CLIP - Contrastive Language-Image Pre-Training
OpenAI's model that understands images from natural language.
When to use CLIP
Use when:
- •Zero-shot image classification (no training data needed)
- •Image-text similarity/matching
- •Semantic image search
- •Content moderation (detect NSFW, violence)
- •Visual question answering
- •Cross-modal retrieval (image→text, text→image)
Metrics:
- •25,300+ GitHub stars
- •Trained on 400M image-text pairs
- •Matches ResNet-50 on ImageNet (zero-shot)
- •MIT License
Use alternatives instead:
- •BLIP-2: Better captioning
- •LLaVA: Vision-language chat
- •Segment Anything: Image segmentation
Quick start
Installation
bash
pip install git+https://github.com/openai/CLIP.git pip install torch torchvision ftfy regex tqdm
Zero-shot classification
python
import torch
import clip
from PIL import Image
# Load model
device = "cuda" if torch.cuda.is_available() else "cpu"
model, preprocess = clip.load("ViT-B/32", device=device)
# Load image
image = preprocess(Image.open("photo.jpg")).unsqueeze(0).to(device)
# Define possible labels
text = clip.tokenize(["a dog", "a cat", "a bird", "a car"]).to(device)
# Compute similarity
with torch.no_grad():
image_features = model.encode_image(image)
text_features = model.encode_text(text)
# Cosine similarity
logits_per_image, logits_per_text = model(image, text)
probs = logits_per_image.softmax(dim=-1).cpu().numpy()
# Print results
labels = ["a dog", "a cat", "a bird", "a car"]
for label, prob in zip(labels, probs[0]):
print(f"{label}: {prob:.2%}")
Available models
python
# Models (sorted by size)
models = [
"RN50", # ResNet-50
"RN101", # ResNet-101
"ViT-B/32", # Vision Transformer (recommended)
"ViT-B/16", # Better quality, slower
"ViT-L/14", # Best quality, slowest
]
model, preprocess = clip.load("ViT-B/32")
| Model | Parameters | Speed | Quality |
|---|---|---|---|
| RN50 | 102M | Fast | Good |
| ViT-B/32 | 151M | Medium | Better |
| ViT-L/14 | 428M | Slow | Best |
Image-text similarity
python
# Compute embeddings
image_features = model.encode_image(image)
text_features = model.encode_text(text)
# Normalize
image_features /= image_features.norm(dim=-1, keepdim=True)
text_features /= text_features.norm(dim=-1, keepdim=True)
# Cosine similarity
similarity = (image_features @ text_features.T).item()
print(f"Similarity: {similarity:.4f}")
Semantic image search
python
# Index images
image_paths = ["img1.jpg", "img2.jpg", "img3.jpg"]
image_embeddings = []
for img_path in image_paths:
image = preprocess(Image.open(img_path)).unsqueeze(0).to(device)
with torch.no_grad():
embedding = model.encode_image(image)
embedding /= embedding.norm(dim=-1, keepdim=True)
image_embeddings.append(embedding)
image_embeddings = torch.cat(image_embeddings)
# Search with text query
query = "a sunset over the ocean"
text_input = clip.tokenize([query]).to(device)
with torch.no_grad():
text_embedding = model.encode_text(text_input)
text_embedding /= text_embedding.norm(dim=-1, keepdim=True)
# Find most similar images
similarities = (text_embedding @ image_embeddings.T).squeeze(0)
top_k = similarities.topk(3)
for idx, score in zip(top_k.indices, top_k.values):
print(f"{image_paths[idx]}: {score:.3f}")
Content moderation
python
# Define categories
categories = [
"safe for work",
"not safe for work",
"violent content",
"graphic content"
]
text = clip.tokenize(categories).to(device)
# Check image
with torch.no_grad():
logits_per_image, _ = model(image, text)
probs = logits_per_image.softmax(dim=-1)
# Get classification
max_idx = probs.argmax().item()
max_prob = probs[0, max_idx].item()
print(f"Category: {categories[max_idx]} ({max_prob:.2%})")
Batch processing
python
# Process multiple images
images = [preprocess(Image.open(f"img{i}.jpg")) for i in range(10)]
images = torch.stack(images).to(device)
with torch.no_grad():
image_features = model.encode_image(images)
image_features /= image_features.norm(dim=-1, keepdim=True)
# Batch text
texts = ["a dog", "a cat", "a bird"]
text_tokens = clip.tokenize(texts).to(device)
with torch.no_grad():
text_features = model.encode_text(text_tokens)
text_features /= text_features.norm(dim=-1, keepdim=True)
# Similarity matrix (10 images × 3 texts)
similarities = image_features @ text_features.T
print(similarities.shape) # (10, 3)
Integration with vector databases
python
# Store CLIP embeddings in Chroma/FAISS
import chromadb
client = chromadb.Client()
collection = client.create_collection("image_embeddings")
# Add image embeddings
for img_path, embedding in zip(image_paths, image_embeddings):
collection.add(
embeddings=[embedding.cpu().numpy().tolist()],
metadatas=[{"path": img_path}],
ids=[img_path]
)
# Query with text
query = "a sunset"
text_embedding = model.encode_text(clip.tokenize([query]))
results = collection.query(
query_embeddings=[text_embedding.cpu().numpy().tolist()],
n_results=5
)
Best practices
- •Use ViT-B/32 for most cases - Good balance
- •Normalize embeddings - Required for cosine similarity
- •Batch processing - More efficient
- •Cache embeddings - Expensive to recompute
- •Use descriptive labels - Better zero-shot performance
- •GPU recommended - 10-50× faster
- •Preprocess images - Use provided preprocess function
Performance
| Operation | CPU | GPU (V100) |
|---|---|---|
| Image encoding | ~200ms | ~20ms |
| Text encoding | ~50ms | ~5ms |
| Similarity compute | <1ms | <1ms |
Limitations
- •Not for fine-grained tasks - Best for broad categories
- •Requires descriptive text - Vague labels perform poorly
- •Biased on web data - May have dataset biases
- •No bounding boxes - Whole image only
- •Limited spatial understanding - Position/counting weak
Resources
- •GitHub: https://github.com/openai/CLIP ⭐ 25,300+
- •Paper: https://arxiv.org/abs/2103.00020
- •Colab: https://colab.research.google.com/github/openai/clip/
- •License: MIT