## Listening to the neural network gradient norms during training

There are, however, other senses that we can use to monitor the training of neural networks, such assound. Sound is one of the perspectives that is currently very poorly explored in the training of neural networks. Human hearing can be very good a distinguishing very small perturbations in characteristics such as rhythm and pitch, even when these perturbations are very short in time or subtle.

You’ll need to installPyAudioandPyTorch以运行该代码（在the end of this post）。

### Training sound with SGD using LR 0.01

This segment represents a training session with gradients from 4 layers during the first 200 steps of the first epoch and using a batch size of 10. The higher the pitch, the higher the norm for a layer, there is a short silence to indicate different batches. Note the gradient increasing during time.

### 使用LR 0.1 SGD培训声

Same as above, but with higher learning rate.

### 使用LR 1.0 SGD培训声and BS 256

Same setting but with a high learning rate of 1.0 and a batch size of 256. Note how the gradients explode and then there are NaNs causing the final sound.

### Training sound with Adam using LR 0.01

This is using Adam in the same setting as the SGD.

## Source code

For those who are interested, here is the entire source code I used to make the sound clips:

进口pyaudio进口numpy np波不进口rt torch import torch.nn as nn import torch.nn.functional as F import torch.optim as optim from torchvision import datasets, transforms class Net(nn.Module): def __init__(self): super(Net, self).__init__() self.conv1 = nn.Conv2d(1, 20, 5, 1) self.conv2 = nn.Conv2d(20, 50, 5, 1) self.fc1 = nn.Linear(4*4*50, 500) self.fc2 = nn.Linear(500, 10) self.ordered_layers = [self.conv1, self.conv2, self.fc1, self.fc2] def forward(self, x): x = F.relu(self.conv1(x)) x = F.max_pool2d(x, 2, 2) x = F.relu(self.conv2(x)) x = F.max_pool2d(x, 2, 2) x = x.view(-1, 4*4*50) x = F.relu(self.fc1(x)) x = self.fc2(x) return F.log_softmax(x, dim=1) def open_stream(fs): p = pyaudio.PyAudio() stream = p.open(format=pyaudio.paFloat32, channels=1, rate=fs, output=True) return p, stream def generate_tone(fs, freq, duration): npsin = np.sin(2 * np.pi * np.arange(fs*duration) * freq / fs) samples = npsin.astype(np.float32) return 0.1 * samples def train(model, device, train_loader, optimizer, epoch): model.train() fs = 44100 duration = 0.01 f = 200.0 p, stream = open_stream(fs) frames = [] for batch_idx, (data, target) in enumerate(train_loader): data, target = data.to(device), target.to(device) optimizer.zero_grad() output = model(data) loss = F.nll_loss(output, target) loss.backward() norms = [] for layer in model.ordered_layers: norm_grad = layer.weight.grad.norm() norms.append(norm_grad) tone = f + ((norm_grad.numpy()) * 100.0) tone = tone.astype(np.float32) samples = generate_tone(fs, tone, duration) frames.append(samples) silence = np.zeros(samples.shape[0] * 2, dtype=np.float32) frames.append(silence) optimizer.step() # Just 200 steps per epoach if batch_idx == 200: break wf = wave.open("sgd_lr_1_0_bs256.wav", 'wb') wf.setnchannels(1) wf.setsampwidth(p.get_sample_size(pyaudio.paFloat32)) wf.setframerate(fs) wf.writeframes(b''.join(frames)) wf.close() stream.stop_stream() stream.close() p.terminate() def run_main(): device = torch.device("cpu") train_loader = torch.utils.data.DataLoader( datasets.MNIST('../data', train=True, download=True, transform=transforms.Compose([ transforms.ToTensor(), transforms.Normalize((0.1307,), (0.3081,)) ])), batch_size=256, shuffle=True) model = Net().to(device) optimizer = optim.SGD(model.parameters(), lr=0.01, momentum=0.5) for epoch in range(1, 2): train(model, device, train_loader, optimizer, epoch) if __name__ == "__main__": run_main()

