Bare-Metal Android Display Pipeline

Transforming a legacy Android smartphone into a dedicated hardware-accelerated display.

This project repurposes an old Android device—specifically the Samsung Galaxy Star—into a lightweight external monitor by bypassing the Android framework entirely. Components such as Zygote, SurfaceFlinger, and the standard application stack are removed from the rendering path, allowing raw video frames to be streamed directly into the Linux framebuffer.

Since the Galaxy Star's CPU cannot efficiently decode modern video formats, all decoding is performed on a host machine. The host converts video frames into either raw RGB data or H.264 streams and sends them to custom native C++ receivers running directly on the device.

Requirements

Rooted Android device with TWRP
Android NDK
FFmpeg
abootimg
cpio
ADB

Architecture Evolution

Version	Transport	Video Format	Notes
v1	ADB	RGB24 Raw Frames	Simple implementation
v2	USB Ethernet (RNDIS)	RGB24 Raw Frames	~2× throughput improvement
v3	USB Ethernet (RNDIS)	H.264 + PCM Audio	Hardware decoding via MediaCodec

Phase 1 — Custom Boot Image

The device boots directly into native services by embedding custom binaries inside the ramdisk and launching them from init.rc.

Note: The Galaxy Star boot partition (mmcblk0p5) is limited to approximately 10 MB, so binary size matters.

Build Service

'/run/media/goku/54F2BAD1F2BAB718/SDK/ndk/29.0.14206865/toolchains/llvm/prebuilt/linux-x86_64/bin/armv7a-linux-androideabi21-clang++' \
main.cpp \
-o temp_display \
-static-libstdc++

rm -rf ramdisk/sbin/temp_display
cp temp_display ramdisk/sbin/temp_display
chmod 755 ramdisk/sbin/temp_display

Repack Boot Image

cd ramdisk

find . | cpio -o -H newc | gzip -9 > ../custom_initrd.img

cd ..

abootimg \
    --create custom_boot.img \
    -f bootimg.cfg \
    -k zImage \
    -r custom_initrd.img

Flash Through TWRP

adb reboot recovery

adb push custom_boot.img /tmp/custom_boot.img

adb shell dd \
    if=/tmp/custom_boot.img \
    of=/dev/block/mmcblk0p5

adb reboot

Restore Stock Boot Image

adb push boot.img /tmp/boot.img

adb shell dd \
    if=/tmp/boot.img \
    of=/dev/block/mmcblk0p5

Phase 2 — Native Framebuffer Receiver (v1)

video_display receives RGB frames from stdin and writes directly to:

/dev/graphics/fb0

The binary is statically linked to avoid dependency issues on older Android versions.

Build

'/run/media/goku/54F2BAD1F2BAB718/SDK/ndk/29.0.14206865/toolchains/llvm/prebuilt/linux-x86_64/bin/armv7a-linux-androideabi21-clang++' \
video.cpp \
-o video_display \
-static \
-O3 \
-mcpu=cortex-a9 \
-mfpu=neon \
-funroll-loops \
-Wl,-s

adb push video_display /data/local/bin/video_display

adb shell chmod 755 /data/local/bin/video_display

Receiver Flow

stdin
  ↓
RGB24 Frames
  ↓
RGB24 → RGBA8888
  ↓
Framebuffer
  ↓
LCD Panel

Phase 3 — ADB Video Streaming (v1)

Video is decoded on the host machine and streamed through ADB.

Streaming Command

ffmpeg \
-re \
-stream_loop -1 \
-i final.mov \
-f rawvideo \
-pix_fmt rgb24 \
-vf "transpose=1,scale=240:320" \
- | adb exec-in /data/local/bin/video_display

Data Path

Video File
    ↓
FFmpeg Decode
    ↓
RGB24 Frame Stream
    ↓
ADB USB Transport
    ↓
video_display
    ↓
Framebuffer (/dev/graphics/fb0)
    ↓
LCD Panel

Key Options

Option	Description
`-re`	Play at native frame rate
`-stream_loop -1`	Infinite playback
`-pix_fmt rgb24`	24-bit RGB output
`transpose=1`	Rotate 90° clockwise
`scale=240:320`	Match panel resolution
`adb exec-in`	Pipe directly into receiver

Phase 4 — USB Ethernet (RNDIS)

ADB transport introduces noticeable overhead.

Version 2 replaces ADB with a dedicated USB Ethernet (RNDIS) connection and TCP-based transport.

Device Network Script

#!/system/bin/sh

setprop sys.usb.config none
sleep 1

setprop sys.usb.config rndis,adb

for i in $(seq 1 10); do
    if [ -d /sys/class/net/rndis0 ] || \
       [ -d /sys/class/net/usb0 ]; then
        break
    fi
    sleep 1
done

sleep 1
ifconfig usb0 192.168.42.2 netmask 255.255.255.0 up 2>/dev/null

sleep 1
ifconfig rndis0 192.168.42.2 netmask 255.255.255.0 up 2>/dev/null

Deploy Script

adb shell 'su -c rm -f /data/local/bin/run.sh'

adb push run.sh /data/local/bin/run.sh

adb shell chmod 755 /data/local/bin/run.sh

Host Configuration

sudo nmcli dev set enp2s0f0u6 managed no

sudo ifconfig enp2s0f0u6 \
192.168.42.1 \
netmask 255.255.255.0 up

Verify Connectivity

ping -c 4 192.168.42.2

Phase 5 — TCP Framebuffer Receiver (v2)

video_display_v2 replaces stdin transport with a TCP server listening on port 5000.

