truepng-mcp

Turn painted transparency checkerboards into real PNG alpha.

truepng-mcp is built for images that look transparent because they contain a gray-and-white PNG checkerboard, but are actually flat RGB images with no usable alpha channel. It detects the painted checker pattern, reconstructs foreground edges, recovers enclosed transparent holes, and writes a genuine RGBA PNG.

This is not a general image-format converter. The primary job is to recover real transparency from fake checkerboard backgrounds embedded in PNG, JPEG, or WebP image pixels.

The input may already have a .png extension and still be fake: PNG is a file format, not proof that transparency exists. A flattened PNG often has mode RGB and the checkerboard is ordinary visible pixel data. The output from truepng has mode RGBA and a verified, used alpha channel.

It provides an on-demand CLI and a standard MCP stdio server for macOS. It never installs a daemon, HTTP server, login item, or background service.

Why this exists

Image previews, generated assets, screenshots, and some download sites may flatten an originally transparent asset onto a visible checkerboard before saving it. A conventional background remover treats this as a generic segmentation problem and may damage colors, pixel-art edges, white foreground details, or holes inside map pins and rings.

truepng treats the checkerboard as a known compositing pattern. It detects the grid period, colors, origin, and parity, then reconstructs transparency against the expected checker color. Enclosed regions are classified with parity coverage, size, density, robust border error, and periodic fit so genuine holes become transparent while white foreground remains intact.

The design is derived from the MIT-licensed image_tools_mcp v1.3.0 checker reconstruction. See UPSTREAM.md.

Related MCP server: FreezeText MCP

Features

• Deterministic fake-checkerboard detection and removal

• Genuine RGBA PNG output from flattened RGB checkerboard images

• Hole-aware recovery for rings, map pins, frames, and glyph counters

• White and light foreground preservation

• Existing-alpha sanitization with hidden RGB cleanup

• Optional BiRefNet and rembg fallback for non-checker images

• CLI and MCP stdio server in one command

• Absolute output paths and structured JSON results

• Atomic PNG writes, safe overwrite behavior, and output validation

• No resident process, daemon, HTTP server, Docker container, or CUDA dependency

Requirements

• macOS on Apple Silicon

• Python 3.11, 3.12, or 3.13

• uv

• Internet access only during installation and the first use of an AI model

Install

Clone the repository, then install the lightweight checker, sanitize, CLI, and MCP routes:

git clone https://github.com/gkhntpbs/truepng-mcp.git
cd truepng-mcp
./install.sh

Installation with the optional AI fallback:

TRUEPNG_INSTALL_AI=1 ./install.sh

The executable is installed at ~/.local/bin/truepng, support files at ~/.local/share/truepng-mcp, and models at ~/.cache/truepng-mcp. Models download only on their first AI use. No CUDA packages are installed and the system Python environment is not modified.

Add to an MCP client

After running ./install.sh, add the installed stdio server globally with one command.

Codex

codex mcp add true-png -- "$HOME/.local/bin/truepng" mcp

Confirm the entry:

codex mcp get true-png

Claude Code

claude mcp add --scope user --transport stdio true-png -- "$HOME/.local/bin/truepng" mcp

Confirm the entry:

claude mcp get true-png

Restart an already-open client session if the tools do not appear immediately.

CLI

truepng input.png
truepng input.jpg -o output.png
truepng convert input.webp --output output.png
truepng input.png --mode checker
truepng input.png --mode ai --model isnet-anime
truepng input.png --mode sanitize
truepng inspect output.png --json
truepng verify output.png --json
truepng doctor

auto preserves and sanitizes used alpha, otherwise tries deterministic checker recovery, and only then starts the isolated AI worker. The AI model is imported in a short-lived child process with compute thread counts fixed to one. Checker and sanitize calls never import ONNX or load a model.

Existing output files are not overwritten by default. A safe numbered name is selected. Use --force for explicit overwrite. Reported paths are absolute.

MCP

The main tool is image_to_true_png with input_path, optional output_path, mode, model, overwrite, and return_preview. return_preview is accepted for schema compatibility but the full image is never embedded in the response. The result includes an absolute path and file URI. Additional tools are inspect_image_transparency and verify_true_png.

Codex ~/.codex/config.toml:

[mcp_servers.true-png]
command = "/Users/ACTUAL_USER/.local/bin/truepng"
args = ["mcp"]

Claude Code:

claude mcp add --scope user --transport stdio true-png -- /Users/ACTUAL_USER/.local/bin/truepng mcp

Claude Desktop, Cursor, and generic MCP clients:

{
  "mcpServers": {
    "true-png": {
      "command": "/Users/ACTUAL_USER/.local/bin/truepng",
      "args": ["mcp"]
    }
  }
}

MCP tools

image_to_true_png:

{
  "input_path": "/absolute/path/input.png",
  "output_path": "/absolute/path/output.png",
  "mode": "auto",
  "model": "birefnet-general",
  "overwrite": false,
  "return_preview": false
}

The server also exposes inspect_image_transparency and verify_true_png.

Processing modes

The default AI model is birefnet-general. birefnet-general-lite, isnet-anime, and other safe rembg model identifiers can be selected with --model.

The AI route is a fallback for inputs that are not fake-checkerboard images. It is not the defining feature of this project. Checker recovery remains deterministic, lighter, and better suited to the core use case.

Resource usage

Checker recovery uses compact byte masks and float arrays instead of full-size Python object grids. It does not import rembg, NumPy, or ONNX. AI inference runs in a short-lived child process, restricts compute libraries to one thread, and is stopped if it exceeds the configurable RSS limit.

TRUEPNG_MAX_AI_RSS_MB=3072 truepng photo.jpg --mode ai

The default AI RSS limit is 4096 MB.

Maintenance

git pull --ff-only
./update.sh
./uninstall.sh

The uninstaller intentionally retains downloaded models. Remove ~/.cache/truepng-mcp manually if they are no longer needed.

Troubleshooting

If truepng is not found, add this to your shell configuration:

export PATH="$HOME/.local/bin:$PATH"

If auto mode reaches AI fallback and reports that rembg is missing, rerun TRUEPNG_INSTALL_AI=1 ./install.sh. If memory pressure matters more than segmentation, use --mode checker or --mode sanitize; these deterministic paths have no model runtime.

Development

uv sync --extra test
uv run pytest -q
uv build

See TESTING.md, ARCHITECTURE.md, and CONTRIBUTING.md.

Releases

Version tags create GitHub Releases automatically:

git tag -a v1.0.0 -m "truepng-mcp v1.0.0"
git push origin v1.0.0

The release workflow runs the test suite, builds the package, and attaches:

• truepng_mcp-1.0.0-py3-none-any.whl

• truepng_mcp-1.0.0.tar.gz

• GitHub-generated source code archives in ZIP and TAR.GZ formats

Security

Path values are normalized and never interpolated into shell commands. Same-file writes and symlink outputs are rejected. See SECURITY.md for vulnerability reporting.

License

MIT. See LICENSE and UPSTREAM.md.