## The effective receptive field on CNNs

Given the interesting recent article on “The Emergence of a Fovea while Learning to Attend”,我决定做一个评论的paper written by Luo, Wenjie et al. called “Understanding the Effective Receptive Field in Deep Convolutional Neural Networks”在那里，他们推出了‘的想法有效的感受野’（ERF），并与凹视觉上卷积神经网络自然会产生令人惊讶的关系。

The receptive field in Convolutional Neural Networks (CNN) is the region of the input space that affects a particular unit of the network. Note that this input region can be not only the input of the network but also output from other units in the network, therefore this receptive field can be calculated relative to the input that we consider and also relative the unit that we are taking into consideration as the “receiver” of this input region. Usually, when the receptive field term is mentioned, it is taking into consideration the final output unit of the network (i.e. a single unit on a binary classification task) in relation to the network input (i.e. input image of the network).

In the figure below, we can see in left the input pixels, after that we have a feature map calculated from the input pixels using a 3×3 convolution filter and then finally the output after another 3×3 filtering. The numbers inside the pixels on the left image represent how many times this pixel was part of a convolution step (each sliding step of the filter). As we can see, some pixels like the central ones will have their information propagated through many different paths in the network, while the pixels on the borders are propagated along a single path.

By looking at the image above, it isn’t that surprising that the effective receptive field impact on the final output computation will look more like aGaussian distributioninstead of a uniform distribution. What is actually more even interesting is that this receptive field isdynamicand changes during the training. The impact of this on the backpropagation is that the central pixels will have a larger gradient magnitude when compared to the border pixels.

In the article written by Luo, Wenjie et al., they devised a way to quantify the effect on each input pixel of the network by calculating the quantity$\压裂{\部分Y} {\局部X_ {I，J}}$that represents how much each pixel$x_{i, j}$contributes to the output$y$.

In thepaper, they did experimentations to visualize the effective receptive field using multiple different architectures, activations, etc. I replicate here the ones that I found most interesting:

As we can see from theFigure 1of thepaper, where they compare the effect of the number of layers, initialization schemes, and different activations, the results are amazing. We can clearly see the Gaussian and also the sparsity added by the ReLU activations.

There are also some comparisons onFigure 3的纸，其中CIFAR-10和CamVid数据集被用于训练网络。

As we can see, the size of the effective receptive field is very dynamic and it is increased by a large margin after the training, which implies, as stated by authors of the paper, that better initialization schemes can be employed to increase the receptive field in the beginning of the training. They actually developed a different initialization scheme and were able to get 30% training speed-up, however, these results weren’t consistent.

PS: Just for the sake of curiosity, some birds that do complex aerial movements such as the hummingbird, have2个foveas而不是单一的一个，这意味着它们不仅对中部地区也对双方的尖锐眼光准确。

I hope you enjoyed the post !

– Christian S. Perone

Cite this article as: Christian S. Perone, "The effective receptive field on CNNs," inTerra Incognita, 12/11/2017,//www.cpetem.com/2017/11/the-effective-receptive-field-on-cnns/.

## Convolutional Neural Networks – Architectural Zoo

Presentation about an “Achitectural Zoo” of different applications and architectures of CNNs. Presented at Machine Learning Meetup in Porto Alegre yesterday.

## Convolutional hypercolumns in Python

If you are following some Machine Learning news, you certainly saw the work done by Ryan Dahl on自动彩色化(Hacker News comments,Reddit comments）。This amazing work uses pixelhypercolumninformation extracted from the VGG-16 network in order to colorize images.Samim还使用的网络来处理黑白视频帧和下面产生的惊人的视频：

Colorizing Black&White Movies with Neural Networks (video by Samim, network by Ryan)

But how does this hypercolumns works ? How to extract them to use on such variety of pixel classification problems ? The main idea of this post is to use the VGG-16 pre-trained network together with Keras and Scikit-Learn in order to extract the pixel hypercolumns and take a superficial look at the information present on it. I’m writing this because I haven’t found anything in Python to do that and this may be really useful for others working on pixel classification, segmentation, etc.

