backend/llama.cpp/tools/mtmd/README-dev.md

# libmtmd dev guide

## History

Please refer to [multimodal.md](../../docs/multimodal.md) for a broader context.

In short:
- `libmtmd` started as a wrapper around `libllava` / `clip.cpp`
- Various components that used to be in `clip.cpp` are moved progressively to mtmd. For example, preprocessor is now part of mtmd

## Terminologies

- mtmd: **M**ul**T**i**M**o**D**al
- bitmap: representing a raw input data, for example: RGB image, PCM audio
- tiles / slices: for llava-uhd-style models, the preprocessor breaks a large input into smaller square images called tiles or slices
- chunk: a mtmd_input_chunk represents a preprocessed input that can then be passed through `mtmd_encode()`

## Pipeline

A typical pipeline of the core libmtmd is as follows:
- A bitmap (RGB image or PCM audio) is created
- Bitmap and the text prompt is provided to `mtmd_tokenize()` that breaks the input into chunks
    - The tokenizer function first expands a "lazy" bitmap if it finds one. Typically, this is used by video, so that one media token corresponds to one input bitmap
    - For models that support "fused" temporal frames like Qwen-VL, the tokenizer tries to merge pair of consecutive frames into one batch
    - The preprocessor will then be called, which produces a list of chunks
    - Depending on the model itself, special tokens will be injected to separate image chunks (i.e. llava-uhd-style models)
- Multiple bitmaps may be batched together to form a larger `mtmd_batch()`
- Single image or batch is encoded, via `mtmd_encode()` or `mtmd_batch_encode()`
- Get the output embeddings

## Helper

We provide a set of helper functions via `mtmd_helper` to make using libmtmd easier. The helper provides:
- Image, audio and video file decoding (for example, decode raw JPEG into RGB bitmap)
- Manage `llama_batch` and calls to `llama_decode`