koboldcpp/tools/mtmd/README-dev.md

libmtmd dev guide

History

Please refer to 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: MulTiMoDal
• 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

Audio generation support

Audio generation is added to mtmd in PR #26254

Currently, we support the 3-stage pipeline below which should cover most TTS models:

• Stage 1: Backbone / Semantic Stage: Backbone model accepts text prompt and reference voice as input
• Stage 2: Acoustic Detail Generator: A model takes the hidden state from backbone and generate audio details (usually as audio codes or mel-spectrogram)
• Stage 3: Waveform Reconstruction: Convert the semantic and acoustic data from previous stages to the final waveform

For example, Qwen3-TTS:

• Reference voice is encoded using ECAPA-TDNN speaker encoder (speaker_encoder)
• Text prompt and reference voice are processed via a backbone (talker.model)
• A model converts sampled semantic token and hidden state from stage 2 into a list of 15 acoustic codes (talker.code_predictor)
• 16 generated codes are converted into waveform (code2wav)

API design constraints

Due to wide variety of audio generation pipelines, the mtmd_gen_audio system is designed to be flexible and reusable by new models.

mtmd_gen_audio is split into 2 main API:

• Core API mtmd.h: handles main inference. Important: the API surface must be stateless; caller must handle state management and audio frame accumulation.
• Helper API mtmd-helper.h: provides a model-agnostic stateful API. Usage example can be found in the tools/tts directory.

Checklist for porting new audio generation models to mtmd

1. Make sure to consult merged PRs about adding new TTS models, especially reviewer comments
   • Example: https://github.com/ggml-org/llama.cpp/pulls?q=is%3Apr+mtmd+tts+is%3Amerged
2. Establish a list of reusable and missing components from the current mtmd implementation.
3. For GGUF conversion:
   • Backbone model should be converted to a normal text model (loadable via libllama)
     • If model used hard-coded embedding row ID, append them to token embeddings and assign token name for them (see qwen3tts.py)
     • If model have a specific output logits head for audio codes (usually semantic code), keep the head as-is and pad the logits at inference time (see src/models/qwen3vl.cpp)
   • Sidecar models (code2wav, bigvgan, etc) must live inside the mmproj GGUF (but can be in different clip_context if necessary)
     • Note: it should use ggml_build_forward_select to select graphs if multiple graphs living in the same context
   • Reuse existing GGUF metadata key name and tensor name whenever possible; think twice before adding extensive changes to GGUF writer. For example, Qwen3-TTS hard-code part of the hparams to clip.cpp as they won't likely to change.
   • For tensor naming:
     • Prefixed with a.* for tensors used by speaker encoder pipeline
     • Prefixed with a.gen.* for generation stages (code / mel-spectrogram / PCM generation)
   • For GGUF metadata:
     • Reuse as many existing keys as possible
     • In most cases, you can hard-code model configs in the model graph class, or in clip_hparams
     • If some values need to be exposed to the mtmd_helper layer, hard-code them in mtmd_helper and distinguish by pipeline and mtmd_gen_audio_info::model_variant if necessary
     • Do NOT add new GGUF metadata or new fields to mtmd_gen_audio_info unless you can prove that you absolutely need them
4. Make sure most of the changes happen inside mtmd-helper-gen.cpp. A good PR looks like this:
   • 10-20% changes is to add new backbone (text) model and conversion
   • 60% changes inside mtmd-helper-gen.cpp
   • 10% changes inside libmtmd and clip.cpp systems
   • The rest downstream code (CLI, server) should have no changes at all
5. Update usage documentation in tools/tts/README.md

IMPORTANT: If your model needs changes that don't fit the existing infrastructure, open an issue first for discussion.

No-go checklist (these will get the PR rejected and require discussion before proceeding):

• Violating the API design constraints stated above
• Adding a new model-specific binary: the API and binary surface must stay model-agnostic