HT203U Thermal

Native Android app for the Vevor/Hti HT-203U USB-C thermal camera with real, calibrated temperature readout — built by reverse-engineering the module live, since the stock apps only show colorized video. Under 1 MB, no native code, no third-party camera libraries.

The big finding

Despite the Hti branding, the HT-203U is a rebranded HIKMICRO Mini2Plus core (USB 2bdf:0102, product string "HikCamera"). It is not an Xtherm/InfiRay device, so none of the published HT-301/T2S+ protocols (zoom-command 0x8004, Xtherm calibration metadata) apply. Everything below was reverse-engineered from the device itself.

Device layout (as discovered)

USB descriptors:

• IF#0: VideoControl — input terminal (id 2), processing unit (id 5), extension unit id 10, 15 controls, GUID {A29E7641-DE04-47E3-8B2B-F4341AFF003B} (vendor controls: FFC/shutter etc. — mapping in progress), interrupt EP 0x83
• IF#1: VideoStreaming — YUY2 uncompressed (7+ frame descriptors), MJPEG, H.264; bulk endpoint 0x81 (maxPkt 512), alt setting 0

The interesting mode is uncompressed 256×392 @ 25 fps (200,704 B/frame), a stacked frame:

The raw block is identified at runtime by chroma signature (the display block has every odd byte = 0x80).

Temperature calibration

Two-point empirical fit (2026-07-26), references measured with a kitchen thermometer:

• Ice-water slurry, 3.9 °C → raw 4405
• Rolling boil, ~100 °C → raw 6979

T(°C) = (raw − 4405) × 0.0373349 + 3.9        (≈ 26.8 counts/°C)

Independent validation on anchors not used in the fit: palm skin → 32.5 °C (expected 32–34), indoor ambient → 25.2 °C (July, plausible). Caveat: absolute offset can drift a few °C across power cycles and during warm-up because FFC (shutter flat-field correction) can't be triggered yet — that lives in the vendor XU and is the current reverse-engineering target.

Why the usual Android UVC libraries fail

libuvc-based libraries (saki/UVCCamera forks, UVCAndroid) fail negotiation on this device with UVC_ERROR_INVALID_MODE (-51) for every mode. This app therefore implements UVC-over-bulk in pure Kotlin (BulkUvc.kt) on the standard UsbManager API:

1. Claim the VS interface, PROBE/COMMIT via controlTransfer (probe response: dwMaxVideoFrameSize=200704, dwMaxPayloadTransferSize=12288, bmFramingInfo=0x03)
2. Read the bulk endpoint, reassemble payloads by header FID/EOF bits

App features

• Live thermal display, ironbow palette, auto-exposure, min/max/center markers
• Calibrated °C/°F readout for min/center/max (+ raw center count in the status line)
• Photo: 4× PNG to Pictures/HT203U + raw u16 frame to Download/HT203U (.raw, 256×192 LE — great for offline analysis)
• Video: H.264 MP4 recording of the live view to Movies/HT203U
• Rotate (90° steps, persisted) + free screen orientation
• Debug tooling: Probe XU (read-only walk of the 15 vendor controls), Dump meta (hex of config/telemetry rows + frame stats), Log (in-app trace + logcat, share/copy)
• Mode auto-cycling if a stream mode delivers no frames
• Runtime CAMERA permission (Android 10+ silently denies USB access to video-class devices without it) + standard USB permission dialog (GrapheneOS-friendly)

Getting the APK

Every push to main builds via GitHub Actions and publishes to the rolling release:

https://github.com/cfbird/HT203U-Thermal/releases/download/latest/app-debug.apk

Or build locally: Gradle 8.9, JDK 17, Android SDK 34 — gradle assembleDebug.

GrapheneOS notes

• Allow USB peripherals: Settings → USB-C port ("Allow new USB peripherals when unlocked")
• Grant Camera permission on first launch, then accept the USB dialog after plugging in

TODO

• Map the XU controls (FFC/shutter trigger first — kills the offset drift; possibly a factory-calibrated temperature mode)
• Finish decoding config rows 192–193 and telemetry row 388 (FPA temp → automatic offset compensation)
• Tap-to-measure spot meter, palettes, emissivity setting
• In-app recalibration UI (single-point offset touch-up against a known reference)

Credits / license

UVC access and radiometry are original code on Android's USB host API. The Xtherm calibration math in Xtherm.kt (kept for genuine Xtherm-family devices) is ported from stawel/ht301_hacklib (GPL-3.0); this project is GPL-3.0.