Test of Adafruit Feather M4 Express' built-in NeoPixel, with modified WheelDim() function.

NeoPixelDim.ino

#include <Adafruit_NeoPixel.h>
#ifdef __AVR__
  #include <avr/power.h>
#endif

// Modified by @erico to test Adafruit Feather M4 Express

// The NeoPixel pin is defined as 6 in examples.
// Adafruit Feather M4 Express uses pin 8 for its built-in  NeoPixel.
#define PIN 8

// Parameter 1 = number of pixels in strip
// Parameter 2 = Arduino pin number (most are valid)
// Parameter 3 = pixel type flags, add together as needed:
//   NEO_KHZ800  800 KHz bitstream (most NeoPixel products w/WS2812 LEDs)
//   NEO_KHZ400  400 KHz (classic 'v1' (not v2) FLORA pixels, WS2811 drivers)
//   NEO_GRB     Pixels are wired for GRB bitstream (most NeoPixel products)
//   NEO_RGB     Pixels are wired for RGB bitstream (v1 FLORA pixels, not v2)
//   NEO_RGBW    Pixels are wired for RGBW bitstream (NeoPixel RGBW products)
Adafruit_NeoPixel strip = Adafruit_NeoPixel(2, PIN, NEO_GRB + NEO_KHZ800);

// IMPORTANT: To reduce NeoPixel burnout risk, add 1000 uF capacitor across
// pixel power leads, add 300 - 500 Ohm resistor on first pixel's data input
// and minimize distance between Arduino and first pixel.  Avoid connecting
// on a live circuit...if you must, connect GND first.

void setup() {
  // This is for Trinket 5V 16MHz, you can remove these three lines if you are not using a Trinket
  #if defined (__AVR_ATtiny85__)
    if (F_CPU == 16000000) clock_prescale_set(clock_div_1);
  #endif
  // End of trinket special code


  strip.begin();
  strip.show(); // Initialize all pixels to 'off'
}

void loop() {
  rainbow(5);
}

void rainbow(uint8_t wait) {
  uint16_t i, j;
  uint8_t maxBright = 4;
  
  for(j=0; j<256; j++) {
    for(i=0; i<strip.numPixels(); i++) {
      strip.setPixelColor(i, WheelDim((i+j) & 255, maxBright));
    }
    strip.show();
    delay(wait);
  }
}

// A version of Wheel that dims pixels down for photos/video
// maxBright is from 0 to 255; low numbers are dimmer.
// NeoPixel brightness is nonlinear, so play with the maxBright number. I used 4 for recent video.
uint32_t WheelDim(byte WheelPos, uint8_t maxBright){
  WheelPos = 255 - WheelPos;
  uint16_t r = 0, g = 0, b = 0;
  
  if(WheelPos < 85) {
    r = 255 - WheelPos * 3;
    b = WheelPos * 3;
  } else if(WheelPos < 170) {
    WheelPos -= 85;
    g = WheelPos * 3;
    b = 255 - WheelPos * 3;
  } else {
    WheelPos -= 170;
    r = WheelPos * 3;
    g = 255 - WheelPos * 3;
  }

  r = r * maxBright / 255;
  g = g * maxBright / 255;
  b = b * maxBright / 255;

  return strip.Color(r, g, b);
}