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Posted to commits@mxnet.apache.org by GitBox <gi...@apache.org> on 2018/05/16 18:19:21 UTC
[GitHub] ThomasDelteil commented on a change in pull request #10900:
[MXNET-414] Tutorial on visualizing CNN decisions using Grad-CAM
ThomasDelteil commented on a change in pull request #10900: [MXNET-414] Tutorial on visualizing CNN decisions using Grad-CAM
URL: https://github.com/apache/incubator-mxnet/pull/10900#discussion_r188721229
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File path: docs/tutorials/vision/cnn_visualization.md
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+# Visualizing Decisions of Convolutional Neural Networks
+
+Convolutional Neural Networks have made a lot of progress in Computer Vision. Their accuracy is as good as humans in some tasks. However it remains hard to explain the predictions of convolutional neural networks.
+
+It is often helpful to be able to explain why a model made the prediction it made. For example when a model misclassifies an image, it is hard to say why without visualizing the network's decision.
+
+<img align="right" src="https://raw.githubusercontent.com/dmlc/web-data/master/mxnet/example/cnn_visualization/volcano_barn_spider.png" alt="Explaining the misclassification of volcano as spider" width=500px/>
+
+Visualizations also help build confidence about the predictions of a model. For example, even if a model correctly predicts birds as birds, we would want to confirm that the model bases its decision on the features of bird and not on the features of some other object that might occur together with birds in the dataset (like leaves).
+
+In this tutorial, we show how to visualize the predictions made by convolutional neural networks using Gradient-weighted Class Activation Mapping. Unlike many other visualization methods, Grad-CAM can be used on a wide variety of CNN model families - CNNs with fully connected layers, CNNs used for structural outputs (e.g. captioning), CNNs used in tasks with multi-model input (e.g. VQA) or reinforcement learning without architectural changes or re-training.
+
+In the rest of this notebook, we will explain how to visualize predictions made by [VGG-16](https://arxiv.org/abs/1409.1556). We begin by importing the required dependencies. `gradcam` module contains the implementation of visualization techniques used in this notebook.
+
+```python
+from __future__ import print_function
+
+import mxnet as mx
+from mxnet import gluon
+
+from matplotlib import pyplot as plt
+import numpy as np
+import cv2
+
+gradcam_file = "gradcam.py"
+base_url = "https://raw.githubusercontent.com/indhub/mxnet/cnnviz/example/cnn_visualization/{}?raw=true"
+mx.test_utils.download(base_url.format(gradcam_file), fname=gradcam_file)
+import gradcam
+```
+
+## Building the network to visualize
+
+Next, we build the network we want to visualize. For this example, we will use the [VGG-16](https://arxiv.org/abs/1409.1556) network. This code was taken from the Gluon [model zoo](https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/gluon/model_zoo/vision/alexnet.py) and refactored to make it easy to switch between `gradcam`'s and Gluon's implementation of ReLU and Conv2D. Same code can be used for both training and visualization with a minor (one line) change.
+
+Notice that we import ReLU and Conv2D from `gradcam` module instead of mxnet.gluon.nn.
+- We use a modified ReLU because we use guided backpropagation for visualization and guided backprop requires ReLU layer to block the backward flow of negative gradients corresponding to the neurons which decrease the activation of the higher layer unit we aim to visualize. Check [this](https://arxiv.org/abs/1412.6806) paper to learn more about guided backprop.
+- We use a modified Conv2D (a wrapper on top of Gluon's Conv2D) because we want to capture the output of a given convolutional layer and its gradients. This is needed to implement Grad-CAM. Check [this](https://arxiv.org/abs/1610.02391) paper to learn more about Grad-CAM.
+
+When you train the network, you could just import `Activation` and `Conv2D` from `gluon.nn` instead. No other part of the code needs any change to switch between training and visualization.
+
+```python
+import os
+from mxnet.gluon.model_zoo import model_store
+
+from mxnet.initializer import Xavier
+from mxnet.gluon.nn import MaxPool2D, Flatten, Dense, Dropout, BatchNorm
+from gradcam import Activation, Conv2D
+
+class VGG(mx.gluon.HybridBlock):
+ def __init__(self, layers, filters, classes=1000, batch_norm=False, **kwargs):
+ super(VGG, self).__init__(**kwargs)
+ assert len(layers) == len(filters)
+ with self.name_scope():
+ self.features = self._make_features(layers, filters, batch_norm)
+ self.features.add(Dense(4096, activation='relu',
+ weight_initializer='normal',
+ bias_initializer='zeros'))
+ self.features.add(Dropout(rate=0.5))
+ self.features.add(Dense(4096, activation='relu',
+ weight_initializer='normal',
+ bias_initializer='zeros'))
+ self.features.add(Dropout(rate=0.5))
+ self.output = Dense(classes,
+ weight_initializer='normal',
+ bias_initializer='zeros')
+
+ def _make_features(self, layers, filters, batch_norm):
+ featurizer = mx.gluon.nn.HybridSequential(prefix='')
+ for i, num in enumerate(layers):
+ for _ in range(num):
+ featurizer.add(Conv2D(filters[i], kernel_size=3, padding=1,
+ weight_initializer=Xavier(rnd_type='gaussian',
+ factor_type='out',
+ magnitude=2),
+ bias_initializer='zeros'))
+ if batch_norm:
+ featurizer.add(BatchNorm())
+ featurizer.add(Activation('relu'))
+ featurizer.add(MaxPool2D(strides=2))
+ return featurizer
+
+ def hybrid_forward(self, F, x):
+ x = self.features(x)
+ x = self.output(x)
+ return x
+```
+
+## Loading pretrained weights
+
+We'll use pre-trained weights (trained on ImageNet) from model zoo instead of training the model from scratch.
