sony_draw

A small Android app that turns the Sony DPT-CP1 — a 13.3” e-ink reader Sony shipped in 2018 and never officially opened to third-party developers — into a usable notepad.

The interesting bit isn’t the drawing UI. It’s that the strokes appear with the same sub-50ms latency as Sony’s first-party DigitalPaperApp, by talking directly to two device-specific layers Sony never documented:

• the EPD waveform controller (DU / GC16 partial / GC16 full — the same modes the framebuffer driver exposes to Sony’s own apps), accessed through reflection into the modified android.view.SurfaceHolder that ships in the DPT-CP1 firmware
• the Direct Handwriting kernel path, which lets the framebuffer driver paint stylus strokes inside a registered allow-area rect without ever leaving kernel space

Both paths were reverse-engineered from the decompiled DigitalPaperApp APK and the /system/lib/libSystemUtil.so shared library pulled off a rooted DPT-CP1.

View on GitHub

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Why this exists

The DPT-CP1 is a beautiful piece of hardware — Carta e-ink, Wacom EMR digitiser, A5 size, weeks of battery. Sony shut down its companion-app support in 2023 and the device shipped locked to a single PDF-reading flow.

Out-of-the-box third-party Android apps can install (the device runs an unmodified-looking AOSP 4.4 underneath), but they have no idea about the EPD waveform controller. So they all draw with the stock Android compositor, which on this device means every stroke is a 450ms full-screen GC16 refresh — completely unusable for handwriting.

sony_draw is the minimum amount of code needed to prove that a third-party app can hit native-latency stylus performance on this device. It’s deliberately small (one Activity, ~700 lines of Java) and exists to be read and ported, not to be a polished product.

How to read these docs

1. Architecture — the four-layer pipeline from MotionEvent to e-ink pixels, with a diagram.
2. E-ink pipeline — EPD waveform modes, why DU during the stroke and GC16 on lift, how EpdHelper reflects into Sony’s framework.
3. Direct Handwriting — the kernel fast-path, the JNI binding to libSystemUtil.so, and the package-name landmine.
4. Build & install — Gradle invocation, the deliberate SDK locks, and how to develop on a non-Sony Android device.
5. Porting to KOReader — using the same primitives to bring stylus support to KOReader’s pencil.koplugin.

Status

Works on a Sony DPT-CP1 running firmware 1.6.50.14130. Not tested on the DPT-RP1 (A4 model) or DPT-S1 — they share the same libSystemUtil.so symbol set per strings output, so it would probably work with minor tuning.