mlx-video/mlx_video/models/ltx_2/README.md

# LTX-2 for MLX

MLX port of [LTX-2](https://huggingface.co/Lightricks/LTX-2), a 19B parameter video generation model from Lightricks with synchronized audio-video support.

## Pipelines

Four pipeline types are available via the `--pipeline` flag:

| Pipeline | Description | CFG | Stages | Speed |
|----------|-------------|-----|--------|-------|
| `distilled` (default) | Fixed sigma schedule, no CFG | No | 2 (8+3 steps) | Fastest |
| `dev` | Dynamic sigmas, constant CFG | Yes | 1 (30 steps) | Medium |
| `dev-two-stage` | Dev + LoRA refinement | Yes (stage 1) | 2 (30+3 steps) | Slow |
| `dev-two-stage-hq` | res_2s sampler + LoRA both stages | Yes (stage 1) | 2 (15+3 steps) | Slow, highest quality |

## Usage

### Text-to-Video (T2V)

```bash
# Distilled (default) - fast, two-stage
uv run mlx_video.generate --prompt "Two dogs wearing sunglasses, cinematic, sunset" -n 97 --width 768

# Dev - single-stage with CFG
uv run mlx_video.generate --pipeline dev --prompt "A cinematic scene" --cfg-scale 3.0

# Dev two-stage - dev + LoRA refinement
uv run mlx_video.generate --pipeline dev-two-stage \
    --prompt "Two dogs of the poodle breed wearing sunglasses, close up, cinematic, sunset" \
    -n 145 --width 1024 --height 768 \
    --model-repo prince-canuma/LTX-2-dev \
    --cfg-scale 3.0 --lora-strength 0.8 \
    --enhance-prompt

# Dev two-stage HQ - res_2s sampler, LoRA both stages (highest quality)
uv run mlx_video.generate --pipeline dev-two-stage-hq \
    --prompt "A cinematic scene of ocean waves at golden hour" \
    --model-repo prince-canuma/LTX-2-dev

# HQ with custom LoRA strengths
uv run mlx_video.generate --pipeline dev-two-stage-hq \
    --prompt "A sunset over mountains" \
    --model-repo prince-canuma/LTX-2-dev \
    --lora-strength-stage-1 0.3 --lora-strength-stage-2 0.6
```

### Image-to-Video (I2V)

```bash
# Distilled I2V
uv run mlx_video.generate --prompt "A person dancing" --image photo.jpg

# Dev I2V
uv run mlx_video.generate --pipeline dev --prompt "Waves crashing" --image beach.png --cfg-scale 3.5
```

### Audio-to-Video (A2V)

Generate video conditioned on an input audio file. Works with all four pipelines. The audio is encoded to latent space and frozen during denoising -- the transformer's cross-attention reads the audio signal to guide video generation.

```bash
# A2V - distilled (default, fastest)
uv run mlx_video.generate --audio-file music.wav --prompt "A band playing music"

# A2V - dev (single-stage with CFG)
uv run mlx_video.generate --pipeline dev --audio-file ocean.wav --prompt "Ocean waves"

# A2V - dev-two-stage (dev + LoRA refinement)
uv run mlx_video.generate --pipeline dev-two-stage --audio-file music.wav \
    --prompt "A band playing music" --model-repo prince-canuma/LTX-2-dev

# A2V - dev-two-stage-hq (highest quality)
uv run mlx_video.generate --pipeline dev-two-stage-hq --audio-file music.wav \
    --prompt "A band playing music" --model-repo prince-canuma/LTX-2-dev

# A2V + I2V (audio + image conditioning)
uv run mlx_video.generate --audio-file rain.wav --image forest.jpg --prompt "Rain in forest"

# A2V with custom start time
uv run mlx_video.generate --audio-file song.mp3 --audio-start-time 30.0 --prompt "Concert"
```

> **Note:** `--audio-file` (A2V) and `--audio` (generate audio) are mutually exclusive. Supported formats: WAV, FLAC, MP3, OGG, and video files with audio tracks.

