downscale PNG and print on bluetooth printer from Windows

thermal_print_png.py

# /// script
# requires-python = ">=3.11"
# dependencies = [
#     "pillow>=11.2.1",
# ]
# ///

# usage: uv run this_script.py <path_to_png>

import sys
import socket
from PIL import Image

def print_png(file_path):
    # Printer config
    printer_mac = "00:02:5B:21:28:53"  # Your printer's MAC address
    printer_width = 384  # pixels, adjust for your printer
    
    # Windows Bluetooth constants
    BTPROTO_RFCOMM = 3

    # Open and process image
    img = Image.open(file_path)
    img = img.convert('1')  # 1-bit pixels, black and white
    # Resize to printer width, keep aspect ratio
    w, h = img.size
    new_h = int((printer_width / w) * h)
    img = img.resize((printer_width, new_h), Image.LANCZOS)

    # Each ESC * line can print up to 24 dots (vertical) at once
    rows = img.height

    # Connect to printer via Bluetooth socket
    try:
        # Create Bluetooth RFCOMM socket
        sock = socket.socket(socket.AF_BLUETOOTH, socket.SOCK_STREAM, BTPROTO_RFCOMM)
        print(f"Connecting to {printer_mac}...")
        
        # Connect to printer (port 1 is standard for SPP)
        sock.connect((printer_mac, 1))
        print("Connected!")
        
        sock.send(b'\x1B\x33\x00')  # Set line spacing

        # For each chunk of 24 rows
        for y in range(0, rows, 24):
            # Prepare 24 rows of data
            slice_height = min(24, rows - y)
            line_data = bytearray()

            for x in range(printer_width):
                # For each column, pack 24 bits into 3 bytes (top byte first)
                column_bytes = [0, 0, 0]
                for b in range(slice_height):
                    pixel = img.getpixel((x, y + b))
                    if pixel == 0:  # Black pixel
                        column_bytes[b // 8] |= (1 << (7 - (b % 8)))
                line_data.extend(column_bytes)

            # ESC * m nL nH [d1...dk]
            m = 33  # 24-dot double-density
            nL = printer_width & 0xFF
            nH = (printer_width >> 8) & 0xFF
            cmd = bytes([0x1B, 0x2A, m, nL, nH])
            sock.send(cmd)
            sock.send(bytes(line_data))
            sock.send(bytes([0x0A]))  # Line feed

        print("Image sent to printer.")
        sock.close()
    except Exception as e:
        import traceback
        print(f"Could not print image: {e}")
        traceback.print_exc()

if __name__ == "__main__":
    if sys.platform != "win32":
        print("This script only works on Windows.")
        sys.exit(1)

    if len(sys.argv) != 2:
        print("Usage: python print_png.py <path_to_png>")
    else:
        print_png(sys.argv[1])