Build

'/run/media/goku/54F2BAD1F2BAB718/SDK/ndk/29.0.14206865/toolchains/llvm/prebuilt/linux-x86_64/bin/armv7a-linux-androideabi21-clang++' \
video_v2.cpp \
-o video_display_v2 \
-static \
-O3 \
-mcpu=cortex-a9 \
-mfpu=neon \
-funroll-loops \
-Wl,-s

adb push video_display_v2 \
/data/local/bin/video_display_v2

adb shell chmod 755 \
/data/local/bin/video_display_v2

Receiver Architecture

TCP Socket (:5000)
    ↓
RGB24 Frame Buffer
    ↓
RGB24 → RGBA8888
    ↓
Framebuffer Mapping
    ↓
/dev/graphics/fb0
    ↓
LCD Panel

Runtime Flow

Open framebuffer device.
Memory-map framebuffer.
Start TCP server on port 5000.
Wait for incoming connection.
Receive complete RGB24 frames.
Convert to framebuffer format.
Render directly to display.
Re-listen automatically after disconnect.

Phase 6 — Raw RGB Streaming over Ethernet (v2)

Host Setup

sudo nmcli dev set enp2s0f0u6 managed no

sudo ifconfig enp2s0f0u6 \
192.168.42.1 \
netmask 255.255.255.0 up

⚠️ Wait until the Android device fully mounts the RNDIS interface before starting FFmpeg. Starting too early can significantly reduce FPS.

Stream Video File

sudo nmcli dev set enp2s0f0u6 managed no

sudo ifconfig enp2s0f0u6 \
192.168.42.1 \
netmask 255.255.255.0 up
ffmpeg \
-re \
-stream_loop -1 \
-i future-trunks-powering-up-dragon-ball-moewalls-com.mp4 \
-f rawvideo \
-pix_fmt rgb24 \
-vf "transpose=2,scale=240:320" \
- | nc 192.168.42.2 5000

Stream OBS Virtual Camera

sudo nmcli dev set enp2s0f0u6 managed no

sudo ifconfig enp2s0f0u6 \
192.168.42.1 \
netmask 255.255.255.0 up
ffmpeg \
-re \
-f v4l2 \
-i /dev/video0 \
-f rawvideo \
-pix_fmt rgb24 \
-vf "transpose=2,scale=240:320" \
- | nc 192.168.42.2 5000

Lower Host CPU Usage

Configure OBS canvas directly to the device resolution and orientation.

sudo nmcli dev set enp2s0f0u6 managed no

sudo ifconfig enp2s0f0u6 \
192.168.42.1 \
netmask 255.255.255.0 up
ffmpeg \
-re \
-f v4l2 \
-i /dev/video0 \
-f rawvideo \
-pix_fmt rgb24 \
- | nc 192.168.42.2 5000

Data Path

Host PC
 ├─ FFmpeg Decode
 ├─ RGB24 Conversion
 └─ TCP Streaming
          │
          ▼
USB Ethernet (RNDIS)
          │
          ▼
video_display_v2
          │
          ▼
Framebuffer (/dev/graphics/fb0)
          │
          ▼
LCD Panel

Phase 7 — MediaCodec Receiver (v3)

Raw RGB transport provides excellent latency but consumes significant bandwidth.

video_display_v3 uses Android's hardware video decoder via MediaCodec and optionally supports PCM audio playback using TinyALSA.

Works exceptionally well for live streams (OBS Virtual Camera). File playback can exhibit additional latency due to encoding and buffering behavior.

Build (Video + Audio)

'/run/media/goku/54F2BAD1F2BAB718/SDK/ndk/29.0.14206865/toolchains/llvm/prebuilt/linux-x86_64/bin/armv7a-linux-androideabi21-clang++' \
video_display_v3.cpp \
-o video_display_v3 \
-O3 \
-mcpu=cortex-a9 \
-mfpu=neon \
-funroll-loops \
-static-libstdc++ \
-I./tinyalsa/include \
-L. \
-Wl,-rpath=/system/lib \
-Wl,-s \
-lmediandk \
-ltinyalsa

Deploy

adb push video_display_v3 \
/data/local/bin/video_display_v3

adb shell chmod 755 \
/data/local/bin/video_display_v3

Phase 8 — H.264 + Audio Streaming (v3)

