Fix CDNA transformation bug and speed up its implementation.

- Fix CDNA transformation bug where transformed channels of color and masks were combined incorrectly.
- Remove for loop over batch size in implementation of CDNA transformation. This speeds up the building of the graph.

video_prediction/prediction_model.py

@@ -261,6 +261,8 @@ def cdna_transformation(prev_image, cdna_input, num_masks, color_channels):
     List of images transformed by the predicted CDNA kernels.
   """
   batch_size = int(cdna_input.get_shape()[0])
+  height = int(prev_image.get_shape()[1])
+  width = int(prev_image.get_shape()[2])
 
   # Predict kernels using linear function of last hidden layer.
   cdna_kerns = slim.layers.fully_connected(
@@ -276,20 +278,22 @@ def cdna_transformation(prev_image, cdna_input, num_masks, color_channels):
   norm_factor = tf.reduce_sum(cdna_kerns, [1, 2, 3], keep_dims=True)
   cdna_kerns /= norm_factor
 
-  cdna_kerns = tf.tile(cdna_kerns, [1, 1, 1, color_channels, 1])
-  cdna_kerns = tf.split(axis=0, num_or_size_splits=batch_size, value=cdna_kerns)
-  prev_images = tf.split(axis=0, num_or_size_splits=batch_size, value=prev_image)
+  # Treat the color channel dimension as the batch dimension since the same
+  # transformation is applied to each color channel.
+  # Treat the batch dimension as the channel dimension so that
+  # depthwise_conv2d can apply a different transformation to each sample.
+  cdna_kerns = tf.transpose(cdna_kerns, [1, 2, 0, 4, 3])
+  cdna_kerns = tf.reshape(cdna_kerns, [DNA_KERN_SIZE, DNA_KERN_SIZE, batch_size, num_masks])
+  # Swap the batch and channel dimensions.
+  prev_image = tf.transpose(prev_image, [3, 1, 2, 0])
 
   # Transform image.
-  transformed = []
-  for kernel, preimg in zip(cdna_kerns, prev_images):
-    kernel = tf.squeeze(kernel)
-    if len(kernel.get_shape()) == 3:
-      kernel = tf.expand_dims(kernel, -1)
-    transformed.append(
-        tf.nn.depthwise_conv2d(preimg, kernel, [1, 1, 1, 1], 'SAME'))
-  transformed = tf.concat(axis=0, values=transformed)
-  transformed = tf.split(axis=3, num_or_size_splits=num_masks, value=transformed)
+  transformed = tf.nn.depthwise_conv2d(prev_image, cdna_kerns, [1, 1, 1, 1], 'SAME')
+
+  # Transpose the dimensions to where they belong.
+  transformed = tf.reshape(transformed, [color_channels, height, width, batch_size, num_masks])
+  transformed = tf.transpose(transformed, [3, 1, 2, 0, 4])
+  transformed = tf.unstack(transformed, axis=-1)
   return transformed