var)
 
     if is_training:
-        def mean_var_with_update():
-            with tf.control_dependencies([ema_apply_op]):
-                return tf.identity(batch_mean), tf.identity(batch_var)
-        mean, var = mean_var_with_update()
+        with tf.control_dependencies([ema_apply_op]):
+            mean, var = tf.identity(batch_mean), tf.identity(batch_var)
     else:
-        mean, var = ema_mean, ema_var
+        batch = tf.cast(tf.shape(x)[0], tf.float32)
+        mean, var = ema_mean, ema_var * batch / (batch - 1) # unbiased variance estimator
 
-    normed = tf.nn.batch_norm_with_global_normalization(x, mean, var, beta, gamma, 1e-4, True)
+    normed = tf.nn.batch_norm_with_global_normalization(x, mean, var, beta, gamma, EPS, True)
     return normed
 

tensorpack/models/image_sample.py

@@ -42,6 +42,9 @@ def sample(img, coords):
 def ImageSample(inputs):
     """
     Sample the template image, using the given coordinate, by bilinear interpolation.
+    It mimics the same behavior described in:
+    Spatial Transformer Networks, http://arxiv.org/abs/1506.02025
+
     inputs: list of [template, mapping]
     template: bxhxwxc
     mapping: bxh2xw2x2  (y, x) real-value coordinates