## 超柱状体

Many algorithms using features from CNNs (Convolutional Neural Networks) usually use the last FC (fully-connected) layer features in order to extract information about certain input. However, the information in the last FC layer may be too coarse spatially to allow precise localization (due to sequences of maxpooling, etc.), on the other side, the first layers may be spatially precise but will lack semantic information. To get the best of both worlds, the authors of thehypercolumn paperdefine the hypercolumn of a pixel as the vector of activations of all CNN units “above” that pixel.

The first step on the extraction of the hypercolumns is to feed the image into the CNN (Convolutional Neural Network) and extract the feature map activations for each location of the image. The tricky part is when the feature maps are smaller than the input image, for instance after a pooling operation, the authors of the paper then do a bilinear upsampling of the feature map in order to keep the feature maps on the same size of the input. There are also the issue with the FC (fully-connected) layers, because you can’t isolate units semantically tied only to one pixel of the image, so the FC activations are seen as 1×1 feature maps, which means that all locations shares the same information regarding the FC part of the hypercolumn. All these activations are then concatenated to create the hypercolumn. For instance, if we take the VGG-16 architecture to use only the first 2 convolutional layers after the max pooling operations, we will have a hypercolumn with the size of:

64 filters(first conv layer before pooling)

+

128个过滤器(池之前第二CONV层) =192 features

Everything sounds cool, but how do we extract hypercolumns in practice ?

## VGG-16

To setup a pretrained VGG-16 network on Keras, you’ll need to download the weights filefrom here(vgg16_weights.h5 file with approximately 500MB) and then setup the architecture and load the downloaded weights using Keras (关于权重文件和体系结构的详细信息here）：

从进口pyplot matplotlib plt theano进口import cv2 import numpy as np import scipy as sp from keras.models import Sequential from keras.layers.core import Flatten, Dense, Dropout from keras.layers.convolutional import Convolution2D, MaxPooling2D from keras.layers.convolutional import ZeroPadding2D from keras.optimizers import SGD from sklearn.manifold import TSNE from sklearn import manifold from sklearn import cluster from sklearn.preprocessing import StandardScaler def VGG_16(weights_path=None): model = Sequential() model.add(ZeroPadding2D((1,1),input_shape=(3,224,224))) model.add(Convolution2D(64, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(64, 3, 3, activation='relu')) model.add(MaxPooling2D((2,2), stride=(2,2))) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(128, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(128, 3, 3, activation='relu')) model.add(MaxPooling2D((2,2), stride=(2,2))) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(256, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(256, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(256, 3, 3, activation='relu')) model.add(MaxPooling2D((2,2), stride=(2,2))) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(512, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(512, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(512, 3, 3, activation='relu')) model.add(MaxPooling2D((2,2), stride=(2,2))) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(512, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(512, 3, 3, activation='relu')) model.add(ZeroPadding2D((1,1))) model.add(Convolution2D(512, 3, 3, activation='relu')) model.add(MaxPooling2D((2,2), stride=(2,2))) model.add(Flatten()) model.add(Dense(4096, activation='relu')) model.add(Dropout(0.5)) model.add(Dense(4096, activation='relu')) model.add(Dropout(0.5)) model.add(Dense(1000, activation='softmax')) if weights_path: model.load_weights(weights_path) return model

As you can see, this is a very simple code to declare the VGG16 architecture and load the pre-trained weights (together with Python imports for the required packages). After that we’ll compile the Keras model:

model = VGG_16('vgg16_weights.h5') sgd = SGD(lr=0.1, decay=1e-6, momentum=0.9, nesterov=True) model.compile(optimizer=sgd, loss='categorical_crossentropy')

Now let’s test the network using an image:

im_original = cv2.resize（cv2.imread（ 'madruga.jpg'），（224，224））IM = im_original.transpose（（2,0,1））IM = np.expand_dims（1M，轴= 0）im_converted= cv2.cvtColor（im_original，cv2.COLOR_BGR2RGB）plt.imshow（im_converted）

Image used

out = model.predict(im) plt.plot(out.ravel())

As you can see, these are the final activations of the softmax layer, the class with the “jersey, T-shirt, tee shirt” category.

