BLIP-2: Vision-Language Pre-training

Comprehensive guide to using Salesforce's BLIP-2 for vision-language tasks with frozen image encoders and large language models.

When to use BLIP-2

Use BLIP-2 when:

• Need high-quality image captioning with natural descriptions
• Building visual question answering (VQA) systems
• Require zero-shot image-text understanding without task-specific training
• Want to leverage LLM reasoning for visual tasks
• Building multimodal conversational AI
• Need image-text retrieval or matching

Key features:

• Q-Former architecture: Lightweight query transformer bridges vision and language
• Frozen backbone efficiency: No need to fine-tune large vision/language models
• Multiple LLM backends: OPT (2.7B, 6.7B) and FlanT5 (XL, XXL)
• Zero-shot capabilities: Strong performance without task-specific training
• Efficient training: Only trains Q-Former (~188M parameters)
• State-of-the-art results: Beats larger models on VQA benchmarks

Use alternatives instead:

• LLaVA: For instruction-following multimodal chat
• InstructBLIP: For improved instruction-following (BLIP-2 successor)
• GPT-4V/Claude 3: For production multimodal chat (proprietary)
• CLIP: For simple image-text similarity without generation
• Flamingo: For few-shot visual learning

Quick start

Installation

# HuggingFace Transformers (recommended)
pip install transformers accelerate torch Pillow

# Or LAVIS library (Salesforce official)
pip install salesforce-lavis

Basic image captioning

import torch
from PIL import Image
from transformers import Blip2Processor, Blip2ForConditionalGeneration

# Load model and processor
processor = Blip2Processor.from_pretrained("Salesforce/blip2-opt-2.7b")
model = Blip2ForConditionalGeneration.from_pretrained(
    "Salesforce/blip2-opt-2.7b",
    torch_dtype=torch.float16,
    device_map="auto"
)

# Load image
image = Image.open("photo.jpg").convert("RGB")

# Generate caption
inputs = processor(images=image, return_tensors="pt").to("cuda", torch.float16)
generated_ids = model.generate(**inputs, max_new_tokens=50)
caption = processor.batch_decode(generated_ids, skip_special_tokens=True)[0]
print(caption)

Visual question answering

# Ask a question about the image
question = "What color is the car in this image?"

inputs = processor(images=image, text=question, return_tensors="pt").to("cuda", torch.float16)
generated_ids = model.generate(**inputs, max_new_tokens=50)
answer = processor.batch_decode(generated_ids, skip_special_tokens=True)[0]
print(answer)

Using LAVIS library

import torch
from lavis.models import load_model_and_preprocess
from PIL import Image

# Load model
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
model, vis_processors, txt_processors = load_model_and_preprocess(
    name="blip2_opt",
    model_type="pretrain_opt2.7b",
    is_eval=True,
    device=device
)

# Process image
image = Image.open("photo.jpg").convert("RGB")
image = vis_processors["eval"](image).unsqueeze(0).to(device)

# Caption
caption = model.generate({"image": image})
print(caption)

# VQA
question = txt_processors["eval"]("What is in this image?")
answer = model.generate({"image": image, "prompt": question})
print(answer)

Core concepts

Architecture overview

BLIP-2 Architecture:
┌─────────────────────────────────────────────────────────────┐
│                        Q-Former                              │
│  ┌─────────────────────────────────────────────────────┐    │
│  │     Learned Queries (32 queries × 768 dim)          │    │
│  └────────────────────────┬────────────────────────────┘    │
│                           │                                  │
│  ┌────────────────────────▼────────────────────────────┐    │
│  │    Cross-Attention with Image Features               │    │
│  └────────────────────────┬────────────────────────────┘    │
│                           │                                  │
│  ┌────────────────────────▼────────────────────────────┐    │
│  │    Self-Attention Layers (Transformer)               │    │
│  └────────────────────────┬────────────────────────────┘    │
└───────────────────────────┼─────────────────────────────────┘
                            │
┌───────────────────────────▼─────────────────────────────────┐
│  Frozen Vision Encoder    │      Frozen LLM                  │
│  (ViT-G/14 from EVA-CLIP) │      (OPT or FlanT5)            │
└─────────────────────────────────────────────────────────────┘

Model variants

Q-Former components

Advanced usage

Batch processing

from PIL import Image
import torch

# Load multiple images
images = [Image.open(f"image_{i}.jpg").convert("RGB") for i in range(4)]
questions = [
    "What is shown in this image?",
    "Describe the scene.",
    "What colors are prominent?",
    "Is there a person in this image?"
]

# Process batch
inputs = processor(
    images=images,
    text=questions,
    return_tensors="pt",
    padding=True
).to("cuda", torch.float16)

# Generate
generated_ids = model.generate(**inputs, max_new_tokens=50)
answers = processor.batch_decode(generated_ids, skip_special_tokens=True)

for q, a in zip(questions, answers):
    print(f"Q: {q}\nA: {a}\n")

Controlling generation

# Control generation parameters
generated_ids = model.generate(
    **inputs,
    max_new_tokens=100,
    min_length=20,
    num_beams=5,              # Beam search
    no_repeat_ngram_size=2,   # Avoid repetition
    top_p=0.9,                # Nucleus sampling
    temperature=0.7,          # Creativity
    do_sample=True,           # Enable sampling
)

# For deterministic output
generated_ids = model.generate(
    **inputs,
    max_new_tokens=50,
    num_beams=5,
    do_sample=False,
)

Memory optimization

# 8-bit quantization
from transformers import BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_8bit=True)

model = Blip2ForConditionalGeneration.from_pretrained(
    "Salesforce/blip2-opt-6.7b",
    quantization_config=quantization_config,
    device_map="auto"
)

# 4-bit quantization (more aggressive)
quantization_config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_compute_dtype=torch.float16
)

model = Blip2ForConditionalGeneration.from_pretrained(
    "Salesforce/blip2-flan-t5-xxl",
    quantization_config=quantization_config,
    device_map="auto"
)

Image-text matching

# Using LAVIS for ITM (Image-Text Matching)
from lavis.models import load_model_and_preprocess

model, vis_processors, txt_processors = load_model_and_preprocess(
    name="blip2_image_text_matching",
    model_type="pretrain",
    is_eval=True,
    device=device
)

image = vis_processors["eval"](raw_image).unsqueeze(0).to(device)
text = txt_processors["eval"]("a dog sitting on grass")

# Get matching score
itm_output = model({"image": image, "text_input": text}, match_head="itm")
itm_scores = torch.nn.functional.softmax(itm_output, dim=1)
print(f"Match probability: {itm_scores[:, 1].item():.3f}")

Feature extraction

# Extract image features with Q-Former
from lavis.models import load_model_and_preprocess

model, vis_processors, _ = load_model_and_prepro