SAI File Documentation

Summary

A PaintTool SAI Image is the native painting document of PaintTool SAI, the lightweight Japanese art program from SYSTEMAX. A .sai file keeps the full multi-layer canvas, and because the format is proprietary and lightly encrypted, practically only PaintTool SAI opens it. Its MIME type is application/octet-stream. To use the artwork elsewhere, open it in SAI and export: PSD keeps the layers, while PNG or JPG flattens it. SAI Ver.2 uses a separate .sai2 extension.

Technical details

Feature	Value
Full name	PaintTool SAI Image
File extension	`.sai`
MIME type	`application/octet-stream`
Format type	Proprietary, lightly encrypted layered painting document
Layer support	Raster layers and SAI vector/linework layers, blend modes, opacity
Developer	SYSTEMAX Software Development (Tokyo, Japan)
Introduced	PaintTool SAI v1 public release, 2008
Platform	Windows only
Open standard	No — proprietary, no published specification
Magic number	None documented (lightly encrypted container; no stable public signature)
Internal structure	Small encrypted filesystem-like container (canvas, layers, thumbnails, metadata)
Reverse-engineered by	libsai (open-source developer library)
Editable by	PaintTool SAI only (no third-party editor reads `.sai`)
Export targets	`.psd` (layers), `.png`, `.jpg`, `.bmp`
Successor extension	`.sai2` (PaintTool SAI Ver.2)
Related extensions	`.sai2`, `.psd`, `.psb`, `.png`, `.clip`
Vendor	`systemax.jp/en/sai/`

Structure at a glance

A .sai has no documented public magic number: the format is proprietary and lightly encrypted, so there is no stable byte signature to match on. Internally it behaves like a small encrypted filesystem — a table of named blocks holding the canvas dimensions and resolution, each raster layer (pixels, blend mode, opacity), SAI’s vector/linework layers (control points and pressure), and per-layer thumbnail previews for the UI. What is known publicly comes from the open-source libsai project, which reverse-engineered the container to decrypt and dump it. Identify a .sai by its extension and origin (PaintTool SAI), not by a header. Note that PaintTool SAI Ver.2 writes a different, separate format, .sai2.

What is a SAI file?

A .sai file is the native working document of PaintTool SAI, a digital painting application developed by SYSTEMAX Software Development in Tokyo and released publicly in 2008. SAI is a small, fast, Windows-only program with a following among illustrators and the anime and manga art community, valued for smooth, natural pen strokes, strong stroke stabilisation and responsive tablet support. The .sai file is where an artist’s work-in-progress lives: it stores the entire multi-layer canvas so the drawing can be reopened and kept editing.

The important thing to understand up front is that .sai is a proprietary, lightly encrypted format with no published specification. That single fact drives almost every practical question about it. No mainstream art program reads .sai directly, so to move artwork out of SAI you open it in SAI and export a standard format. This article stays honest about what is publicly known: the internal layout was not documented by SYSTEMAX, and the details below come from the extension’s known behaviour and the open-source libsai reverse-engineering project, not from an official spec.

What a SAI file contains

A .sai is not a flat bitmap; it is a layered document that preserves the state of the canvas. It holds the canvas dimensions and resolution, and then a stack of layers. SAI supports two kinds. Raster layers are ordinary pixel layers, each carrying its own blend mode and opacity. Vector, or linework, layers are SAI’s editable line layers: instead of pixels they store control points and pressure information, so a line can be reshaped or re-weighted after it is drawn. Alongside the layers, the file keeps a small thumbnail preview for each layer, which is what SAI’s layer panel shows.

Because the linework layers and blend/opacity settings are all inside the .sai, keeping the original file is the only way to preserve full editability. Any export to PNG or JPG merges the layers into a single flat image and discards the vector data, which is why the recurring advice is to never delete your .sai originals even after you have exported a finished picture.

Why the format is closed: an encrypted container

Rather than a documented header followed by chunks, a .sai behaves like a small encrypted filesystem: a table of named internal blocks, one for the canvas, one per layer, one per thumbnail, with a light encryption layer over the whole thing. There is no reliable public magic number to test for, so tools cannot sniff a .sai by its first bytes the way they can a PNG or a ZIP. The only reason any of this internal shape is known publicly is libsai, an open-source library that decrypts and dumps the container for developers. libsai is an extraction and analysis tool, not a viewer or editor; it lets a programmer read the structure, but it does not open the artwork in a usable way for an artist.

Opening a SAI file: PaintTool SAI is effectively the only option

The honest answer to “how do I open a .sai without PaintTool SAI” is that you essentially cannot. Photoshop does not read it, and neither do Clip Studio Paint, GIMP or Krita. The practical opener is PaintTool SAI itself: it is inexpensive, Windows-only, and SYSTEMAX offers a free 31-day trial plus a Ver.2 technical-preview build (which also opens .sai). Inside SAI you use File › Open to view and fully edit every layer. If you have received someone else’s .sai and do not want to install SAI, the realistic route is to ask the sender to export the artwork to PSD or PNG for you.

Exporting out of SAI: PSD keeps layers, PNG and JPG flatten

Getting artwork out of SAI is done from inside SAI with File › Export As, and the right target depends on the goal. PSD (Photoshop document) is the most useful export because it keeps the separate layers, so you can continue editing in Photoshop, Clip Studio Paint or GIMP. SAI can also open PSD files, which is the usual round-trip with Photoshop users. PNG produces a flat image that keeps transparency, ideal for posting online. JPG is a flat, lossy copy with no transparency, and is a poor choice for line art you might re-edit because its compression artefacts degrade sharp edges. BMP is a flat uncompressed export for the rare tool that specifically wants it. SAI has no native PDF export, so if you need a PDF you export a PNG or PSD first and then place or print that from another program.

SAI Ver.1 versus SAI Ver.2 and the .sai2 extension

A common “why won’t my file open” case is a version mismatch between programs, not corruption. PaintTool SAI Ver.2, the newer technical-preview build, saves to a different extension, .sai2, which SAI Ver.1 cannot open. If a file is a .sai2, open it in SAI Ver.2. A plain .sai opens in both Ver.1 and Ver.2.

Frequently asked questions

How do I open a SAI file without PaintTool SAI?

Realistically you cannot, because the format is proprietary and no other mainstream app reads it. Use PaintTool SAI (there is a free 31-day trial), or ask the sender to export the artwork as PSD or PNG. The libsai developer library can extract the data but is not a viewer.

How do I convert a SAI file to PSD?

Open it in PaintTool SAI and choose File › Export As › .psd. The PSD keeps your separate layers, so you can keep editing in Photoshop, Clip Studio Paint or GIMP.

Why won’t my .sai open — is it a .sai2?

PaintTool SAI Ver.2 saves to a separate .sai2 extension that SAI Ver.1 cannot read. Open a .sai2 in the newer SAI Ver.2 technical-preview build.

References

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