---

name: ffmpeg-modal-containers description: Complete Modal.com FFmpeg deployment system for serverless video processing. PROACTIVELY activate for: (1) Modal.com FFmpeg container setup, (2) GPU-accelerated video encoding on Modal (NVIDIA, NVENC), (3) Parallel video processing with Modal map/starmap, (4) Volume mounting for large video files, (5) CPU vs GPU container cost optimization, (6) apt_install/pip_install for FFmpeg, (7) Python subprocess FFmpeg patterns, (8) Batch video transcoding at scale, (9) Modal pricing for video workloads, (10) Audio/video processing with Whisper. Provides: Image configuration examples, GPU container patterns, parallel processing code, volume usage, cost comparisons, production-ready FFmpeg deployments. Ensures: Efficient, scalable video processing on Modal serverless infrastructure.

Quick Reference

Container Type	Image Setup	GPU	Use Case
CPU (debian_slim)	.apt_install("ffmpeg")	No	Batch processing, I/O-bound tasks
GPU (debian_slim)	.apt_install("ffmpeg").pip_install("torch")	Yes	ML inference, not NVENC
GPU (CUDA image)	from_registry("nvidia/cuda:...")	Yes	Full CUDA toolkit, NVENC possible

GPU Type	Price/Hour	NVENC	Best For
T4	~$0.59	Yes (Turing)	Inference + encoding
A10G	~$1.10	Yes (Ampere)	4K encoding, ML
L40S	~$1.95	Yes (Ada)	Heavy ML + video
H100	~$4.25	Yes (Hopper)	Training, overkill for video

When to Use This Skill

Use for serverless video processing:

• Batch transcoding that needs to scale to hundreds of containers
• Parallel video processing with Modal's map/starmap
• GPU-accelerated encoding (with limitations on NVENC)
• Cost-effective burst processing (pay only for execution time)
• Integration with ML models (Whisper, video analysis)

Key decision: Modal excels at parallel CPU workloads and ML inference on GPU. For pure hardware NVENC encoding, verify GPU capabilities first.

---

FFmpeg on Modal.com (2025)

Complete guide to running FFmpeg on Modal's serverless Python platform with CPU and GPU containers.

Overview

Modal is a serverless platform for running Python code in the cloud with:

• Sub-second cold starts - Containers spin up in milliseconds
• Elastic GPU capacity - Access T4, A10G, L40S, H100 GPUs
• Parallel processing - Scale to thousands of containers instantly
• Pay-per-use - Billed by CPU cycle, not idle time

Modal vs Traditional Cloud

Feature	Modal	Traditional VMs
Cold start	<1 second	Minutes
Scaling	Automatic to 1000s	Manual setup
Billing	Per execution	Per hour
GPU access	gpu="any" decorator	Complex provisioning
Setup	Python decorators	Infrastructure as code

Basic FFmpeg Setup

CPU Container (Simplest)

import modal
import subprocess
from pathlib import Path

app = modal.App("ffmpeg-processor")

# Create image with FFmpeg installed
ffmpeg_image = modal.Image.debian_slim(python_version="3.12").apt_install("ffmpeg")

@app.function(image=ffmpeg_image)
def transcode_video(input_bytes: bytes, output_format: str = "mp4") -> bytes:
    """Transcode video to specified format."""
    import tempfile

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input"
        output_path = Path(tmpdir) / f"output.{output_format}"

        # Write input file
        input_path.write_bytes(input_bytes)

        # Run FFmpeg
        result = subprocess.run([
            "ffmpeg", "-y",
            "-i", str(input_path),
            "-c:v", "libx264",
            "-preset", "veryfast",
            "-crf", "23",
            "-c:a", "aac",
            "-b:a", "128k",
            "-movflags", "+faststart",
            str(output_path)
        ], capture_output=True, text=True)

        if result.returncode != 0:
            raise RuntimeError(f"FFmpeg error: {result.stderr}")

        return output_path.read_bytes()

@app.local_entrypoint()
def main():
    # Read local file
    video_bytes = Path("input.mp4").read_bytes()

    # Process remotely on Modal
    output_bytes = transcode_video.remote(video_bytes)

    # Save result locally
    Path("output.mp4").write_bytes(output_bytes)
    print("Transcoding complete!")

Running Your First Modal App

# Install Modal
pip install modal

# Authenticate (one-time)
modal setup

# Run the app
modal run your_script.py

GPU Containers

Basic GPU Setup for ML + FFmpeg

import modal

app = modal.App("ffmpeg-gpu")

