blip-2-vision-language

# 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

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

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

### Basic image captioning

```python
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

```python
# 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

```python
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

| Model | LLM Backend | Size | Use Case |
|-------|-------------|------|----------|
| `blip2-opt-2.7b` | OPT-2.7B | ~4GB | General captioning, VQA |
| `blip2-opt-6.7b` | OPT-6.7B | ~8GB | Better reasoning |
| `blip2-flan-t5-xl` | FlanT5-XL | ~5GB | Instruction following |
| `blip2-flan-t5-xxl` | FlanT5-XXL | ~13GB | Best quality |

### Q-Former components

| Component | Description | Parameters |
|-----------|-------------|------------|
| Learned queries | Fixed set of learnable embeddings | 32 × 768 |
| Image transformer | Cross-attention to vision features | ~108M |
| Text transformer | Self-attention for text | ~108M |
| Linear projection | Maps to LLM dimension | Varies |

## Advanced usage

### Batch processing

```python
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: