rt-alpha-video

rt-alpha-video turns a folder of PNG frames with alpha into:

folder-name.webm (VP9 + alpha)
folder-name-hevc.mov (HEVC + alpha, tagged hvc1 for Safari)
folder-name.txt (ready-to-paste HTML + JS snippet that auto-detects WebM/HEVC at runtime)

Perfect for replacing heavy PNG image sequences with lightweight transparent video on the web.

Platform: macOS only
Input: PNG frames with alpha (RGBA)
Output: WebM + optional HEVC + snippet

Launch Web Helper

Use the browser-based helper to drag and drop your PNG frames, auto-detect the pattern and FPS, verify alpha, preview the loop, and generate a ready-to-run rt-alpha-video command.
Read more about the helper in Web helper.

1. Requirements
2. What the CLI actually does
3. Installation
- 3.1. Global install
- 3.2. One-off usage with npx
4. Basic usage
5. CLI options (full)
6. Default quality settings
7. Input expectations (PNG + alpha)
8. Using the generated snippet
9. Troubleshooting
10. Typical workflow
11. Web helper
12. Changelog
13. License

1. Requirements

1.1. OS and runtime

macOS (Apple Silicon or Intel)
Node.js ≥ 16

Check Node:

node -v

1.2. FFmpeg

Your FFmpeg build must include:

libvpx-vp9
hevc_videotoolbox (for HEVC + alpha via VideoToolbox)

Check:

ffmpeg -version

1.3. Homebrew (optional but recommended)

If FFmpeg is missing but Homebrew exists, the CLI will offer to install FFmpeg automatically via:

brew install ffmpeg

If Homebrew is not available, you’ll need to install FFmpeg manually.

2. What the CLI actually does

Given a PNG sequence, rt-alpha-video will:

Detect the file pattern and start index Finds a common prefix/suffix between first and last filenames and infers the numeric run (e.g. Frame_%05d.png + starting index).
Encode transparent video outputs
- .webm (VP9 + alpha, yuva420p)
- .mov (HEVC + alpha via hevc_videotoolbox, tagged hvc1), unless --no-hevc is provided or the encoder is unavailable.
Generate an HTML snippet (.txt) A ready-to-paste snippet that auto-selects WebM vs HEVC at runtime based on codec support (Safari / iOS vs others).
Print detailed stats Frame count, total PNG size, animation duration, final WebM/HEVC sizes, and per-encode times.

3. Installation

3.1. Global install

Install globally and use the rt-alpha-video command:

npm install -g @rethink-js/alpha-video-cli

3.2. One-off usage with npx

Run once without installing globally:

npx @rethink-js/alpha-video-cli --input ./frames --fps 50

4. Basic usage

rt-alpha-video --input ./frames --fps 50

Output folder:

frames/dist/
  frames.webm
  frames-hevc.mov
  frames.txt

The output folder defaults to <input>/dist.
The base name defaults to the input folder name (normalized).

5. CLI options (full)

Required

Flag	Description
`--input <folder>`	Directory containing PNG frames
`--fps <number>`	Playback framerate

Optional

Flag	Description
`--output <folder>`	Custom output directory (default: `<input>/dist`)
`--pattern <pattern>`	Frame pattern e.g. `Frame_%05d.png` (otherwise inferred from filenames)
`--start <number>`	First frame index for the pattern (`-start_number` in FFmpeg). Defaults from detection
`--end <number>`	Last frame index to include (inclusive). Limits encoding to this range
`--width <number>`	Resize video width; if height is omitted, a square output is produced
`--height <number>`	Resize video height; if width is omitted, a square output is produced
`--name <basename>`	Base name for output files (default: input folder name, lowercased, spaces → dashes)
`--webm-quality <1-100>`	Logical WebM quality (maps to CRF internally; default: 50)
`--hevc-quality <1-100>`	Logical HEVC alpha quality (maps to `-alpha_quality` internally; default: 90)
`--no-hevc`	Disable HEVC `.mov` encoding (WebM only)
`--quiet`	Suppress progress bars and extra logs (FFmpeg still runs, but with minimal console output)
`--help`, `-h`	Help menu

Notes:

If you pass only width or only height, the CLI produces a square output and warns if you upscale above the source PNG dimensions.
--end does not change pattern detection; it only clamps how many frames are actually encoded.

6. Default quality settings

WebM (VP9 + alpha)

Logical default: 50 (--webm-quality 50)
Internally mapped to: CRF ≈ 28
CRF range used: 18 → 38 (Lower CRF = higher quality, larger files.)

HEVC (VideoToolbox, alpha)

Logical default: 90 (--hevc-quality 90)
Internally mapped to: -alpha_quality 0.9
Range: 0.1 → 1.0 (Higher = better alpha quality, larger files.)

7. Input expectations (PNG + alpha)

Your PNG frames should:

Include an alpha channel (RGBA)
Follow consistent numbering (with a stable prefix/suffix)
Use a detectable pattern, e.g. Frame_00001.png → Frame_00350.png
Or you explicitly provide the pattern with --pattern

Example:

Frame_00000.png
Frame_00001.png
Frame_00002.png
...
Frame_00349.png

The CLI:

Reads the first and last PNG filenames,
Finds the shared prefix and suffix,
Detects the numeric run in the middle,
Builds a pattern like Frame_%05d.png,
And infers the starting index (e.g. 0).

If multiple numeric segments exist, it picks the first segment that actually changes between the first and last filenames.

8. Using the generated snippet

A .txt file is generated alongside your video encodes. This file contains:

A minimal HTML document
A <video> element with a <source> child
A small script that:
- Detects Safari / iOS vs other browsers
- Checks actual codec support
- Picks the best between WebM (VP9) and HEVC (hvc1) at runtime
- Attempts to autoplay safely

Example excerpt (simplified):

<video id="rt-alpha-video" muted autoplay loop playsinline poster="">
  <source src="" type="" />
</video>

<script>
  (function () {
    var video = document.getElementById("rt-alpha-video");
    if (!video) return;
    var sourceEl = video.querySelector("source");
    if (!sourceEl) return;

    var webmSrc = "my-animation.webm";
    var hevcSrc = "my-animation-hevc.mov";

    var ua = navigator.userAgent || "";
    var isIOS = /iPad|iPhone|iPod/.test(ua);
    var isSafariDesktop = /^((?!chrome|android).)*safari/i.test(ua);
    var isSafariEngine = isIOS || isSafariDesktop;

    var canPlayHevc = video.canPlayType('video/mp4; codecs="hvc1"');
    var canPlayWebm =
      video.canPlayType('video/webm; codecs="vp9"') ||
      video.canPlayType("video/webm");

    var finalSrc = webmSrc;
    var finalType = 'video/webm; codecs="vp9"';

    if (isSafariEngine && canPlayHevc) {
      finalSrc = hevcSrc;
      finalType = 'video/mp4; codecs="hvc1"';
    } else if (!canPlayWebm && canPlayHevc) {
      finalSrc = hevcSrc;
      finalType = 'video/mp4; codecs="hvc1"';
    }

    sourceEl.src = finalSrc;
    sourceEl.type = finalType;

    video.load();
    var p = video.play();
    if (p && typeof p.then === "function") {
      p.catch(function () {});
    }
  })();
</script>

To use:

Copy the contents of your-base-name.txt.
Paste into your project:
- Either as a full HTML file for testing, or
- Extract just the <video> + <script> block into your component/template.
Update webmSrc / hevcSrc (and poster, if desired) to match your hosting paths.

9. Troubleshooting

9.1. FFmpeg missing

Error example:

ffmpeg is not installed and Homebrew is not available.

Fix:

Install FFmpeg (via Homebrew or manually).
Re-run rt-alpha-video.

9.2. No PNG files found

If you see:

No .png files found in input directory: <path>

Check:

The --input path is correct.
The directory actually contains .png files.
The extension is .png (not .PNG with some mismatch — though case-insensitive matching is used).

9.3. Could not infer pattern

If automatic pattern detection fails:

Could not infer numeric pattern from files.
First file: Frame_final.png

Fix: specify the pattern manually:

rt-alpha-video \
  --input ./frames \
  --fps 50 \
  --pattern "Frame_%05d.png" \
  --start 0

9.4. HEVC not generated

Possible reasons:

You passed --no-hevc
hevc_videotoolbox encoder is not available in your FFmpeg build
Your FFmpeg build is too old or compiled without VideoToolbox

The CLI will still encode WebM and print a message such as:

Skipping HEVC encode: hevc_videotoolbox encoder not available in ffmpeg on this system.

10. Typical workflow

Minimal:

rt-alpha-video \
  --input "/path/to/frames" \
  --fps 50

Full-options example:

rt-alpha-video \
  --input "/path/to/frames" \
  --output "/path/to/output" \
  --fps 50 \
  --pattern "Frame_%05d.png" \
  --start 0 \
  --end 349 \
  --width 500 \
  --height 500 \
  --name "frame-preview" \
  --webm-quality 72 \
  --hevc-quality 95

Then:

Drop the .webm and .mov files into your preferred hosting (CDN, static folder, etc.).
Copy HTML/JS from the generated .txt file into your site.

11. Web helper

A visual companion tool for rt-alpha-video (no installation required — runs entirely in your browser).

Launch Web Helper

https://rethink-js.github.io/alpha-video-cli-web-helper

Repository

https://github.com/Rethink-JS/alpha-video-cli-web-helper

The web helper provides:

Drag and drop folder analysis Detects sequences directly from your PNG files.
Pattern detection Automatically identifies patterns (for example %05d), start frames, and end frames using the same logic as the CLI.
FPS auto-detection Attempts to estimate the most likely frame rate (24, 25, 30, 48, 50, 60) from total frame count.
Transparency verification Checks the alpha channel of your frames so you don’t accidentally encode opaque videos.
Live canvas preview Loops the sequence so you can confirm smoothness and transitions before encoding.
Smart defaults Mirrors CLI defaults (FPS, quality settings, naming).
One-click command generation Outputs a full, ready-to-run terminal command string for rt-alpha-video.

Privacy and security

100% client-side.
No files ever uploaded.
Uses browser memory only.
No external servers or third-party processing.

Browser path limitations

Due to browser security, the helper cannot read absolute paths (such as /Users/username/Desktop/project/frames). It only sees relative folder names (for example frames).

The helper will suggest a relative path like ./frames.
If you run the command from the parent directory of your frames folder, this works as-is.
Otherwise, replace --input with your actual absolute path.

Tip for macOS: drag a folder from Finder into Terminal to paste its absolute path.

12. Changelog

1.2.0

Improved pattern detection logic:
- Uses a shared prefix/mid/suffix comparison between first and last frames
- Handles multiple numeric segments more robustly
Added explicit --width and --height flags with square-output behaviour when only one dimension is set
Added a warning when requested resize dimensions upscale beyond the source PNG size
Extended summary output with:
- Total PNG size
- Per-encode duration for WebM and HEVC
Updated HTML preview snippet to:
- Use <source> with runtime codec selection
- Prefer HEVC (hvc1) on Safari / iOS when supported, otherwise WebM
Minor improvements to progress output and quiet mode handling

1.1.0

Added --end (frame end index)
Updated defaults:
- Defaults now map exactly to previous internal quality
Added full Web Helper documentation

1.0.0

Stable public release
Pattern detection
WebM + HEVC encoding
Snippet generation
Stronger errors

0.1.0

Initial preview
Basic encoding pipeline

13. License

MIT License

Package: @rethink-js/alpha-video-cli GitHub: https://github.com/Rethink-JS/alpha-video-cli

by Rethink JS https://github.com/Rethink-JS

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package.json		package.json

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