pwm-lightness 1.0.0

A tiny library to calculate CIE 1931 lightness tables for eyeball friendly PWM LED brightness.

Provides lightness correction tables for eyeball pleasing LED brightness.

Want a smooth fade on your pulsing LEDs or get lovely antialiasing on LED matrix fonts? You need to correct your raw linear brightness values for human eyeball persistence of vision perception sensitivity.

Otherwise known as the CIE 1931 Lightness curve. It is also covered in many books. The research was done in the 1920s, winding up as one part of the International Commission on Illumination (CIE) 1931 standard.

Use it to smoothly fade an LED:
This example assumes CircuitPython or adafruit-blinka.

import time, board, pulseio, pwm_lightness
PWM = pwm_lightness.get_pwm_table(0xffff, max_input=100)
output_pin = pulseio.PWMOut(board.D13)  # or analogio.AnalogOut(A0) & .value
while True:
    for v in range(100, -1, -1):
        output_pin.duty_cycle = PWM[v]
        time.sleep(0.02)
    for v in range(1, 100):
        output_pin.duty_cycle = PWM[v]
        time.sleep(0.02)

It is also useful for the RGB values you send to programmable LED strips. Those are also linear fraction of on-time PWM.

Use it with Pillow to make an antialiased font shine rather than look like a bold blur:
This example assumes a RPi connected to an I2C Adafruit charlieplexed LED matrix.

import pwm_lightness
PWM = pwm_lightness.get_pwm_table(60)  # 0..255 is our max LED PWM value.
import board, busio, adafruit_is31fl3731, PIL
i2c = busio.I2C(board.SCL, board.SDA)
led_matrix = adafruit_is31fl3731.CharlieBonnet(i2c)
font = PIL.ImageFont.truetype('fonts/RobotoCondensed-Regular.ttf', 16)
image = PIL.Image.new('L', (led_matrix.height, led_matrix.width), 0)
draw = PIL.ImageDraw.Draw(image)
# fill=255 gives us the most antialiasing detail to work with, we control
# overall brightness via the max range in our PWM table.
draw.text((0,-1), '?', fill=255, font=font)
image = image.point(PWM)  # Corrects linear 0..255 values for PWM lightness.
image = image.transpose(PIL.Image.ROTATE_90)  # Match the matrix orientation.
led_matrix.image(image)  # Send pixels to our LED display.

This code does work on CircuitPython or MicroPython microcontrollers so long as your build has floating point enabled. In microcontroller environments recomputing a table on device is often overkill. Precompute the table(s) you need offline and store them as data to save precious RAM, no on device floating point required.