Real-time desktop mirroring from X11 to a 3.5" ST7796 (320x480) display and capacitive touch support for FT6336. This project uses the bcm2835 hardware library and MIT-SHM (XShm) for low-latency screen capture and I2C for touch coordinate output.
- Real-time Mirroring: Captures X11 root window and mirrors it to SPI.
- Partial Updates: Sends only modified regions (dirty rectangles) to save SPI bandwidth.
- ST7796 Optimized: Full 320x480 resolution support with 16-bit RGB565.
- Capacitive Touch: Real-time X/Y coordinate logging via I2C (FT6336/FT6236).
- Synchronized Rotation: Integrated rotation defines for both LCD and Touch.
- High Performance: Uses Shared Memory (XShm) to minimize CPU overhead.
[Image of Raspberry Pi 4 GPIO pinout diagram]
| Function | Pin (Physical) | GPIO (BCM) | Notes |
|---|---|---|---|
| VCC | 1 or 17 | 3.3V | Power |
| GND | 6, 9, 14... | GND | Ground |
| SCL | 23 | 11 | SPI Clock |
| SDA | 19 | 10 | SPI MOSI |
| CS | 24 | 8 | SPI Chip Select |
| DC | 18 | 24 | Data/Command |
| RST | 15 | 22 | Reset |
| BL | 11 | 17 | Backlight |
| I2C SDA | 3 | 2 | Touch Data |
| I2C SCL | 5 | 3 | Touch Clock |
- Raspberry Pi OS: X11 environment (Wayland is not supported).
- SPI & I2C enabled: Enable via
sudo raspi-config. - bcm2835 library: Hardware access library.
- X11 Libraries: For screen capture.
sudo apt update
sudo apt install libx11-dev libxext-devInstall bcm2835 Library
wget [http://www.airspayce.com/mikem/bcm2835/bcm2835-latest.tar.gz](http://www.airspayce.com/mikem/bcm2835/bcm2835-latest.tar.gz)
tar zxvf bcm2835-latest.tar.gz
cd bcm2835-*
./configure
make
sudo make installUsing Makefile
make runManual Compilation
gcc -O3 -o mirror_screen mirror_screen.c -lX11 -lXext -lbcm2835 -lXtst
sudo ./mirror_screenAdjust the ORIENTATION define at the top of the code to match your mounting:
- 0: Portrait (320x480)
- 1: Landscape (480x320)
- 2: Portrait Inverted (320x480)
- 3: Landscape Inverted (480x320)
The code automatically swaps
DISP_W/DISP_Hand re-calculates touch coordinates based on this value to ensure they always match the visual output.
The default I2C address:
- For FT6336U: Use
0x38.
- Init: Initializes the BCM2835 library, setting up SPI (32MHz) for the display and I2C (100kHz) for the touch panel.
- LCD Sync: Unlocks the ST7796 specific extended command set (Command 2) and configures the
MADCTLregister for the selected rotation. - Capture: Utilizes the
XShmGetImageextension for high-speed desktop framebuffer grabbing directly from the X11 server. - Processing: Downscales the captured frame to the display resolution and converts pixels from RGB888 to RGB565 Big-Endian.
- Diffing: Compares the current frame with the previous one to identify "dirty" regions (rectangles that actually changed).
- Touch: Polls the I2C bus for touch events, extracts raw data, and applies rotation matrices to align coordinates with the LCD.
- Output: Prints real-time touch coordinates to the terminal and pushes visual updates to the LCD via SPI.
Free to use for personal and educational projects.