+
+```python
+vgg_spec = {11: ([1, 1, 2, 2, 2], [64, 128, 256, 512, 512]),
+ 13: ([2, 2, 2, 2, 2], [64, 128, 256, 512, 512]),
+ 16: ([2, 2, 3, 3, 3], [64, 128, 256, 512, 512]),
+ 19: ([2, 2, 4, 4, 4], [64, 128, 256, 512, 512])}
+
+def get_vgg(num_layers, pretrained=False, ctx=mx.cpu(),
+ root=os.path.join('~', '.mxnet', 'models'), **kwargs):
+ layers, filters = vgg_spec[num_layers]
+ net = VGG(layers, filters, **kwargs)
+ if pretrained:
+ from mxnet.gluon.model_zoo.model_store import get_model_file
+ batch_norm_suffix = '_bn' if kwargs.get('batch_norm') else ''
+ net.load_params(get_model_file('vgg%d%s'%(num_layers, batch_norm_suffix),
+ root=root), ctx=ctx)
+ return net
+
+def vgg16(**kwargs):
+ return get_vgg(16, **kwargs)
+```
+
+## Preprocessing and other helpers
+
+We'll resize the input image to 224x224 before feeding it to the network. We normalize the images using the same parameters ImageNet dataset was normalised using to create the pretrained model. These parameters are published [here](https://mxnet.incubator.apache.org/api/python/gluon/model_zoo.html). We use `transpose` to convert the image to channel-last format.
+
+Note that we do not hybridize the network. This is because we want `gradcam.Activation` and `gradcam.Conv2D` to behave differently at different times during the execution. For example, `gradcam.Activation` will do the regular backpropagation while computing the gradient of the topmost convolutional layer but will do guided backpropagation when computing the gradient of the image.
+
+```python
+image_sz = (224, 224)
+
+def preprocess(data):
+ data = mx.image.imresize(data, image_sz[0], image_sz[1])
+ data = data.astype(np.float32)
+ data = data/255
+ data = mx.image.color_normalize(data,
+ mean=mx.nd.array([0.485, 0.456, 0.406]),
+ std=mx.nd.array([0.229, 0.224, 0.225]))
+ data = mx.nd.transpose(data, (2,0,1))
+ return data
+
+network = vgg16(pretrained=True, ctx=mx.cpu())
+```
+
+We define some helpers to read image files from disk and display multiple images in a row in Jupyter notebook.
+
+```python
+def read_image_mxnet(path):
+ with open(path, 'rb') as fp:
+ img_bytes = fp.read()
+ return mx.img.imdecode(img_bytes)
+
+def read_image_cv(path):
+ return cv2.resize(cv2.cvtColor(cv2.imread(path), cv2.COLOR_BGR2RGB), image_sz)
+
+def show_images(pred_str, images):
+ titles = [pred_str, 'Grad-CAM', 'Guided Grad-CAM', 'Saliency Map']
+ num_images = len(images)
+ fig=plt.figure(figsize=(15,15))
+ rows, cols = 1, num_images
+ for i in range(num_images):
+ fig.add_subplot(rows, cols, i+1)
+ plt.xlabel(titles[i])
+ plt.imshow(images[i], cmap='gray' if i==num_images-1 else None)
+ plt.show()
+```
+
+Given an image, the network predicts a probability distribution over all categories. The most probable category can be found by applying the `argmax` operation. This gives an integer corresponding to the category. We still need to convert this to a human readable category name to know what category the network predicted. [Synset](http://data.mxnet.io/models/imagenet/synset.txt) file contains the mapping between Imagenet category index and category name. We'll download the synset file, load it in a list to convert category index to human readable category names.
+
+```python
+synset_url = "http://data.mxnet.io/models/imagenet/synset.txt"
+synset_file_name = "synset.txt"
+mx.test_utils.download(synset_url, fname=synset_file_name)
+synset = []
+with open('synset.txt', 'r') as f:
+ synset = [l.rstrip().split(' ', 1)[1].split(',')[0] for l in f]
+
+def get_class_name(cls_id):
+ return "%s (%d)" % (synset[cls_id], cls_id)
+
+def run_inference(net, data):
+ out = net(data)
+ return out.argmax(axis=1).asnumpy()[0].astype(int)
Review comment:
Suggestion: there is also a `topk()` function that is quite useful especially when getting several predictions and could be worth publicizing.
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