How to use multithreading for LCD output on the raspberry pi

Question

Writing to the 16x2 LCD display on the raspberryp pi can take some time to finish, especially with the module I wrote that automatically scrolls text that exceeds the length of the display.

I need to use multithreading, or something similar, to send the output to the display and continue with the rest of the program. I've tried a couple things with multithreading, but haven't quite got it.

This is the working code without any multithreading. The method I want to be multithreaded is "TextToLCD.ProcessFrameBuffer".

piBell.py

#!/usr/bin/env python3

import time
import rekognition
import TextToLCD
import PiPhoto
import json
import logging
import re
import threading
from queue import Queue

logFormatter = logging.Formatter("%(asctime)s [%(name)-8.8s]/[%(funcName)-12.12s] [%(levelname)-5.5s]  %(message)s")
rootLogger = logging.getLogger('piBell')

fileHandler = logging.FileHandler("{0}/{1}.log".format("./", "piBell"), 'a')
fileHandler.setFormatter(logFormatter)
rootLogger.addHandler(fileHandler)

consoleHandler = logging.StreamHandler()
consoleHandler.setFormatter(logFormatter)
rootLogger.addHandler(consoleHandler)

reFace = re.compile('face|head|selfie|portrait|person', re.IGNORECASE)

def main(debugMode='INFO'):
    TextToLCD.Clear()
    rootLogger.setLevel(debugMode)
    imgRotation = 270
    imgPath = './'
    imgName = 'image.jpg'

    TextToLCD.ProcessFrameBuffer(["Scanning:", "................."], debugMode)
    PiPhoto.GetPhoto(imgPath + imgName, imgRotation, "INFO")

    rootLogger.info("Sending image to rekognition.")
    TextToLCD.ProcessFrameBuffer(["Processing","................."], debugMode)

    jsonLabels = rekognition.get_labels(imgPath + imgName)
    rootLogger.info("Obtained JSON payload from rekognition.")
    rootLogger.debug(json.dumps(jsonLabels))

    if len(json.dumps(jsonLabels)) > 0:
        if IsFace(jsonLabels):
            if TestFace(imgPath + imgName):
                TextToLCD.ProcessFrameBuffer(['Hello', '      :)'], debugMode)

                celeb = IsCelebrity(imgPath + imgName)
                if celeb:
                    TextToLCD.ProcessFrameBuffer(["You look like:", celeb], debugMode)
            else:
                rootLogger.info("No face detected.")
                TextToLCD.ProcessFrameBuffer(['No face detected', '       :('], debugMode)

        else:
            rootLogger.info("No face detected.")
            TextToLCD.ProcessFrameBuffer(['No face detected', '       :('], debugMode)
    else:
        rootLogger.error("JSON payload from rekognition was empty.")

def IsFace(jsonPayload):
    for value in jsonPayload:
        rootLogger.info("Label: " + value['Name'] + ", Confidence: " +  str(round(value['Confidence'])))
        rootLogger.debug(json.dumps(jsonPayload))

        if reFace.match(value['Name']) and round(value['Confidence']) > 75:
            rootLogger.info("Possible face match.")
            return True
    return False

def TestFace(img):
    jsonFaces = rekognition.get_faces(img)
    rootLogger.debug(json.dumps(jsonFaces))

    if len(json.dumps(jsonFaces)) > 2:
        for item in jsonFaces:
            if item['Confidence']:
                if item['Confidence'] > 75:
                    rootLogger.info("Face detected. Confidence: " + str(round(item['Confidence'])))
                    return True
    else:
        rootLogger.info("No facial data obtained.")

    return False

def IsCelebrity(img):
    celebMatchAccuracy = 25
    jsonCelbFaces = rekognition.get_celebrities(img)
    rootLogger.debug(json.dumps(jsonCelbFaces))

    if len(json.dumps(jsonCelbFaces)) > 2:
        for item in jsonCelbFaces:
            if item['MatchConfidence']:
                if item['MatchConfidence'] > celebMatchAccuracy and item['Name']:
                    rootLogger.info("Celebirity match detected: " + item['Name'] + ", Confidence: " + str(round(item['MatchConfidence'])))

                    return item['Name']
    else:
        rootLogger.info("No celebirity match found.")

    return False


if __name__ == "__main__":
    main('INFO')

I did write the TextToLCD module myself. I can post it if you wish. I didn't include it at first as I didn't think it would be necessary. — japtain.cack, Oct 21 '17 at 17:57
Might be easier to use [`asyncio`](https://docs.python.org/3/library/asyncio.html) instead of `threading`. — Lawrence D'Oliveiro, Oct 22 '17 at 02:38

score 0 · Answer 1 · answered Oct 21 '17 at 17:51

First of all, it would be nice to see your LCD functions. You are using hardware, so hardware is a limited resource. For this you will need some kind of access control, this could be implemented with a Lock Object or Event Object.

You have two choices when you used the display:

Run the current writing to the end

Interrupt the current writing

import threading

import time
def parallelWithLock(lock:threading.Lock, name:str):

    with lock:
         for i in range(5):
         print(f"{name}: {i}")
         time.sleep(0.5)
         # doWantYouWant(...)

def parallelWithInterrupt(event:threading.Event,lock:threading.Lock,name:str):


    event.set()
    i = 0
    with lock:
        event.clear()
        while True:# or writing
             if event.isSet():
                  print(f"{name} Interrupted!")
                  break
             print(f"{name}: {i}")
             time.sleep(1)
             i += 1
             #doWantYouWant(...)

 if __name__ == '__main__':
      lock = threading.Lock()

      t1 = threading.Thread(target=parallelWithLock,args=(lock,"Thread_1"))
      t2 = threading.Thread(target=parallelWithLock,args=(lock,"Thread_2"))
      t1.start()
      t2.start()
      t1.join()
      t2.join()
      event = threading.Event()
      lock = threading.Lock()
      t3 = threading.Thread(target=parallelWithInterrupt,args=(event,lock,"Thread_3"))
      t4 = threading.Thread(target=parallelWithInterrupt,args=(event,lock,"Thread_4"))
      t5 = threading.Thread(target=parallelWithInterrupt, args=(event,lock, "Thread_4"))
      t3.start()
      time.sleep(5)
      t4.start()
      time.sleep(3)
      t5.start()
      t3.join()
      t4.join()
      time.sleep(2)
      event.set()
      t5.join()

How to use multithreading for LCD output on the raspberry pi

piBell.py

1 Answers1