OBS Live Stream

Video (TCP 5000)

sudo nmcli dev set enp2s0f0u6 managed no

sudo ifconfig enp2s0f0u6 \
192.168.42.1 \
netmask 255.255.255.0 up
adb shell tinymix 6 1
adb shell tinymix 14 14
ffmpeg \
-f v4l2 -i /dev/video0 \
-f alsa -i default \
-map 0:v \
-c:v libx264 \
-preset ultrafast \
-tune zerolatency \
-pix_fmt yuv420p \
-g 60 \
-crf 18 \
-f h264 \
tcp://192.168.42.2:5000 \
-map 1:a \
-c:a pcm_s16le \
-ar 48000 \
-ac 2 \
-f s16le \
tcp://192.168.42.2:5001

Video File Streaming

sudo nmcli dev set enp2s0f0u6 managed no

sudo ifconfig enp2s0f0u6 \
192.168.42.1 \
netmask 255.255.255.0 up
adb shell tinymix 6 1
adb shell tinymix 14 14
ffmpeg \
-re \
-stream_loop -1 \
-i final.mov \
-vf "transpose=2,scale=240:320:flags=bicubic" \
-map 0:v \
-c:v libx264 \
-preset superfast \
-tune zerolatency \
-profile:v baseline \
-crf 12 \
-pix_fmt yuv420p \
-f h264 \
tcp://192.168.42.2:5000 \
-map 0:a \
-c:a pcm_s16le \
-ar 48000 \
-ac 2 \
-f s16le \
tcp://192.168.42.2:5001

Receiver Architecture

TCP H.264 Stream
        ↓
MediaCodec
        ↓
Hardware Decode
        ↓
Framebuffer / Surface
        ↓
LCD Panel

TCP PCM Audio
        ↓
TinyALSA
        ↓
Audio HAL
        ↓
Speaker

Phase 9 — Audio Configuration

On the Galaxy Star, audio outputs are disabled by default.

Enable the speaker using and up volume to full:

adb shell tinymix 6 1
adb shell tinymix 14 15

Mixer Controls

Control 6 : Speaker Function
Default   : Off
Enabled   : On

Relevant controls:

Control	Description
6	Speaker Function
7	Earpiece Function
8	HeadPhone Function
0	Master Playback Volume
5	HeadPhone Playback Volume
14	Inter PA Playback Volume

Performance Results

v1 (ADB)

Direct framebuffer rendering
No Android UI
No SurfaceFlinger
No MediaCodec
No Java runtime
Simple deployment

v2 (RNDIS + Raw RGB)

Removes ADB overhead
Approximately 2× higher throughput
Stable 61 FPS
Automatic reconnect support
Lower CPU usage

v3 (RNDIS + MediaCodec)

Hardware-accelerated H.264 decoding
Significantly lower network bandwidth
Optional audio playback
Excellent performance for live streams
More complex transport and synchronization

Final Architecture

Host PC
 ├─ FFmpeg
 │   ├─ Decode
 │   ├─ Scale
 │   ├─ Rotate
 │   └─ Encode (optional)
 │
 └─ TCP Streaming
          │
          ▼
USB Ethernet (RNDIS)
          │
          ▼
video_display_v2 / video_display_v3
          │
          ▼
Framebuffer / MediaCodec
          │
          ▼
LCD Panel

The result is a legacy Android smartphone transformed into a dedicated framebuffer-driven display appliance with virtually none of the standard Android graphics stack in the rendering path.

Name		Name	Last commit message	Last commit date
Latest commit History 10 Commits
ramdisk		ramdisk
tinyalsa		tinyalsa
README.md		README.md
boot.img		boot.img
bootimg.cfg		bootimg.cfg
custom_boot.img		custom_boot.img
custom_initrd.img		custom_initrd.img
htop-arm		htop-arm
initrd.img		initrd.img
libtinyalsa.so		libtinyalsa.so
main.cpp		main.cpp
run.sh		run.sh
video.cpp		video.cpp
video_display		video_display
video_display_v2		video_display_v2
video_display_v3		video_display_v3
video_display_v3.cpp		video_display_v3.cpp
video_v2.cpp		video_v2.cpp
zImage		zImage

Folders and files

Latest commit

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Bare-Metal Android Display Pipeline

Requirements

Architecture Evolution

Phase 1 — Custom Boot Image

Build Service

Repack Boot Image

Flash Through TWRP

Restore Stock Boot Image

Phase 2 — Native Framebuffer Receiver (v1)

Build

Receiver Flow

Phase 3 — ADB Video Streaming (v1)

Streaming Command

Data Path

Key Options

Phase 4 — USB Ethernet (RNDIS)

Device Network Script

Deploy Script

Host Configuration

Verify Connectivity

Phase 5 — TCP Framebuffer Receiver (v2)

Build

Receiver Architecture

Runtime Flow

Phase 6 — Raw RGB Streaming over Ethernet (v2)

Host Setup

Stream Video File

Stream OBS Virtual Camera

Lower Host CPU Usage

Data Path

Phase 7 — MediaCodec Receiver (v3)

Build (Video + Audio)

Deploy

Phase 8 — H.264 + Audio Streaming (v3)

OBS Live Stream

Video (TCP 5000)

Video File Streaming

Receiver Architecture

Phase 9 — Audio Configuration

Mixer Controls

Performance Results

v1 (ADB)

v2 (RNDIS + Raw RGB)

v3 (RNDIS + MediaCodec)

Final Architecture

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