# GPU image with FFmpeg and PyTorch
gpu_image = (
    modal.Image.debian_slim(python_version="3.12")
    .apt_install("ffmpeg")
    .pip_install("torch", "torchaudio", "transformers")
)

@app.function(image=gpu_image, gpu="T4")
def transcribe_and_process(audio_bytes: bytes) -> dict:
    """Transcribe audio with Whisper, then process with FFmpeg."""
    import tempfile
    import torch
    from transformers import pipeline

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input.mp3"
        input_path.write_bytes(audio_bytes)

        # GPU-accelerated transcription
        transcriber = pipeline(
            model="openai/whisper-base",
            device="cuda"
        )
        result = transcriber(str(input_path))

        # FFmpeg audio normalization (CPU-based in this setup)
        normalized_path = Path(tmpdir) / "normalized.mp3"
        subprocess.run([
            "ffmpeg", "-y",
            "-i", str(input_path),
            "-af", "loudnorm=I=-16:TP=-1.5:LRA=11",
            str(normalized_path)
        ], check=True)

        return {
            "transcription": result["text"],
            "normalized_audio": normalized_path.read_bytes()
        }

Full CUDA Toolkit for Advanced GPU Features

For NVENC or full CUDA toolkit requirements:

import modal

cuda_version = "12.4.0"
flavor = "devel"  # Full toolkit
os_version = "ubuntu22.04"
tag = f"{cuda_version}-{flavor}-{os_version}"

# Full CUDA image with FFmpeg
cuda_ffmpeg_image = (
    modal.Image.from_registry(f"nvidia/cuda:{tag}", add_python="3.12")
    .entrypoint([])  # Remove base image entrypoint
    .apt_install(
        "ffmpeg",
        "git",
        "libglib2.0-0",
        "libsm6",
        "libxrender1",
        "libxext6",
        "libgl1",
    )
    .pip_install("numpy", "Pillow")
)

app = modal.App("ffmpeg-cuda")

@app.function(image=cuda_ffmpeg_image, gpu="A10G")
def gpu_transcode(input_bytes: bytes) -> bytes:
    """Transcode video with GPU acceleration if available."""
    import subprocess
    import tempfile
    from pathlib import Path

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input.mp4"
        output_path = Path(tmpdir) / "output.mp4"

        input_path.write_bytes(input_bytes)

        # Check for NVENC support
        check_result = subprocess.run(
            ["ffmpeg", "-encoders"],
            capture_output=True,
            text=True
        )

        has_nvenc = "h264_nvenc" in check_result.stdout

        if has_nvenc:
            # GPU encoding with NVENC
            cmd = [
                "ffmpeg", "-y",
                "-hwaccel", "cuda",
                "-hwaccel_output_format", "cuda",
                "-i", str(input_path),
                "-c:v", "h264_nvenc",
                "-preset", "p4",
                "-cq", "23",
                "-c:a", "aac",
                "-b:a", "128k",
                str(output_path)
            ]
        else:
            # Fallback to CPU encoding
            cmd = [
                "ffmpeg", "-y",
                "-i", str(input_path),
                "-c:v", "libx264",
                "-preset", "veryfast",
                "-crf", "23",
                "-c:a", "aac",
                "-b:a", "128k",
                str(output_path)
            ]

        result = subprocess.run(cmd, capture_output=True, text=True)
        if result.returncode != 0:
            raise RuntimeError(f"FFmpeg error: {result.stderr}")

        return output_path.read_bytes()

Important Note on NVENC Support

Modal's GPU containers use NVIDIA GPUs primarily for ML inference. NVENC video encoding support depends on:

1. FFmpeg build - Must include --enable-nvenc
2. NVIDIA drivers - Must expose video encoding capabilities
3. Container setup - May require NVIDIA_DRIVER_CAPABILITIES=compute,video,utility

For guaranteed NVENC support, use a custom Docker image or verify with:

@app.function(image=cuda_ffmpeg_image, gpu="T4")
def check_nvenc():
    """Check NVENC availability."""
    import subprocess

    # Check GPU
    gpu_result = subprocess.run(["nvidia-smi"], capture_output=True, text=True)
    print("GPU Info:", gpu_result.stdout)

    # Check FFmpeg encoders
    enc_result = subprocess.run(
        ["ffmpeg", "-encoders"],
        capture_output=True,
        text=True
    )

    nvenc_encoders = [line for line in enc_result.stdout.split('\n') if 'nvenc' in line]
    print("NVENC Encoders:", nvenc_encoders)

    return {
        "has_nvenc": len(nvenc_encoders) > 0,
        "encoders": nvenc_encoders
    }

Parallel Video Processing

Modal's killer feature for video processing is parallel execution across many containers.

