ai-generated-image-detector

🔍 AI-Generated Image Detector

Multi-Branch Frequency-Aware Detector: SwinV2 + SRM + DCT + FFT

A robust AI-generated image detector that combines semantic understanding with frequency-domain forensic analysis to detect AI-generated images from any source — including high-quality outputs from Stable Diffusion, DALL-E, Midjourney, Flux, and 4,800+ other generators.

🏗️ Architecture

This model uses a novel 4-branch fusion architecture for maximum detection robustness:

Input Image (256×256)
         │
    ┌────┼─────────────┬──────────────┬──────────────┐
    │    │             │              │              │
    ▼    ▼             ▼              ▼              ▼
 SwinV2  SRM HPF   DCT Analyzer  FFT Analyzer
 (768d)  (256d)     (22d)         (36d)
    │    │             │              │
    │    └─────────────┼──────────────┘
    │           Freq Features (314d)
    │                  │
    │           Freq Projection (128d)
    │                  │
    └──────────────────┘
              │
        Fusion MLP (896d → 512 → 128 → 2)
              │
        Real / AI-Generated

Branch 1: SwinV2-Tiny Backbone (Semantic Features)

• Pretrained microsoft/swinv2-tiny-patch4-window8-256
• Captures high-level semantic inconsistencies (e.g., unnatural textures, impossible geometry)
• 768-dimensional feature vector

Branch 2: SRM High-Pass Filter Bank (Forensic Residuals)

• 30 fixed Spatial Rich Model (SRM) filters from image forensics literature
• Based on Fridrich & Kodovsky (2012) — the gold standard in steganalysis
• Includes: 1st/2nd/3rd order derivatives, Laplacians, SPAM filters, edge detectors, Gabor-like directional filters
• Detects subtle manipulation artifacts invisible in RGB space
• Zero learnable parameters in the filter bank → maximum generalization
• Processed through a lightweight CNN encoder (30→64→128→256 channels)

Branch 3: DCT Frequency Band Analysis

• 2D Discrete Cosine Transform on 32×32 image patches
• Extracts 8 frequency band energy statistics (mean + std per band)
• Computes spectral centroid (center of mass of frequency distribution)
• Measures high-to-low frequency energy ratio — AI images often have anomalous ratios
• Captures DC component statistics across patches
• 22-dimensional feature vector

Branch 4: FFT Radial Power Spectrum

• 2D Fast Fourier Transform with Hanning window (reduces spectral leakage)
• Azimuthally averaged power spectrum in 32 radial bins
• Measures deviation from natural 1/f² power law — natural images follow this law, AI-generated images deviate
• Extracts: log spectrum, spectral slope, intercept, residual std, residual max
• Detects upsampling artifacts and periodic patterns from generator architectures
• 36-dimensional feature vector

Fusion

• Frequency features (SRM + DCT + FFT = 314d) → projected to 128d
• Concatenated with SwinV2 semantic features (768d) → 896d
• MLP classifier with dropout (0.3, 0.1) and label smoothing (0.1)

Total parameters: ~28.6M (compact enough for real-time inference)

📊 Training Dataset

OwensLab/CommunityForensics-Small (CVPR 2025)

• 556,000 images (278K real + 278K AI-generated)
• 4,803 different AI generators — the most diverse training set ever used
• Real images from: LAION, ImageNet, COCO, FFHQ, CelebA, MetFaces, AFHQ, and more
• AI images from: All Stable Diffusion variants, DeepFloyd, StyleGAN 1/2/3, BigGAN, VQDM, and thousands of community models

Social Media Robustness Augmentation

During training, images are augmented with:

• Random JPEG compression (QF 30-95) — simulates Instagram/Twitter/WhatsApp compression
• Gaussian blur (σ 0.1-2.0) — simulates re-encoding artifacts
• Downscale-upscale (0.5x-0.9x) — simulates re-upload quality loss
• Standard color jitter, random crops, and horizontal flips

🚀 Training

Requirements

pip install transformers torch torchvision datasets evaluate accelerate trackio pillow scikit-learn

Run Training

# Full training on GPU (recommended: A10G 24GB or better)
python train.py \
    --num_train_epochs 5 \
    --per_device_train_batch_size 16 \
    --gradient_accumulation_steps 4 \
    --learning_rate 2e-5 \
    --hub_model_id your-username/ai-image-detector

# Quick test run
python train.py --test_mode

# Custom settings
python train.py \
    --max_train_samples 50000 \
    --num_train_epochs 3 \
    --per_device_train_batch_size 8 \
    --image_size 256

Training Hyperparameters

🔮 Inference

Single Image

import torch
from train import FrequencyAwareDetector
from torchvision.transforms import Compose, Resize, CenterCrop, ToTensor, Normalize
from PIL import Image

# Load model
model = FrequencyAwareDetector()
state_dict = torch.load("model_state_dict.pt", map_location="cpu")
model.load_state_dict(state_dict)
model.eval()

# Preprocess
transform = Compose([
    Resize((288, 288)),
    CenterCrop((256, 256)),
    ToTensor(),
    Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225]),
])

img = Image.open("test.jpg").convert("RGB")
pixel_values = transform(img).unsqueeze(0)

# Predict
with torch.no_grad():
    output = model(pixel_values=pixel_values)
    probs = torch.softmax(output["logits"], dim=1)
    pred = probs.argmax(dim=1).item()

labels = {0: "Real", 1: "AI-Generated"}
print(f"Prediction: {labels[pred]} ({probs[0][pred]:.2%} confidence)")

Command Line

# Single image
python inference.py --image photo.jpg

# URL
python inference.py --image https://example.com/image.png

# Batch (entire directory)
python inference.py --image_dir ./photos/

📚 Scientific Background

Why Frequency Analysis?

AI-generated images contain subtle artifacts that are invisible to the human eye but detectable in the frequency domain:

1. Upsampling Artifacts: Diffusion models and GANs use transposed convolutions and upsampling layers that leave periodic patterns in the frequency spectrum
2. 1/f² Deviation: Natural images follow a characteristic 1/f² power spectrum (Fourier). AI images deviate from this, especially at mid-to-high frequencies
3. DCT Block Patterns: The generation process creates non-natural distributions of DCT coefficients across image patches
4. Noise Residuals: SRM filters reveal that AI images have fundamentally different noise patterns than camera-captured images

Key References

1. AIDE (2024): "A Sanity Check for AI-generated Image Detection" — arxiv:2406.19435. DCT patch selection + SRM + CLIP fusion achieves 92.77% on AIGCDetectBenchmark.
2. CommunityForensics (CVPR 2025): "Using Thousands of Generators to Train Fake Image Detectors" — arxiv:2411.04125. Training on diverse generators (4803+) dramatically improves cross-generator generalization.
3. SRM Filters: Fridrich & Kodovsky (2012) — "Rich Models for Steganalysis of Digital Images". The standard filter bank for image forensics.
4. UnivFD: Ojha et al. (2023) — "Towards Universal Fake Image Detectors". CLIP features for zero-shot detection.

📁 Repository Structure

├── train.py          # Full training script with model architecture
├── inference.py      # Easy-to-use inference script
├── detector_config.json  # Model configuration
├── model_state_dict.pt   # Trained weights (after training)
└── README.md         # This file

License

Apache 2.0