Sending negative values via pyserial problem

Question

I need to send mouse coordinates from python to arduino. As you know there is X and Y axis and there is some negative values like -15 or -10 etc. on those axis. Arduino's serial only accepts Bytes so bytes are limited with 0 to 256. My problem is starts right here. I cant send negative values from python to arduino. Here is my code for python :

def mouse_move(x, y):
    pax = [x,y]
    arduino.write(pax)
    print(pax)

For example when x or y is negative value like -5 , Program crashes because byte array is 0-256 .

here is my arduino's code:

#include <Mouse.h>

byte bf[2];
void setup() {
  Serial.begin(9600);
  Mouse.begin();
}

void loop() {
  if (Serial.available() > 0) {
    Serial.readBytes(bf, 2);
    Mouse.move(bf[0], bf[1], 0);
    Serial.read();
  }
}

Answer 1

You need to send more bytes to represent each number. Let's say you use 4 bytes per number. Please note that this code needs to be adapted to the arduino endianess. On python side you would have to do something like:

def mouse_move(x, y):
    bytes = x.to_bytes(4, byteorder = 'big') + y.to_bytes(4, byteorder = 'big')
    arduino.write(bytes)

    print(pax)

On receiver side you need to reconstruct the number from their bytes constituants something like:

byte bytes[4] 
void loop() {
  int x,y; /* use arduino int type of size 4 bytes  */
  if (Serial.available() > 0) {
    Serial.readBytes(bytes, 4);
    x = bytes[0] << 24 | bytes[1] << 16 |  bytes[2] << 8 |  bytes[0]
    Serial.readBytes(bytes, 4);
    y = bytes[0] << 24 | bytes[1] << 16 |  bytes[2] << 8 |  bytes[0]
    Mouse.move(x, y, 0);
    Serial.read();
  }
}