Batch Processing with map()

import modal
from pathlib import Path

app = modal.App("batch-transcode")

ffmpeg_image = modal.Image.debian_slim().apt_install("ffmpeg")

@app.function(image=ffmpeg_image, timeout=600)
def transcode_single(video_bytes: bytes, video_id: str) -> tuple[str, bytes]:
    """Transcode a single video."""
    import subprocess
    import tempfile

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input"
        output_path = Path(tmpdir) / "output.mp4"

        input_path.write_bytes(video_bytes)

        subprocess.run([
            "ffmpeg", "-y",
            "-i", str(input_path),
            "-c:v", "libx264",
            "-preset", "fast",
            "-crf", "23",
            "-c:a", "aac",
            str(output_path)
        ], check=True, capture_output=True)

        return video_id, output_path.read_bytes()

@app.local_entrypoint()
def main():
    # Prepare batch of videos
    video_files = list(Path("videos").glob("*.mp4"))
    inputs = [(f.read_bytes(), f.stem) for f in video_files]

    # Process all videos in parallel (up to 100 containers)
    results = list(transcode_single.starmap(inputs))

    # Save results
    for video_id, output_bytes in results:
        Path(f"output/{video_id}.mp4").write_bytes(output_bytes)
        print(f"Processed: {video_id}")

Frame-by-Frame Parallel Processing

For maximum parallelism, process frames independently:

import modal
from pathlib import Path

app = modal.App("parallel-frames")

ffmpeg_image = modal.Image.debian_slim().apt_install("ffmpeg")

@app.function(image=ffmpeg_image)
def extract_frames(video_bytes: bytes, fps: int = 1) -> list[bytes]:
    """Extract frames from video."""
    import subprocess
    import tempfile

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input.mp4"
        input_path.write_bytes(video_bytes)

        # Extract frames
        subprocess.run([
            "ffmpeg", "-y",
            "-i", str(input_path),
            "-vf", f"fps={fps}",
            f"{tmpdir}/frame_%04d.png"
        ], check=True, capture_output=True)

        # Read all frames
        frames = []
        for frame_path in sorted(Path(tmpdir).glob("frame_*.png")):
            frames.append(frame_path.read_bytes())

        return frames

@app.function(image=ffmpeg_image)
def process_frame(frame_bytes: bytes, frame_id: int) -> bytes:
    """Process a single frame (add watermark, filter, etc.)."""
    import subprocess
    import tempfile

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "frame.png"
        output_path = Path(tmpdir) / "processed.png"

        input_path.write_bytes(frame_bytes)

        # Apply processing (example: add text overlay)
        subprocess.run([
            "ffmpeg", "-y",
            "-i", str(input_path),
            "-vf", f"drawtext=text='Frame {frame_id}':fontsize=24:fontcolor=white:x=10:y=10",
            str(output_path)
        ], check=True, capture_output=True)

        return output_path.read_bytes()

@app.function(image=ffmpeg_image)
def combine_frames(frames: list[bytes], fps: int = 24) -> bytes:
    """Combine processed frames back into video."""
    import subprocess
    import tempfile

    with tempfile.TemporaryDirectory() as tmpdir:
        # Write frames
        for i, frame_bytes in enumerate(frames):
            frame_path = Path(tmpdir) / f"frame_{i:04d}.png"
            frame_path.write_bytes(frame_bytes)

        output_path = Path(tmpdir) / "output.mp4"

        subprocess.run([
            "ffmpeg", "-y",
            "-framerate", str(fps),
            "-i", f"{tmpdir}/frame_%04d.png",
            "-c:v", "libx264",
            "-pix_fmt", "yuv420p",
            str(output_path)
        ], check=True, capture_output=True)

        return output_path.read_bytes()

@app.local_entrypoint()
def main():
    video_bytes = Path("input.mp4").read_bytes()

    # Step 1: Extract frames (single container)
    frames = extract_frames.remote(video_bytes, fps=24)
    print(f"Extracted {len(frames)} frames")

    # Step 2: Process frames in parallel (many containers)
    args = [(frame, i) for i, frame in enumerate(frames)]
    processed_frames = list(process_frame.starmap(args))
    print(f"Processed {len(processed_frames)} frames")

    # Step 3: Combine frames (single container)
    output = combine_frames.remote(processed_frames, fps=24)

    Path("output.mp4").write_bytes(output)
    print("Video processing complete!")

