pydiffuse

A python library for generating media with diffusion.

Development

The project uses uv. With it installed:

git clone git@github.com:samirelanduk/pydiffuse.git
cd pydiffuse
uv sync
pre-commit install

uv sync creates the virtualenv in .venv and installs the project with its dev dependencies from the lockfile. pre-commit install sets up the ruff check and format hooks, which CI also enforces.

Then, to run the checks:

uv run python -m unittest discover
uv run pyright
uv run pre-commit run --all-files

CLI

The library exposes most of its functionality as a Command-Line Interface.

pydiffuse <command>

CLIP

CLIP turns a text prompt into a set of vectors that represent its semantic meaning, which diffusion models use to condition image generation. The process is split into three commands - tokenising, embedding, and encoding - each of which writes its output to a file that the next one reads. See docs/clip.md for an explanation of what each stage does.

The embedding and encoding steps need model weights in safetensors format - typically a Stable Diffusion checkpoint, whose CLIP text encoder tensors are stored under cond_stage_model.

Tokenising

tokenize converts a prompt into the integer token IDs that CLIP uses.

pydiffuse tokenize "a photo of a lighthouse"

This writes two files. tokens.json is a list of lists of token IDs, where each inner list is a chunk of exactly 77 tokens - prompts longer than that are split over as many chunks as they need, and the last chunk is padded:

[[49406, 320, 1125, 539, 320, 13717, 49407, 49407, ...]]

mappings.json has the same shape, but pairs each token ID with the substring it came from, so you can see how the prompt was split up:

[[["<|startoftext|>", 49406], ["a</w>", 320], ["photo</w>", 1125], ...]]

The library bundles the standard CLIP tokenizer, so --tokenizer is only needed if your model was trained with a different vocabulary.

Embedding

embed looks up the vector for each token, and adds the vector for its position in the chunk. The result represents each token in isolation, with no context from the rest of the prompt.

pydiffuse embed tokens.json model.safetensors

This writes embedding.pt, a torch.save-d tensor of shape (chunks, 77, width), where width is the embedding width of the model (768 for Stable Diffusion 1.x). The model must contain tensors whose keys end in token_embedding.weight and position_embedding.weight, and it must use the same vocabulary as the tokenizer that produced the tokens.

Encoding

encode runs the embeddings through the CLIP transformer layers, so that each vector is adjusted by the tokens before it. This is the conditioning that gets fed to a diffusion model.

pydiffuse encode embedding.pt model.safetensors

This writes conditioning.pt, a tensor of the same shape as the embedding. The model must contain the attention, MLP and layer norm tensors for every encoder layer under cond_stage_model, plus the final layer norm - any number of layers is supported.