Class Activation Map methods implemented in Pytorch
pip install gradcam
Method  What it does 

GradCAM  Weight the 2D activations by the average gradient 
GradCAM++  Like GradCAM but uses second order gradients 
XGradCAM  Like GradCAM but scale the gradients by the normalized activations 
AblationCAM  Zero out activations and measure how the output drops (this repository includes a fast batched implementation) 
ScoreCAM  Perbutate the image by the scaled activations and measure how the output drops 
EigenCAM  Takes the first principle component of the 2D Activations (no class discrimination, but seems to give great results) 
EigenGradCAM  Like EigenCAM but with class discrimination: First principle component of Activations*Grad. Looks like GradCAM, but cleaner 
What makes the network think the image label is 'pug, pugdog' and 'tabby, tabby cat':
Combining GradCAM with Guided Backpropagation for the 'pug, pugdog' class:
More Visual Examples
Resnet50:
Category  Image  GradCAM  AblationCAM  ScoreCAM 

Dog  
Cat 
Vision Transfomer (Deit Tiny):
Category  Image  GradCAM  AblationCAM  ScoreCAM 

Dog  
Cat 
Swin Transfomer (Tiny window:7 patch:4 inputsize:224):
Category  Image  GradCAM  AblationCAM  ScoreCAM 

Dog  
Cat 
It seems that GradCAM++ is almost the same as GradCAM, in most networks except VGG where the advantage is larger.
Network  Image  GradCAM  GradCAM++  ScoreCAM  AblationCAM  EigenCAM 

VGG16  
Resnet50 
Chosing the Target Layer
You need to choose the target layer to compute CAM for. Some common choices are:
 Resnet18 and 50: model.layer4[1]
 VGG and densenet161: model.features[1]
 mnasnet1_0: model.layers[1]
 ViT: model.blocks[1].norm1
 SwinT: model.layers[1].blocks[1].norm1
Using from code as a library
from pytorch_grad_cam import GradCAM, ScoreCAM, GradCAMPlusPlus, AblationCAM, XGradCAM, EigenCAM
from pytorch_grad_cam.utils.image import show_cam_on_image
from torchvision.models import resnet50
model = resnet50(pretrained=True)
target_layer = model.layer4[1]
input_tensor = # Create an input tensor image for your model..
# Note: input_tensor can be a batch tensor with several images!
# Construct the CAM object once, and then reuse it on many images:
cam = GradCAM(model=model, target_layer=target_layer, use_cuda=args.use_cuda)
# If target_category is None, the highest scoring category
# will be used for every image in the batch.
# target_category can also be an integer, or a list of different integers
# for every image in the batch.
target_category = 281
# You can also pass aug_smooth=True and eigen_smooth=True, to apply smoothing.
grayscale_cam = cam(input_tensor=input_tensor, target_category=target_category)
# In this example grayscale_cam has only one image in the batch:
grayscale_cam = grayscale_cam[0, :]
visualization = show_cam_on_image(rgb_img, grayscale_cam)
Smoothing to get nice looking CAMs
To reduce noise in the CAMs, and make it fit better on the objects, two smoothing methods are supported:

aug_smooth=True
Test time augmentation: increases the run time by x6.
Applies a combination of horizontal flips, and mutiplying the image by [1.0, 1.1, 0.9].
This has the effect of better centering the CAM around the objects.

eigen_smooth=True
First principle component of
activations*weights
This has the effect of removing a lot of noise.
AblationCAM  aug smooth  eigen smooth  aug+eigen smooth 