Modal Volumes for Large Files

For video files too large to pass as function arguments, use Modal Volumes:

Volume Setup and Usage

import modal
from pathlib import Path

app = modal.App("video-volume")

# Create persistent volume for video storage
video_volume = modal.Volume.from_name("video-storage", create_if_missing=True)

ffmpeg_image = modal.Image.debian_slim().apt_install("ffmpeg")

@app.function(
    image=ffmpeg_image,
    volumes={"/data": video_volume},
    timeout=1800  # 30 minutes for large files
)
def transcode_from_volume(input_filename: str, output_filename: str):
    """Transcode video from volume to volume."""
    import subprocess

    input_path = Path("/data") / input_filename
    output_path = Path("/data") / output_filename

    if not input_path.exists():
        raise FileNotFoundError(f"Input file not found: {input_path}")

    subprocess.run([
        "ffmpeg", "-y",
        "-i", str(input_path),
        "-c:v", "libx264",
        "-preset", "medium",
        "-crf", "22",
        "-c:a", "aac",
        "-b:a", "192k",
        str(output_path)
    ], check=True, capture_output=True)

    # Commit changes to volume (important!)
    video_volume.commit()

    return f"Transcoded: {output_filename}"

@app.function(volumes={"/data": video_volume})
def list_videos():
    """List all videos in the volume."""
    videos = list(Path("/data").glob("*.mp4"))
    return [v.name for v in videos]

@app.local_entrypoint()
def main():
    # Upload a file to the volume first
    # modal volume put video-storage local_video.mp4 video.mp4

    # Then transcode
    result = transcode_from_volume.remote("video.mp4", "video_transcoded.mp4")
    print(result)

    # List files
    files = list_videos.remote()
    print("Files in volume:", files)

Uploading to Volumes

# Upload file to volume
modal volume put video-storage local_video.mp4 video.mp4

# Download file from volume
modal volume get video-storage video_transcoded.mp4 local_output.mp4

# List volume contents
modal volume ls video-storage

Volume Best Practices

@app.function(
    volumes={"/data": video_volume},
    ephemeral_disk=50 * 1024  # 50 GB ephemeral disk for temp files
)
def process_large_video(input_filename: str):
    """Process large video with ephemeral disk for temp storage."""
    import subprocess
    import shutil

    # Copy from volume to ephemeral disk for faster I/O
    input_volume_path = Path("/data") / input_filename
    temp_input = Path("/tmp") / input_filename
    shutil.copy(input_volume_path, temp_input)

    temp_output = Path("/tmp") / "output.mp4"

    # Process on fast ephemeral disk
    subprocess.run([
        "ffmpeg", "-y",
        "-i", str(temp_input),
        "-c:v", "libx264",
        "-preset", "slow",  # Higher quality, more processing
        "-crf", "18",
        str(temp_output)
    ], check=True, capture_output=True)

    # Copy result back to volume
    output_volume_path = Path("/data") / f"processed_{input_filename}"
    shutil.copy(temp_output, output_volume_path)

    # Commit to persist
    video_volume.commit()

    return str(output_volume_path)

Cost Optimization

CPU vs GPU Pricing

# Modal pricing (approximate, 2025):
# - CPU: ~$0.000024/vCPU-second
# - Memory: ~$0.0000025/GiB-second
# - T4 GPU: ~$0.000164/second ($0.59/hour)
# - A10G GPU: ~$0.000306/second ($1.10/hour)
# - L40S GPU: ~$0.000542/second ($1.95/hour)

# For a 10-second video transcode:
# - CPU (4 cores, 10 seconds): ~$0.001
# - GPU (T4, 2 seconds): ~$0.0003

# For 1000 videos:
# - CPU: ~$1.00, parallelized across 100 containers = ~10 seconds wall time
# - GPU: ~$0.30, but harder to parallelize

# Recommendation: Use CPU for transcoding, GPU for ML inference

Resource Configuration

@app.function(
    image=ffmpeg_image,
    cpu=4,          # 4 CPU cores
    memory=8192,    # 8 GB RAM
    timeout=300,    # 5 minute timeout
)
def optimized_transcode(video_bytes: bytes) -> bytes:
    """Transcode with optimized resource allocation."""
    # Use all available CPU cores
    subprocess.run([
        "ffmpeg", "-y",
        "-threads", "4",  # Match CPU allocation
        "-i", "input.mp4",
        "-c:v", "libx264",
        "-preset", "fast",
        "-crf", "23",
        "output.mp4"
    ], check=True)