### Audio-Video Generation (experimental)

Generate synchronized audio alongside video from scratch:

```bash
uv run mlx_video.generate --prompt "Ocean waves crashing" --audio
uv run mlx_video.generate --pipeline dev --prompt "A jazz band playing" --audio --enhance-prompt

# With full guidance (STG + modality_scale, matches PyTorch defaults)
uv run mlx_video.generate --pipeline dev --prompt "Ocean waves crashing" --audio \
    --stg-scale 1.0 --stg-blocks 29 --modality-scale 3.0
```

### LoRA

LoRA weights can be loaded from a file, directory, or HuggingFace repo:

```bash
# From HuggingFace repo
uv run mlx_video.generate --pipeline dev-two-stage \
    --prompt "Camera dolly out of a forest" \
    --lora-path Lightricks/LTX-2-19b-LoRA-Camera-Control-Dolly-Out \
    --lora-strength 1.0

# From local file
uv run mlx_video.generate --pipeline dev-two-stage \
    --prompt "A scene" \
    --lora-path ./my-lora/weights.safetensors

# From local directory (auto-detects .safetensors file)
uv run mlx_video.generate --pipeline dev-two-stage \
    --prompt "A scene" \
    --lora-path ./LTX-2-distilled/lora
```

### Upscaling

```bash
# Upscale an image 2x
uv run mlx_video.upscale --input photo.png --output upscaled.png

# Upscale a video 2x
uv run mlx_video.upscale --input video.mp4 --output upscaled.mp4

# Upscale with refinement (higher quality, requires text prompt)
uv run mlx_video.upscale --input video.mp4 --output upscaled.mp4 --refine --prompt "A cinematic scene"
```

## CLI Options

### General

| Option | Default | Description |
|--------|---------|-------------|
| `--prompt`, `-p` | (required) | Text description of the video |
| `--pipeline` | `distilled` | Pipeline type: `distilled`, `dev`, `dev-two-stage`, or `dev-two-stage-hq` |
| `--height`, `-H` | 512 | Output height (divisible by 64 for two-stage, 32 for dev) |
| `--width`, `-W` | 512 | Output width (divisible by 64 for two-stage, 32 for dev) |
| `--num-frames`, `-n` | 33 | Number of frames (must be 1 + 8*k) |
| `--seed`, `-s` | 42 | Random seed for reproducibility |
| `--fps` | 24 | Frames per second |
| `--output-path`, `-o` | output.mp4 | Output video path |
| `--model-repo` | Lightricks/LTX-2 | HuggingFace model repository |
| `--text-encoder-repo` | None | Separate text encoder repo (if not in model repo) |
| `--save-frames` | false | Save individual frames as images |
| `--enhance-prompt` | false | Enhance prompt using Gemma |
| `--image`, `-i` | None | Conditioning image for I2V |
| `--image-strength` | 1.0 | Conditioning strength for I2V |
| `--audio`, `-a` | false | Enable synchronized audio generation |
| `--audio-file` | None | Path to audio file for A2V conditioning |
| `--audio-start-time` | 0.0 | Start time in seconds for audio file |
| `--tiling` | `auto` | VAE tiling mode: `auto`, `none`, `aggressive`, `conservative` |
| `--stream` | false | Stream frames as they decode |

### Dev / Dev-Two-Stage

| Option | Default | Description |
|--------|---------|-------------|
| `--steps` | 30 | Number of denoising steps |
| `--cfg-scale` | 3.0 | CFG guidance scale |
| `--cfg-rescale` | 0.7 | CFG rescale factor (reduces over-saturation) |
| `--negative-prompt` | (default) | Negative prompt for CFG |
| `--apg` | false | Use Adaptive Projected Guidance (more stable for I2V) |
| `--stg-scale` | 0.0 | STG scale (PyTorch default: 1.0, requires `--audio`) |
| `--stg-blocks` | None | Transformer blocks for STG ([29] for LTX-2, [28] for LTX-2.3) |
| `--modality-scale` | 1.0 | Cross-modal guidance scale (PyTorch default: 3.0, requires `--audio`) |

### Dev-Two-Stage LoRA

| Option | Default | Description |
|--------|---------|-------------|
| `--lora-path` | auto-detect | Path to LoRA file, directory, or HuggingFace repo |
| `--lora-strength` | 1.0 | LoRA merge strength |

### Dev-Two-Stage HQ

| Option | Default | Description |
|--------|---------|-------------|
| `--lora-strength-stage-1` | 0.25 | LoRA strength for stage 1 |
| `--lora-strength-stage-2` | 0.5 | LoRA strength for stage 2 |

HQ defaults: 15 steps (vs 30), `cfg-rescale` 0.45 (vs 0.7), STG disabled. Uses the res_2s second-order sampler (2 model evals per step) for better quality at the same compute budget.

