serial_demo.py

import numpy as np
import cv2 as cv
import matplotlib.pyplot as plt
from pathlib import Path

import easyocr
from serial import Serial

from recognize_plates import get_plate_number

def receive_plate_image(serial: Serial) -> np.ndarray:

    # TODO:
    # NEED TO ADD SOME SORT OF SYNCHRONIZATION IN HERE.
    # I.e., the arduino needs to send some "START" sequence/string that this
    # function will block until it reads that sequence! Then it can carry on
    # with everything else.
    START_SEQUENCE = "NEW_IMAGE"
    serial.read_until(expected=START_SEQUENCE)

    # Constants for the small protocol we're using to send the plate images.
    HEIGHT_BYTES = 1 # Sent as a uint8_t
    WIDTH_BYTES = 1 # Sent as a uint8_t
    NUM_CHANNELS = 3 # RGB image, 3 channels. RGB order.
    PLATE_BYTE_COUNT_BYTES = 4 # Sent as a uint32_t

    # 1. Read height and width.
    height_px = int.from_bytes(serial.read(HEIGHT_BYTES), byteorder="big")
    width_px = int.from_bytes(serial.read(WIDTH_BYTES), byteorder="big")
    # 2. Read in the number of bytes comprising the plate image that will be
    #    sent.
    num_bytes = int.from_bytes(serial.read(PLATE_BYTE_COUNT_BYTES), byteorder="big")
    print(f"Received metadata: Height: {height_px}, Width: {width_px}, num_bytes: {num_bytes}")
    # 3. Read as many bytes as instructed in the previous step here.
    plate_image_bytes = serial.read(num_bytes)

    # Now, need to construct the image from the above bytes.
    # Per our small "protocol," the image bytes should be sent starting with the
    # first row and first column, then first row second column, etc. For
    # simplicity, we're assuming that our images are grayscale. So, Grayscale 8
    # bit--meaing we have h rows, w columns, and an 8 bit value (one byte) at
    # each h,w (y,x) position.
    # Starter example: https://stackoverflow.com/a/17170855 and
    # https://stackoverflow.com/a/28235794
    # np.fromstring preferred here, as it makes a copy of the buffer, rather
    # than just creating a view of it (dangerous if the underlying bytes change).
    image_np = np.fromstring(string=plate_image_bytes, dtype=np.int8)
    # TODO: Test this. Not sure if this is going to behave as expected here.
    image = np.reshape(image_np, newshape=(height_px, width_px, NUM_CHANNELS), order='C')
    # image = cv.imdecode(buf=image_np, flags=cv.IMREAD_GRAYSCALE)

    return image

if __name__ == "__main__":
    
    # Create a new OCR pipeline to use for the lifetime of the demo.
    print("Instantiating new OCR pipeline")
    reader = easyocr.Reader(lang_list=["en"], gpu=False)

    # While true...
    #   receive a new plate
    #   pass the plate through the OCR pipeline.
    #   print out resulting text.

    # Open a serial session/fd to receive data from the specified serial port.
    # This will exist for the duration of the demo.
    serial_port = "COM4"
    baud_rate = 9600 # 115200 for BLE sense, 9600 for ESP32.
    with Serial(port=serial_port, baudrate=baud_rate) as serial:
        print(f"Successfully started communication on serial port {serial_port} with baud rate {baud_rate}")
        while True:
            print(f"Waiting to receive another image...")   
            # Read plate image from the serial port.
            plate_image = receive_plate_image(serial=serial)
            print(f"Successfully received new plate image.")
            # TODO Could display it here.
            cv.imshow(winname="Received Plate Image", mat=plate_image)
            # Pass the plate image into the OCR pipeline.
            plate_number = get_plate_number(plate_image=plate_image, reader=reader)
            print(f"Hub received plate number {plate_number} from meter!")