## Extracting arbitrary feature maps

get_feature = theano.function([model.layers[0].input], model.layers[3].get_output(train=False), allow_input_downcast=False) feat = get_feature(im) plt.imshow(feat[0][2])

Feature Map

In the example above, I’m compiling a Theano function to get the 3 layer (a convolutional layer) feature map and then showing only the 3rd feature map. Here we can see the intensity of the activations. If we get feature maps of the activations from the final layers, we can see that the extracted features are more abstract, like eyes, etc. Look at this example below from the 15th convolutional layer:

get_feature = theano.function([model.layers[0].input], model.layers[15].get_output(train=False), allow_input_downcast=False) feat = get_feature(im) plt.imshow(feat[0][13])

As you can see, this second feature map is extracting more abstract features. And you can also note that the image seems to be more stretched when compared with the feature we saw earlier, that is because the the first feature maps has 224×224 size and this one has 56×56 due to the downscaling operations of the layers before the convolutional layer, and that is why we lose a lot of spatial information.

## Extracting hypercolumns

Now finally let’s extract the hypercolumns of arbitrary set of layers. To do that, we will define a function to extract these hypercolumns:

def extract_hypercolumn(model, layer_indexes, instance): layers = [model.layers[li].get_output(train=False) for li in layer_indexes] get_feature = theano.function([model.layers[0].input], layers, allow_input_downcast=False) feature_maps = get_feature(instance) hypercolumns = [] for convmap in feature_maps: for fmap in convmap[0]: upscaled = sp.misc.imresize(fmap, size=(224, 224), mode="F", interp='bilinear') hypercolumns.append(upscaled) return np.asarray(hypercolumns)

As we can see, this function will expect three parameters: the model itself, an list of layer indexes that will be used to extract the hypercolumn features and an image instance that will be used to extract the hypercolumns. Let’s now test the hypercolumn extraction for the first 2 convolutional layers:

layers_extract = [3, 8] hc = extract_hypercolumn(model, layers_extract, im)

That’s it, we extracted the hypercolumn vectors for each pixel. The shape of this “hc” variable is: (192L, 224L, 224L), which means that we have a 192-dimensional hypercolumn for each one of the 224×224 pixel (a total of 50176 pixels with 192 hypercolumn feature each).

Let’s plot the average of the hypercolumns activations for each pixel:

ave = np.average(hc.transpose(1, 2, 0), axis=2) plt.imshow(ave)

Ad you can see, those first hypercolumn activations are all looking like edge detectors, let’s see how these hypercolumns looks like for the layers 22 and 29:

layers_extract = [22, 29] hc = extract_hypercolumn(model, layers_extract, im) ave = np.average(hc.transpose(1, 2, 0), axis=2) plt.imshow(ave)

As we can see now, the features are really more abstract and semantically interesting but with spatial information a little fuzzy.

Remember that you can extract the hypercolumns using all the initial layers and also the final layers, including the FC layers. Here I’m extracting them separately to show how they differ in the visualization plots.

## Simple hypercolumn pixel clustering

Now, you can do a lot of things, you can use these hypercolumns to classify pixels for some task, to do automatic pixel colorization, segmentation, etc. What I’m going to do here just as an experiment, is to use the hypercolumns (from the VGG-16 layers 3, 8, 15, 22, 29) and then cluster it using KMeans with 2 clusters:

m = hc.transpose(1,2,0).reshape(50176, -1) kmeans = cluster.KMeans(n_clusters=2, max_iter=300, n_jobs=5, precompute_distances=True) cluster_labels = kmeans .fit_predict(m) imcluster = np.zeros((224,224)) imcluster = imcluster.reshape((224*224,)) imcluster = cluster_labels plt.imshow(imcluster.reshape(224, 224), cmap="hot")

Now you can imagine how useful hypercolumns can be to tasks like keypoints extraction, segmentation, etc. It’s a very elegant, simple and useful concept.

I hope you liked it !

– Christian S. Perone

Cite this article as: Christian S. Perone, "Convolutional hypercolumns in Python," inTerra Incognita，2016年11月1日，//www.cpetem.com/2016/01/convolutional-hypercolumns-in-python/.