
Image Feature Extraction
[[open-in-colab]]
Image feature extraction is the task of extracting semantically meaningful features given an image. This has many use cases, including image similarity and image retrieval. Moreover, most computer vision models can be used for image feature extraction, where one can remove the task-specific head (image classification, object detection etc) and get the features. These features are very useful on a higher level: edge detection, corner detection and so on. They may also contain information about the real world (e.g. what a cat looks like) depending on how deep the model is. Therefore, these outputs can be used to train new classifiers on a specific dataset.
In this guide, you will:

Learn to build a simple image similarity system on top of the image-feature-extraction pipeline.
Accomplish the same task with bare model inference.

Image Similarity using image-feature-extraction Pipeline
We have two images of cats sitting on top of fish nets, one of them is generated. 
thon
from PIL import Image
import requests
img_urls = ["https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/cats.png", "https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/cats.jpeg"]
image_real = Image.open(requests.get(img_urls[0], stream=True).raw).convert("RGB")
image_gen = Image.open(requests.get(img_urls[1], stream=True).raw).convert("RGB")

Let's see the pipeline in action. First, initialize the pipeline. If you don't pass any model to it, the pipeline will be automatically initialized with google/vit-base-patch16-224. If you'd like to calculate similarity, set pool to True.
thon
import torch
from transformers import pipeline
DEVICE = torch.device('cuda' if torch.cuda.is_available() else 'cpu')
pipe = pipeline(task="image-feature-extraction", model_name="google/vit-base-patch16-384", device=DEVICE, pool=True)

To infer with pipe pass both images to it.
python
outputs = pipe([image_real, image_gen])
The output contains pooled embeddings of those two images.
thon
get the length of a single output
print(len(outputs[0][0]))
show outputs
print(outputs)
768
[[[-0.03909236937761307, 0.43381670117378235, -0.06913255900144577,

To get the similarity score, we need to pass them to a similarity function. 
thon
from torch.nn.functional import cosine_similarity
similarity_score = cosine_similarity(torch.Tensor(outputs[0]),
                                     torch.Tensor(outputs[1]), dim=1)
print(similarity_score)
tensor([0.6043])

If you want to get the last hidden states before pooling, avoid passing any value for the pool parameter, as it is set to False by default. These hidden states are useful for training new classifiers or models based on the features from the model.
python
pipe = pipeline(task="image-feature-extraction", model_name="google/vit-base-patch16-224", device=DEVICE)
output = pipe(image_real)
Since the outputs are unpooled, we get the last hidden states where the first dimension is the batch size, and the last two are the embedding shape.
thon
import numpy as np
print(np.array(outputs).shape)
(1, 197, 768)

Getting Features and Similarities using AutoModel
We can also use AutoModel class of transformers to get the features. AutoModel loads any transformers model with no task-specific head, and we can use this to get the features.
thon
from transformers import AutoImageProcessor, AutoModel
processor = AutoImageProcessor.from_pretrained("google/vit-base-patch16-224")
model = AutoModel.from_pretrained("google/vit-base-patch16-224").to(DEVICE)

Let's write a simple function for inference. We will pass the inputs to the processor first and pass its outputs to the model.
python
def infer(image):
  inputs = processor(image, return_tensors="pt").to(DEVICE)
  outputs = model(**inputs)
  return outputs.pooler_output
We can pass the images directly to this function and get the embeddings.
python
embed_real = infer(image_real)
embed_gen = infer(image_gen)
We can get the similarity again over the embeddings.
thon
from torch.nn.functional import cosine_similarity
similarity_score = cosine_similarity(embed_real, embed_gen, dim=1)
print(similarity_score)
tensor([0.6061], device='cuda:0', grad_fn=)
```