Visualizing what convnets learn
Author: fchollet
Date created: 2020/05/29
Last modified: 2020/05/29
Description: Displaying the visual patterns that convnet filters respond to.
Introduction
In this example, we look into what sort of visual patterns image classification models learn. We'll be using the ResNet50V2 model, trained on the ImageNet dataset.
Our process is simple: we will create input images that maximize the activation of specific filters in a target layer (picked somewhere in the middle of the model: layer conv3_block4_out). Such images represent a visualization of the pattern that the filter responds to.
Setup
import os os.environ["KERAS_BACKEND"] = "tensorflow" import keras import numpy as np import tensorflow as tf # The dimensions of our input image img_width = 180 img_height = 180 # Our target layer: we will visualize the filters from this layer. # See `model.summary()` for list of layer names, if you want to change this. layer_name = "conv3_block4_out" Build a feature extraction model
# Build a ResNet50V2 model loaded with pre-trained ImageNet weights model = keras.applications.ResNet50V2(weights="imagenet", include_top=False) # Set up a model that returns the activation values for our target layer layer = model.get_layer(name=layer_name) feature_extractor = keras.Model(inputs=model.inputs, outputs=layer.output) Set up the gradient ascent process
The "loss" we will maximize is simply the mean of the activation of a specific filter in our target layer. To avoid border effects, we exclude border pixels.
def compute_loss(input_image, filter_index): activation = feature_extractor(input_image) # We avoid border artifacts by only involving non-border pixels in the loss. filter_activation = activation[:, 2:-2, 2:-2, filter_index] return tf.reduce_mean(filter_activation) Our gradient ascent function simply computes the gradients of the loss above with regard to the input image, and update the update image so as to move it towards a state that will activate the target filter more strongly.
@tf.function def gradient_ascent_step(img, filter_index, learning_rate): with tf.GradientTape() as tape: tape.watch(img) loss = compute_loss(img, filter_index) # Compute gradients. grads = tape.gradient(loss, img) # Normalize gradients. grads = tf.math.l2_normalize(grads) img += learning_rate * grads return loss, img Set up the end-to-end filter visualization loop
Our process is as follow:
- Start from a random image that is close to "all gray" (i.e. visually netural)
- Repeatedly apply the gradient ascent step function defined above
- Convert the resulting input image back to a displayable form, by normalizing it, center-cropping it, and restricting it to the [0, 255] range.
def initialize_image(): # We start from a gray image with some random noise img = tf.random.uniform((1, img_width, img_height, 3)) # ResNet50V2 expects inputs in the range [-1, +1]. # Here we scale our random inputs to [-0.125, +0.125] return (img - 0.5) * 0.25 def visualize_filter(filter_index): # We run gradient ascent for 20 steps iterations = 30 learning_rate = 10.0 img = initialize_image() for iteration in range(iterations): loss, img = gradient_ascent_step(img, filter_index, learning_rate) # Decode the resulting input image img = deprocess_image(img[0].numpy()) return loss, img def deprocess_image(img): # Normalize array: center on 0., ensure variance is 0.15 img -= img.mean() img /= img.std() + 1e-5 img *= 0.15 # Center crop img = img[25:-25, 25:-25, :] # Clip to [0, 1] img += 0.5 img = np.clip(img, 0, 1) # Convert to RGB array img *= 255 img = np.clip(img, 0, 255).astype("uint8") return img Let's try it out with filter 0 in the target layer:
from IPython.display import Image, display loss, img = visualize_filter(0) keras.utils.save_img("0.png", img) This is what an input that maximizes the response of filter 0 in the target layer would look like:
display(Image("0.png")) 
Visualize the first 64 filters in the target layer
Now, let's make a 8x8 grid of the first 64 filters in the target layer to get of feel for the range of different visual patterns that the model has learned.
# Compute image inputs that maximize per-filter activations # for the first 64 filters of our target layer all_imgs = [] for filter_index in range(64): print("Processing filter %d" % (filter_index,)) loss, img = visualize_filter(filter_index) all_imgs.append(img) # Build a black picture with enough space for # our 8 x 8 filters of size 128 x 128, with a 5px margin in between margin = 5 n = 8 cropped_width = img_width - 25 * 2 cropped_height = img_height - 25 * 2 width = n * cropped_width + (n - 1) * margin height = n * cropped_height + (n - 1) * margin stitched_filters = np.zeros((width, height, 3)) # Fill the picture with our saved filters for i in range(n): for j in range(n): img = all_imgs[i * n + j] stitched_filters[ (cropped_width + margin) * i : (cropped_width + margin) * i + cropped_width, (cropped_height + margin) * j : (cropped_height + margin) * j + cropped_height, :, ] = img keras.utils.save_img("stiched_filters.png", stitched_filters) from IPython.display import Image, display display(Image("stiched_filters.png")) Processing filter 0 Processing filter 1 Processing filter 2 Processing filter 3 Processing filter 4 Processing filter 5 Processing filter 6 Processing filter 7 Processing filter 8 Processing filter 9 Processing filter 10 Processing filter 11 Processing filter 12 Processing filter 13 Processing filter 14 Processing filter 15 Processing filter 16 Processing filter 17 Processing filter 18 Processing filter 19 Processing filter 20 Processing filter 21 Processing filter 22 Processing filter 23 Processing filter 24 Processing filter 25 Processing filter 26 Processing filter 27 Processing filter 28 Processing filter 29 Processing filter 30 Processing filter 31 Processing filter 32 Processing filter 33 Processing filter 34 Processing filter 35 Processing filter 36 Processing filter 37 Processing filter 38 Processing filter 39 Processing filter 40 Processing filter 41 Processing filter 42 Processing filter 43 Processing filter 44 Processing filter 45 Processing filter 46 Processing filter 47 Processing filter 48 Processing filter 49 Processing filter 50 Processing filter 51 Processing filter 52 Processing filter 53 Processing filter 54 Processing filter 55 Processing filter 56 Processing filter 57 Processing filter 58 Processing filter 59 Processing filter 60 Processing filter 61 Processing filter 62 Processing filter 63 
Image classification models see the world by decomposing their inputs over a "vector basis" of texture filters such as these.
See also this old blog post for analysis and interpretation.