Running the example script:
Usage: python cam.py imagepath <path_to_image> method <method>
To use with CUDA:
python cam.py imagepath <path_to_image> usecuda
You can choose between:
GradCAM
, ScoreCAM
, GradCAMPlusPlus
, AblationCAM
, XGradCAM
and EigenCAM
.
Some methods like ScoreCAM and AblationCAM require a large number of forward passes, and have a batched implementation.
You can control the batch size with
cam.batch_size =
How does it work with Vision Transformers
See usage_examples/vit_example.py
In ViT the output of the layers are typically BATCH x 197 x 192. In the dimension with 197, the first element represents the class token, and the rest represent the 14x14 patches in the image. We can treat the last 196 elements as a 14x14 spatial image, with 192 channels.
To reshape the activations and gradients to 2D spatial images, we can pass the CAM constructor a reshape_transform function.
This can also be a starting point for other architectures that will come in the future.
GradCAM(model=model, target_layer=target_layer, reshape_transform=reshape_transform)
def reshape_transform(tensor, height=14, width=14):
result = tensor[:, 1 : , :].reshape(tensor.size(0),
height, width, tensor.size(2))
# Bring the channels to the first dimension,
# like in CNNs.
result = result.transpose(2, 3).transpose(1, 2)
return result
Which target_layer should we chose for Vision Transformers?
Since the final classification is done on the class token computed in the last attention block, the output will not be affected by the 14x14 channels in the last layer. The gradient of the output with respect to them, will be 0!
We should chose any layer before the final attention block, for example:
target_layer = model.blocks[1].norm1
How does it work with Swin Transformers
See usage_examples/swinT_example.py
In Swin transformer base the output of the layers are typically BATCH x 49 x 1024. We can treat the last 49 elements as a 7x7 spatial image, with 1024 channels.
To reshape the activations and gradients to 2D spatial images, we can pass the CAM constructor a reshape_transform function.
This can also be a starting point for other architectures that will come in the future.
GradCAM(model=model, target_layer=target_layer, reshape_transform=reshape_transform)
def reshape_transform(tensor, height=7, width=7):
result = tensor.reshape(tensor.size(0),
height, width, tensor.size(2))
# Bring the channels to the first dimension,
# like in CNNs.
result = result.transpose(2, 3).transpose(1, 2)
return result
Which target_layer should we chose for Swin Transformers?
Since the swin transformer is different from ViT, it does not contains cls_token
as present in ViT,
therefore we will use all the 7x7 images we get from the last block of the last layer.
We should chose any layer before the final attention block, for example:
target_layer = model.layers[1].blocks[1].norm1
Citation
If you use this for research, please cite. Here is an example BibTeX entry:
@misc{jacobgilpytorchcam,
title={PyTorch library for CAM methods},
author={Jacob Gildenblat and contributors},
year={2021},
publisher={GitHub},
howpublished={\url{https://github.com/jacobgil/pytorchgradcam}},
}
References
https://arxiv.org/abs/1610.02391
GradCAM: Visual Explanations from Deep Networks via Gradientbased Localization Ramprasaath R. Selvaraju, Michael Cogswell, Abhishek Das, Ramakrishna Vedantam, Devi Parikh, Dhruv Batra
https://arxiv.org/abs/1710.11063
GradCAM++: Improved Visual Explanations for Deep Convolutional Networks Aditya Chattopadhyay, Anirban Sarkar, Prantik Howlader, Vineeth N Balasubramanian
https://arxiv.org/abs/1910.01279
ScoreCAM: ScoreWeighted Visual Explanations for Convolutional Neural Networks Haofan Wang, Zifan Wang, Mengnan Du, Fan Yang, Zijian Zhang, Sirui Ding, Piotr Mardziel, Xia Hu
https://ieeexplore.ieee.org/abstract/document/9093360/
Ablationcam: Visual explanations for deep convolutional network via gradientfree localization. Saurabh Desai and Harish G Ramaswamy. In WACV, pages 972–980, 2020
https://arxiv.org/abs/2008.02312
Axiombased GradCAM: Towards Accurate Visualization and Explanation of CNNs Ruigang Fu, Qingyong Hu, Xiaohu Dong, Yulan Guo, Yinghui Gao, Biao Li
https://arxiv.org/abs/2008.00299
EigenCAM: Class Activation Map using Principal Components Mohammed Bany Muhammad, Mohammed Yeasin