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

Project description

pwm_lightness module

Generates lightness correction tables for eyeball pleasing LED brightness.

Antialiased question mark in portrait mode on a CharlieWing

Want a smooth fade on your pulsing LEDs? Want antialiasing on LED matrix fonts? Use this library to correct your raw linear brightness values for human eyeball persistence of vision perception sensitivity.

The adjustment is 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.

This module does work on CircuitPython (and MicroPython if you build with floating point enabled). In microcontroller environments recomputing a table on device is often overkill. Precompute the table(s) you need using this library and store them as data to save precious RAM.

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)
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 similarly useful with the RGB values you send to programmable LED strips.

Improve antialiased fonts or other 8-bit images

Shown here using Pillow to make an antialiased font shine rather than appearing as 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.  Control
# brightness using the max_output value passed to get_pwm_table() above.
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.

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