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Model Interpretability using Captum

Captum helps you understand how the data features impact your model predictions or neuron activations, shedding light on how your model operates.

Using Captum, you can apply a wide range of state-of-the-art feature attribution algorithms such as Guided GradCam and Integrated Gradients in a unified way.

In this recipe you will learn how to use Captum to: * attribute the predictions of an image classifier to their corresponding image features. * visualize the attribution results.

Before you begin

Make sure Captum is installed in your active Python environment. Captum is available both on GitHub, as a pip package, or as a conda package. For detailed instructions, consult the installation guide at https://captum.ai/

For a model, we use a built-in image classifier in PyTorch. Captum can reveal which parts of a sample image support certain predictions made by the model.

import torchvision
from torchvision import transforms
from PIL import Image
import requests
from io import BytesIO

model = torchvision.models.resnet18(pretrained=True).eval()

response = requests.get("https://image.freepik.com/free-photo/two-beautiful-puppies-cat-dog_58409-6024.jpg")
img = Image.open(BytesIO(response.content))

center_crop = transforms.Compose([
 transforms.Resize(256),
 transforms.CenterCrop(224),
])

normalize = transforms.Compose([
    transforms.ToTensor(),               # converts the image to a tensor with values between 0 and 1
    transforms.Normalize(                # normalize to follow 0-centered imagenet pixel rgb distribution
     mean=[0.485, 0.456, 0.406],
     std=[0.229, 0.224, 0.225]
    )
])
input_img = normalize(center_crop(img)).unsqueeze(0)

Computing Attribution

Among the top-3 predictions of the models are classes 208 and 283 which correspond to dog and cat.

Let us attribute each of these predictions to the corresponding part of the input, using Captum’s Occlusion algorithm.

from captum.attr import Occlusion

occlusion = Occlusion(model)

strides = (3, 9, 9)               # smaller = more fine-grained attribution but slower
target=208,                       # Labrador index in ImageNet
sliding_window_shapes=(3,45, 45)  # choose size enough to change object appearance
baselines = 0                     # values to occlude the image with. 0 corresponds to gray

attribution_dog = occlusion.attribute(input_img,
                                       strides = strides,
                                       target=target,
                                       sliding_window_shapes=sliding_window_shapes,
                                       baselines=baselines)


target=283,                       # Persian cat index in ImageNet
attribution_cat = occlusion.attribute(input_img,
                                       strides = strides,
                                       target=target,
                                       sliding_window_shapes=sliding_window_shapes,
                                       baselines=0)

Besides Occlusion, Captum features many algorithms such as Integrated Gradients, Deconvolution, GuidedBackprop, Guided GradCam, DeepLift, and GradientShap. All of these algorithms are subclasses of Attribution which expects your model as a callable forward_func upon initialization and has an attribute(...) method which returns the attribution result in a unified format.

Let us visualize the computed attribution results in case of images.

Visualizing the Results

Captum’s visualization utility provides out-of-the-box methods to visualize attribution results both for pictorial and for textual inputs.

import numpy as np
from captum.attr import visualization as viz

# Convert the compute attribution tensor into an image-like numpy array
attribution_dog = np.transpose(attribution_dog.squeeze().cpu().detach().numpy(), (1,2,0))

vis_types = ["heat_map", "original_image"]
vis_signs = ["all", "all"], # "positive", "negative", or "all" to show both
# positive attribution indicates that the presence of the area increases the prediction score
# negative attribution indicates distractor areas whose absence increases the score

_ = viz.visualize_image_attr_multiple(attribution_dog,
                                      center_crop(img),
                                      vis_types,
                                      vis_signs,
                                      ["attribution for dog", "image"],
                                      show_colorbar = True
                                     )


attribution_cat = np.transpose(attribution_cat.squeeze().cpu().detach().numpy(), (1,2,0))

_ = viz.visualize_image_attr_multiple(attribution_cat,
                                      center_crop(img),
                                      ["heat_map", "original_image"],
                                      ["all", "all"], # positive/negative attribution or all
                                      ["attribution for cat", "image"],
                                      show_colorbar = True
                                     )

If your data is textual, visualization.visualize_text() offers a dedicated view to explore attribution on top of the input text. Find out more at http://captum.ai/tutorials/IMDB_TorchText_Interpret

Final Notes

Captum can handle most model types in PyTorch across modalities including vision, text, and more. With Captum you can: * Attribute a specific output to the model input as illustrated above. * Attribute a specific output to a hidden-layer neuron (see Captum API reference). * Attribute a hidden-layer neuron response to the model input (see Captum API reference).

For complete API of the supported methods and a list of tutorials, consult our website http://captum.ai

Another useful post by Gilbert Tanner: https://gilberttanner.com/blog/interpreting-pytorch-models-with-captum

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