tf.nn.conv2d vs tf.layers.conv2d

Question

Is there any advantage in using tf.nn.* over tf.layers.*?

Most of the examples in the doc use tf.nn.conv2d, for instance, but it is not clear why they do so.

Answer 1

As GBY mentioned, they use the same implementation.

There is a slight difference in the parameters.

For tf.nn.conv2d:

filter: A Tensor. Must have the same type as input. A 4-D tensor of shape [filter_height, filter_width, in_channels, out_channels]

For tf.layers.conv2d:

filters: Integer, the dimensionality of the output space (i.e. the number of filters in the convolution).

I would use tf.nn.conv2d when loading a pretrained model (example code: https://github.com/ry/tensorflow-vgg16), and tf.layers.conv2d for a model trained from scratch.

Answer 2

For convolution, they are the same. More precisely, tf.layers.conv2d (actually _Conv) uses tf.nn.convolution as the backend. You can follow the calling chain of: tf.layers.conv2d>Conv2D>Conv2D.apply()>_Conv>_Conv.apply()>_Layer.apply()>_Layer.\__call__()>_Conv.call()>nn.convolution()...

Answer 3

As others mentioned the parameters are different especially the "filter(s)". tf.nn.conv2d takes a tensor as a filter, which means you can specify the weight decay (or maybe other properties) like the following in cifar10 code. (Whether you want/need to have weight decay in conv layer is another question.)

kernel = _variable_with_weight_decay('weights',
                                     shape=[5, 5, 3, 64],
                                     stddev=5e-2,
                                     wd=0.0)
conv = tf.nn.conv2d(images, kernel, [1, 1, 1, 1], padding='SAME')

I'm not quite sure how to set weight decay in tf.layers.conv2d since it only take an integer as filters. Maybe using kernel_constraint?

On the other hand, tf.layers.conv2d handles activation and bias automatically while you have to write additional codes for these if you use tf.nn.conv2d.

Answer 4

All of these other replies talk about how the parameters are different, but actually, the main difference of tf.nn and tf.layers conv2d is that for tf.nn, you need to create your own filter tensor and pass it in. This filter needs to have the size of: [kernel_height, kernel_width, in_channels, num_filters]

Essentially, tf.nn is lower level than tf.layers. Unfortunately, this answer is not applicable anymore is tf.layers is obselete

Answer 5

DIFFERENCES IN PARAMETER:

Using tf.layer* in a code:

# Convolution Layer with 32 filters and a kernel size of 5
conv1 = tf.layers.conv2d(x, 32, 5, activation=tf.nn.relu) 
# Max Pooling (down-sampling) with strides of 2 and kernel size of 2
conv1 = tf.layers.max_pooling2d(conv1, 2, 2)

Using tf.nn* in a code: ( Notice we need to pass weights and biases additionally as parameters )

strides = 1
# Weights matrix looks like: [kernel_size(=5), kernel_size(=5), input_channels (=3), filters (= 32)]
# Similarly bias = looks like [filters (=32)]
out = tf.nn.conv2d(input, weights, padding="SAME", strides = [1, strides, strides, 1])
out = tf.nn.bias_add(out, bias)
out = tf.nn.relu(out)

Answer 6

Take a look here:tensorflow > tf.layers.conv2d

and here: tensorflow > conv2d

As you can see the arguments to the layers version are:

tf.layers.conv2d(inputs, filters, kernel_size, strides=(1, 1), padding='valid', data_format='channels_last', dilation_rate=(1, 1), activation=None, use_bias=True, kernel_initializer=None, bias_initializer=tf.zeros_initializer(), kernel_regularizer=None, bias_regularizer=None, activity_regularizer=None, trainable=True, name=None, reuse=None)

and the nn version:

tf.nn.conv2d(input, filter, strides, padding, use_cudnn_on_gpu=None, data_format=None, name=None)

I think you can choose the one with the options you want/need/like!