## How It Works

### Distilled Pipeline (default)
1. **Stage 1**: Generate at half resolution with 8 denoising steps (fixed sigmas)
2. **Upsample**: 2x spatial upsampling via LatentUpsampler
3. **Stage 2**: Refine at full resolution with 3 denoising steps
4. **Decode**: VAE decoder converts latents to RGB video

### Dev Pipeline
1. **Generate**: Full resolution with configurable steps and constant CFG
2. **Decode**: VAE decoder converts latents to RGB video

### Dev Two-Stage Pipeline
1. **Stage 1**: Dev denoising at half resolution with CFG
2. **Upsample**: 2x spatial upsampling via LatentUpsampler
3. **Stage 2**: Distilled refinement at full resolution with LoRA weights (3 steps, no CFG)
4. **Decode**: VAE decoder converts latents to RGB video

### Dev Two-Stage HQ Pipeline
1. **Stage 1**: res_2s denoising at half resolution with CFG + LoRA@0.25 (15 steps, 2 evals/step)
2. **Upsample**: 2x spatial upsampling via LatentUpsampler
3. **Stage 2**: res_2s refinement at full resolution with LoRA@0.5 (3 steps, no CFG)
4. **Decode**: VAE decoder converts latents to RGB video

The res_2s sampler uses an exponential Rosenbrock-type Runge-Kutta integrator with SDE noise injection, producing higher quality results than Euler at the same compute budget (~30 total model evaluations).

### Audio-to-Video (A2V) Conditioning

A2V works by encoding input audio into the same latent space as generated audio, then **freezing** those latents during denoising:

1. Load audio file, resample to 16kHz, compute mel-spectrogram
2. `AudioEncoder(mel_spec)` produces audio latents `(B, 8, T, 16)`
3. Normalize via `PerChannelStatistics`
4. Freeze during denoising: `timesteps=0`, `sigma=0`, skip Euler/RK updates
5. Transformer's A2V cross-attention reads frozen audio to guide video generation
6. Output: denoised video + original input audio waveform (skip audio VAE decode)

## Converting Models

Convert original Lightricks/LTX-2 weights to the modular mlx-video format:

```bash
# Convert distilled model
uv run python -m mlx_video.models.ltx_2.convert \
    --source Lightricks/LTX-2 --output ./LTX-2-distilled --variant distilled

# Convert dev model
uv run python -m mlx_video.models.ltx_2.convert \
    --source Lightricks/LTX-2 --output ./LTX-2-dev --variant dev
```

This extracts 7 components from the monolithic checkpoint:

```
LTX-2-distilled/
├── transformer/          # DiT transformer (19B params)
├── vae/
│   ├── decoder/          # Video VAE decoder
│   └── encoder/          # Video VAE encoder
├── audio_vae/
│   ├── decoder/          # Audio VAE decoder
│   └── encoder/          # Audio VAE encoder
├── vocoder/              # Mel-spectrogram to waveform
└── text_projections/     # Text embedding projections
```

Pre-converted weights are available on HuggingFace:
- [prince-canuma/LTX-2-distilled](https://huggingface.co/prince-canuma/LTX-2-distilled)
- [prince-canuma/LTX-2-dev](https://huggingface.co/prince-canuma/LTX-2-dev)
- [prince-canuma/LTX-2.3-distilled](https://huggingface.co/prince-canuma/LTX-2.3-distilled)
- [prince-canuma/LTX-2.3-dev](https://huggingface.co/prince-canuma/LTX-2.3-dev)

