Systems

This section walks the architectural building blocks of the application. Each page describes a single subsystem in modules/ (or, in the case of Lora, a single built-in extension) and how to extend or modify it.

The grouping mirrors how the code actually behaves at runtime, not the directory layout — there is no modules/processing/ package, but "the processing pipeline" is a clear conceptual unit spanning modules/processing.py, modules/sd_samplers*.py, and parts of modules/scripts.py.

System	Owns	Page
UI	Gradio Blocks construction; the txt2img/img2img/Extras/Settings tabs	ui.md
Processing pipeline	The process_images loop, StableDiffusionProcessing dataclasses	processing.md
Models	Checkpoint discovery, loading, switching, hashing, VAE	models.md
Samplers and schedulers	Wrappers around k-diffusion / LCM / timestep samplers; sigma schedules	samplers-and-schedulers.md
SD hijack	Monkey-patches on the ldm package for prompt syntax and attention	sd-hijack.md
Scripts and extensions	The plugin system (alwayson, txt2img-only, img2img-only)	scripts-and-extensions.md
Script callbacks	The 30+ named hooks that scripts and extensions subscribe to	script-callbacks.md
Extra networks	Lora/embedding/hypernetwork browser and prompt syntax	extra-networks.md
Postprocessing	Upscalers, face restoration, the Extras tab	postprocessing.md

Active contributors across these systems (top-of-history): AUTOMATIC1111, w-e-w, Aarni Koskela, Kohaku-Blueleaf, catboxanon, light-and-ray, missionfloyd, brkirch.

How they fit together

graph TD
    UI[ui.py / ui_*.py] -->|StableDiffusionProcessing| Processing
    API[modules/api/api.py] -->|StableDiffusionProcessing| Processing
    Processing -->|loads| Models
    Processing -->|create_sampler| Samplers
    Processing -->|run| Scripts
    Models -->|hijack| SDHijack[sd_hijack.py]
    SDHijack --> Samplers
    Scripts -->|on_*| ScriptCallbacks
    Scripts -->|extra_network_data| ExtraNetworks
    ExtraNetworks -->|patch UNet| Models
    Processing -->|after sampling| Postprocessing
    Postprocessing -->|saves| Images[images.py]

The most central files in the repository are modules/shared.py (process-wide globals), modules/processing.py (the pipeline), modules/scripts.py (the script runtime), and modules/script_callbacks.py (the extension API). Almost every other module imports at least one of those.