When to Use GPU

Task	Recommendation	Reason
Transcoding only	CPU	libx264 is fast, parallelizes well
Whisper transcription	GPU	ML inference, 10x+ faster
Video analysis (YOLO)	GPU	ML inference required
Thumbnail generation	CPU	Simple extraction
Audio normalization	CPU	No GPU benefit
NVENC encoding	GPU (verify)	May not be available

Production Patterns

Error Handling and Retries

import modal
from modal import Retries

app = modal.App("production-ffmpeg")

ffmpeg_image = modal.Image.debian_slim().apt_install("ffmpeg")

@app.function(
    image=ffmpeg_image,
    retries=Retries(
        max_retries=3,
        initial_delay=1.0,
        backoff_coefficient=2.0,
    ),
    timeout=600,
)
def reliable_transcode(video_bytes: bytes) -> bytes:
    """Transcode with automatic retries."""
    import subprocess
    import tempfile

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input"
        output_path = Path(tmpdir) / "output.mp4"

        input_path.write_bytes(video_bytes)

        result = subprocess.run([
            "ffmpeg", "-y",
            "-i", str(input_path),
            "-c:v", "libx264",
            "-preset", "fast",
            "-crf", "23",
            str(output_path)
        ], capture_output=True, text=True)

        if result.returncode != 0:
            # Log error for debugging
            print(f"FFmpeg stderr: {result.stderr}")
            raise RuntimeError(f"FFmpeg failed: {result.returncode}")

        return output_path.read_bytes()

Webhook Integration

import modal

app = modal.App("ffmpeg-webhook")

ffmpeg_image = modal.Image.debian_slim().apt_install("ffmpeg", "curl")

@app.function(image=ffmpeg_image)
def transcode_with_webhook(
    video_bytes: bytes,
    webhook_url: str,
    job_id: str
) -> str:
    """Transcode and notify webhook on completion."""
    import subprocess
    import tempfile
    import json

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input"
        output_path = Path(tmpdir) / "output.mp4"

        input_path.write_bytes(video_bytes)

        try:
            subprocess.run([
                "ffmpeg", "-y",
                "-i", str(input_path),
                "-c:v", "libx264",
                "-preset", "fast",
                "-crf", "23",
                str(output_path)
            ], check=True, capture_output=True)

            status = "success"
            output_size = output_path.stat().st_size

        except subprocess.CalledProcessError as e:
            status = "failed"
            output_size = 0

        # Notify webhook
        payload = json.dumps({
            "job_id": job_id,
            "status": status,
            "output_size": output_size
        })

        subprocess.run([
            "curl", "-X", "POST",
            "-H", "Content-Type: application/json",
            "-d", payload,
            webhook_url
        ], check=True)

        return status

Web Endpoint

import modal
from fastapi import FastAPI, UploadFile, BackgroundTasks
from fastapi.responses import StreamingResponse
import io

app = modal.App("ffmpeg-api")

ffmpeg_image = (
    modal.Image.debian_slim()
    .apt_install("ffmpeg")
    .pip_install("fastapi[standard]", "python-multipart")
)

web_app = FastAPI()

@web_app.post("/transcode")
async def transcode_endpoint(file: UploadFile):
    """HTTP endpoint for video transcoding."""
    import subprocess
    import tempfile

    with tempfile.TemporaryDirectory() as tmpdir:
        input_path = Path(tmpdir) / "input"
        output_path = Path(tmpdir) / "output.mp4"

        # Save uploaded file
        content = await file.read()
        input_path.write_bytes(content)

        # Transcode
        subprocess.run([
            "ffmpeg", "-y",
            "-i", str(input_path),
            "-c:v", "libx264",
            "-preset", "ultrafast",
            "-crf", "28",
            str(output_path)
        ], check=True, capture_output=True)

        # Stream response
        output_bytes = output_path.read_bytes()
        return StreamingResponse(
            io.BytesIO(output_bytes),
            media_type="video/mp4",
            headers={"Content-Disposition": "attachment; filename=output.mp4"}
        )

@app.function(image=ffmpeg_image)
@modal.asgi_app()
def fastapi_app():
    return web_app

Audio Processing with Whisper

Complete pattern for audio transcription and processing:

import modal

app = modal.App("whisper-ffmpeg")

