Different activations with same image

def get_object_context_features(object_image, context_image):
    data = (ImageList.from_folder('/Users/Siddharth/Projects/Wisconsin/causal_discovery/discovering_causal_signaals_in_images/mscoco/dummy/{}'.format('cat'))
    data.train_dl.batch_size = 1
    fnames = data.items
    feature_extractor = cnn_learner(data, models.resnet18, metrics=[accuracy])
    model = feature_extractor.model
    feature_extractor.model = nn.Sequential(*(list(model.children())[:-1]), *(list(model.children())[-1][:-4]))
    tfms = transforms.Compose([
    #transforms.Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225])
    object_img_tensor = tfms(PIL.Image.fromarray(object_image))
    context_img_tensor = tfms(PIL.Image.fromarray(context_image))
    object_image_features = feature_extractor.model(object_img_tensor[None])
    context_image_features = feature_extractor.model(context_img_tensor[None])
    return object_image_features, context_image_features

I’m trying to get activations of the pre-final layer(containing 512 neurons) from a pre-trained resnet18 model. The function takes two images as input and these images passed through the feature extraction network mentioned before.

Every time I run this code on the same images, I get different activations. Is this normal? Isn’t this a pre-trained model on ImageNet so shouldn’t the activations be constant on the same image?

try displaying object_img_tensor and context_img_tensor to make sure no random transformation is going on.

Here is the object image before and after applying the transform for one of the images. I dont understand why 9 images show up

Screenshot 2020-02-11 at 8.06.02 PM

It looks like your feature_extractor model contains the backbone and piece of the head.

feature_extractor.model = nn.Sequential(*(list(model.children())[:-1]), *(list(model.children())[-1][:-4]))

here the list(model.children())[-1] refers to the model’s head.
When you call cnn_learner the head is created, initialized randomly and attached to the pretrained backbone.

To get the backbone you can simply use learner.model[0] right after creating the learner using cnn_learner.

Makes sense!
I want to obtain activations from the pre-final layer before the ReLu non linearity is applied so that I get an output of 512 features. How do I get intermediate layer activations like I mentioned above?

The last layer of backbone is a linear layer. Have a look at the model structure printed here. Last layers of the backbone are Conv and BatchNorm:

But the output of backbone is not 512 features. It’s 512 feature maps. Each feature map is a rectangle of some size(depending on the input image size).

I’m sorry, by pre-final layer I meant the Linear layer numbered at 4 in the Head of this model