## Model Specifications

- **Transformer**: 48 layers, 32 attention heads, 128 dim per head (19B parameters)
- **Latent channels**: 128
- **Patch size**: 4 (for VAE patchify/unpatchify)
- **Text encoder**: Gemma 3 with 3840-dim output
- **RoPE**: Split mode with double precision (LTX-2.3) or standard (LTX-2)
- **Audio VAE**: Encoder (~35M), Decoder (~50M), Vocoder (~13M)

### Audio VAE Architecture

```
Audio Encoder: mel-spectrogram -> latents (B, 8, T, 16)
  - Channel multipliers: (1, 2, 4)
  - ResNet blocks with optional attention
  - GroupNorm or PixelNorm normalization
  - Optional causal convolutions

Audio Decoder: latents -> mel-spectrogram
  - Mirrors encoder with upsampling path
  - Per-channel statistics for latent normalization

Vocoder: mel-spectrogram -> waveform (~13M params)
  - HiFi-GAN style architecture
  - Upsample rates: [6, 5, 2, 2, 2]
  - ResBlock1 with dilations [1, 3, 5]
```

## Project Structure

```
mlx_video/models/ltx_2/
├── __init__.py
├── config.py             # LTXModelConfig, AudioEncoderModelConfig, AudioDecoderModelConfig
├── convert.py            # Weight conversion from Lightricks/LTX-2
├── generate.py           # Unified generation pipeline (T2V, I2V, A2V, +Audio)
├── postprocess.py        # Video post-processing
├── samplers.py           # Euler and res_2s samplers
├── utils.py              # Shared utilities (get_model_path, load_safetensors, etc.)
├── ltx.py                # Main LTXModel (DiT transformer with AV support)
├── transformer.py        # Transformer blocks, Modality dataclass
├── attention.py          # Multi-head attention with RoPE
├── feed_forward.py       # Feed-forward layers
├── adaln.py              # Adaptive Layer Normalization
├── rope.py               # Rotary Position Embeddings (split/combined)
├── text_projection.py    # Text embedding projection
├── text_encoder.py       # Text encoder with AV embeddings support
├── upsampler.py          # LatentUpsampler for 2-stage generation
├── conditioning/
│   ├── keyframe.py       # Image-to-video keyframe conditioning
│   └── latent.py         # Video-to-video latent conditioning
├── video_vae/
│   ├── decoder.py        # VAE decoder with timestep conditioning
│   ├── encoder.py        # VAE encoder for image/video encoding
│   ├── convolution.py    # CausalConv3d, CausalConv2d
│   ├── ops.py            # patchify, unpatchify, PerChannelStatistics
│   ├── resnet.py         # ResBlock3D, ResBlockGroup
│   ├── sampling.py       # DepthToSpaceUpsample, SpaceToDepthDownsample
│   └── video_vae.py      # Full VAE (encoder + decoder)
└── audio_vae/
    ├── audio_vae.py      # Audio encoder and decoder
    ├── audio_processor.py # Mel-spectrogram computation (librosa)
    ├── vocoder.py        # Mel-spectrogram to waveform synthesis
    ├── ops.py            # AudioPatchifier, PerChannelStatistics
    ├── resnet.py         # ResNet blocks for audio
    ├── attention.py      # Attention blocks for audio VAE
    ├── normalization.py  # Normalization layers
    ├── causal_conv_2d.py # Causal 2D convolutions
    ├── downsample.py     # Downsampling layers
    └── upsample.py       # Upsampling layers
```

## LTX-2 vs LTX-2.3

LTX-2.3 introduces prompt-conditioned adaptive layer normalization (adaln):

| Feature | LTX-2 | LTX-2.3 |
|---------|--------|---------|
| AdaLN | Standard | Prompt-conditioned (`has_prompt_adaln=True`) |
| Attention gate | None | `2.0 * sigmoid(gate_logits)` |
| Scale-shift table | 6 params | 9 params (+ cross-attn Q) |
| Text encoder connectors | 2 blocks | 8 blocks with gate_logits |
| Feature extractor | V1 (batch-level) | V2 (per-token RMSNorm) |
| RoPE | Standard | Double precision |
| STG blocks | [29] | [28] |
| Text encoder repo | Included | Separate (`--text-encoder-repo`) |