# Image with FFmpeg and Whisper dependencies
whisper_image = (
    modal.Image.debian_slim(python_version="3.12")
    .apt_install("ffmpeg")
    .pip_install(
        "transformers[torch]",
        "accelerate",
        "torch",
        "torchaudio",
    )
)

@app.function(image=whisper_image, gpu="T4", timeout=600)
def transcribe_video(video_bytes: bytes) -> dict:
    """Extract audio from video and transcribe with Whisper."""
    import subprocess
    import tempfile
    from transformers import pipeline

    with tempfile.TemporaryDirectory() as tmpdir:
        video_path = Path(tmpdir) / "video.mp4"
        audio_path = Path(tmpdir) / "audio.wav"

        video_path.write_bytes(video_bytes)

        # Extract audio with FFmpeg
        subprocess.run([
            "ffmpeg", "-y",
            "-i", str(video_path),
            "-vn",                    # No video
            "-acodec", "pcm_s16le",   # WAV format
            "-ar", "16000",           # 16kHz for Whisper
            "-ac", "1",               # Mono
            str(audio_path)
        ], check=True, capture_output=True)

        # Transcribe with Whisper
        transcriber = pipeline(
            "automatic-speech-recognition",
            model="openai/whisper-base",
            device="cuda"
        )

        result = transcriber(str(audio_path))

        return {
            "text": result["text"],
            "audio_duration": get_duration(str(audio_path))
        }

def get_duration(audio_path: str) -> float:
    """Get audio duration using FFprobe."""
    import subprocess
    import json

    result = subprocess.run([
        "ffprobe",
        "-v", "quiet",
        "-print_format", "json",
        "-show_format",
        audio_path
    ], capture_output=True, text=True)

    data = json.loads(result.stdout)
    return float(data["format"]["duration"])

Troubleshooting

Common Issues

FFmpeg not found:

# Verify FFmpeg is installed in your image
@app.function(image=ffmpeg_image)
def check_ffmpeg():
    import subprocess
    result = subprocess.run(["ffmpeg", "-version"], capture_output=True, text=True)
    print(result.stdout)
    return result.returncode == 0

Out of memory:

# Increase memory allocation
@app.function(image=ffmpeg_image, memory=16384)  # 16 GB
def process_large_video(video_bytes: bytes):
    pass

Timeout errors:

# Increase timeout for long operations
@app.function(image=ffmpeg_image, timeout=3600)  # 1 hour
def transcode_4k_video(video_bytes: bytes):
    pass

Volume not persisting:

# Always call commit() after writing to volume
@app.function(volumes={"/data": video_volume})
def write_to_volume():
    Path("/data/output.mp4").write_bytes(data)
    video_volume.commit()  # Critical!

Debugging FFmpeg Commands

@app.function(image=ffmpeg_image)
def debug_transcode(video_bytes: bytes):
    """Transcode with full debugging output."""
    import subprocess

    result = subprocess.run([
        "ffmpeg", "-y",
        "-v", "verbose",  # Verbose logging
        "-i", "input.mp4",
        "-c:v", "libx264",
        "output.mp4"
    ], capture_output=True, text=True)

    print("STDOUT:", result.stdout)
    print("STDERR:", result.stderr)
    print("Return code:", result.returncode)

    return result.returncode == 0

Best Practices

1. Use CPU for transcoding - GPU is overkill for most encoding
2. Parallelize with map/starmap - Process many files simultaneously
3. Use Volumes for large files - Avoid passing large data as arguments
4. Set appropriate timeouts - Video processing can be slow
5. Commit Volume changes - Always call commit() after writes
6. Use ephemeral disk - For temp files during processing
7. Monitor costs - Track execution time and resource usage
8. Handle errors gracefully - FFmpeg can fail on corrupt inputs
9. Use fast presets for testing - Switch to slower for production
10. Verify GPU capabilities - NVENC may not be available

Related Skills

• ffmpeg-opencv-integration - For FFmpeg + OpenCV combined pipelines, including:
  • BGR/RGB color format conversion (OpenCV=BGR, FFmpeg=RGB)
  • Frame coordinate gotchas (img[y,x] not img[x,y])
  • ffmpegcv for GPU-accelerated video I/O (NVDEC/NVENC)
  • VidGear for multi-threaded streaming
  • Decord for ML batch video loading (2x faster than OpenCV)
  • PyAV for frame-level precision
  • Parallel frame processing patterns with Modal map()

References

• Modal Documentation
• Modal Examples - Blender Video
• Modal CUDA Guide
• Modal Volumes
• Modal Pricing