CLIP: Learning Transferable Visual Models From Natural Language Supervision

These are my notes and explanation on the CLIP paper, a model that uses a very clever contrastive training regime that maps both textual and image inputs into a shared latent universe, to then apply semantically meaningful distance notions and generate images or text.

The basic idea is: we have a model that maps text prompts to a latent space (like a sentence embedding. In this case, a Transformer). We also have a model that maps images into a latent space (here a ResNet or a Transformer) of the same dimension d.

Finally, what we do is take a (massive, crawled from the internet) dataset of images and their captions, and train both models to increase the cosine similarity between a caption and its corresponding image. After backpropagating for a long while, we obtain a new embedding space where a text’s representation and its corresponding image’s are ‘close’ in terms of cosine similarity (angle), and far from other arbitrary captions and pictures.

This is achieved through a similar loss as is used for word embeddings. It is contrastive: we take a pair (image, caption) and also make a spurious pair with the image and a random caption, and one with the caption and a random image. Then we use a loss that penalizes the true pair for diverging from a similarity of 1, and the spurious pairs for having a similarity bigger than 0.

Here ends my explanation and begin the quotes and figures from the paper.

Paper quotes and summary

State-of-the-art computer vision systems are trained to predict a fixed set of predetermined object categories. This restricted form of supervision limits their generality and usability since additional labeled data is needed to specify any other visual concept. Learning directly from raw text about images is a promising alternative which leverages a much broader source of supervision.

We demonstrate that the simple pre-training task of predicting which caption goes with which image is an efficient and scalable way to learn SOTA image representations from scratch on a dataset of 400 million (image, text) pairs collected from the internet.

we match the accuracy of the original ResNet-50 on ImageNet zero-shot without needing to use any of the 1.28 million training examples it was trained on.

The line of work of semi-supervised learning using images with a hashtag related to a imagenet label, or pretraining on a very big dataset noisily labelled (JFT-300M) represents the current pragmatic middle ground between learning from a limited amount of supervised “gold-labels” and learning from practically unlimited amounts of raw text.

Both works limit their supervision to 1000 and 18291 classes respectively. Natural language is able to express, and therefore supervise, a much wider set of visual concepts through its generality. Both approaches also use static softmax classifiers to perform prediction and lack a mechanism for dynamic outputs. This severely curtails their flexibility and limits their “zero-shot” capabilities.

We create a new dataset of 400 million (image, text) pairs and demonstrate that a simplified version of ConVIRT trained from scratch, which we call CLIP, for Contrastive Language-Image Pre-training, is an efficient method of learning from natural language supervision.

CLIP grows capable of competitive zero-shot transfer performance in a battery of benchmarks.

We constructed a new dataset of 400 million (image, text) pairs collected form a variety of publicly available sources on the Internet. To attempt to cover as broad a set of visual concepts as possible, we search for (image, text) pairs as part of the construction process whose text includes one of a set of 500,000 queries (words occurring at least 100 times in the English version of Wikipedia) and take up to 20k images per query. The resulting dataset has a similar total word count as the WebText dataset used to train GPT-2.

Recent work in contrastive representation learning for images has found that contrastive objectives can learn better representations than their equivalent predictive objective (predicting the embedding instead of the discrete word is better).

Noting these findings, we explored training a system to solve the potentially easier proxy task of predicting only which text as a whole is paired with which image and not the exact words of that text. Starting with the same bag-of-words encoding baseline, we swapped the predictive objective for a contrastive objective in Figure 2 and observed a further 4x efficiency improvement in the rate of zero-shot transfer to ImageNet.

Given a batch of N (image, text) pairs, CLIP is trained to predict which of the N × N possible (image, text) pairings across a batch actually occurred. To do this, CLIP learns a multi-modal embedding space by jointly training an image encoder and text encoder to maximize the cosine similarity of the image and text embeddings of the N real pairs in the batch while minimizing the cosine similarity of the embeddings of the N2 − N incorrect pairings.

We optimize a symmetric cross entropy loss over these similarity scores.


Two versions for image encoder:

The text encoder is a Transformer with input capped at 76 characters. The text sequence is bracketed with [SOS] and [EOS] tokens and the activations of the highest layer of the transformer at the [EOS] token are treated as the feature representation of the text which is layer normalized and then linearly projected into the multi-modal embedding space.

“To save additional memory, gradient checkpointing (Griewank & Walther, 2000; Chen et al., 2016), half-precision Adam statistics (Dhariwal et al., 2020), and half-precision stochastically rounded text encoder weights were used.” (gotta look all of these up).

Every step of CLIP pre-training can be viewed as optimizing the performance of a randomly created proxy to a computer vision dataset which contains 1 example per class and has 32,768 total classes defined via natural language descriptions. For zero-shot evaluation, we cache the zero-shot classifier once it has been computed by the text encoder and reuse it for all subsequent predictions.

Another issue we encountered is that it’s relatively rare in our pre-training dataset for the text paired with the image to be just a single word. Usually the text is a full sentence describing the image in some way. To help bridge this distribution gap, we found that using the prompt template “A photo of a {label}.” to be a good default that helps specify the text is about the content of the image. This often improves performance over the baseline of using only the label text. For instance, just using this prompt improves accuracy on ImageNet by 1.3%.

We’ve observed ensembling -by averaging embedding representations over many different textual prompts and caching them, which quickly gets amortized- across many generated zero-shot classifiers to reliably improve performance.

For fine-grained classification, it works to add, “a photo of a X, a type of Y”.

Using a linear probe, CLIP beats other models in a few-shot context (up to 16 instances), and interestingly its 0-shot approach beats few shots up to 4. Zero-shot CLIP performs competitively against fully supervised Linear Probe on ResNet50 on a wide array of tasks (wins in 16/27 datasets).

However, models trained with CLIP scale very well and the largest model we trained (ResNet-50x64) slightly outperforms the best performing existing model (a Noisy Student EfficientNet-L2) on both overall score and compute efficiency -on linear probe accuracy over 27 datasets, a standard benchmark-. We also find that CLIP vision transformers are about 3x more compute efficient than CLIP ResNets, which allows us to reach higher overall performance within our compute budget and use it for the majority of datasets.

Note that by this point, a CLIP ViT has a transformer for textual representation, a transformer for image representation, and just does dot product between them, so it’s transformers all the way down. Linear probe CLIP ViT beats other models in 21/27 tasks.

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25 Oct 2021 - importance: 8