by this point i already had already sent out for a small reel of APA102 addressable leds. my eventual plan is to populate a custom board that will bolt to the acrylic panels. but for now i need to prototype with them. i tried hand soldering some wires, that failed...miserably... I didn't even take photos. i also didnt have any breakout boards for the 6 pin apa102s. but, the 8 pin soic breakouts i made for other things was close enough. 

i wired up two together and got down to the business of making a simple hue fader. now, here is where i fell down the rabbit hole. APA102s use RGB to control color, and i wanted Hue, Saturation, Value (HSV). the math to do this isnt particularly complicated, but doing it on an 8 bit processor (like the atmega328p) can be a bit time consuming. 

ill save you the rabbit hole: http://www.vagrearg.org/content/hsvrgb has a great write up of the fastest way to get from one to the other. including Open Source code for the atmega328p.

for this first step though a simple modification to the strandTest arduino library from adafruit was enough.

#include <Arduino.h>
#include <Adafruit_DotStar.h>
#include <SPI.h>         // COMMENT OUT THIS LINE FOR GEMMA OR TRINKET

#define NUMPIXELS 2 // Number of LEDs in strip

bool showing;

void setup() {
   uint8_t rgb[3];
   uint8_t val;
   //volatile to force debugger to cache the value.
   volatile uint16_t top,x; 


  strip.begin(); // Initialize pins for output
  strip.show();  // Turn all LEDs off ASAP
  for (int i=0;i<NUMPIXELS;i++) strip.setPixelColor(i,0);

  rgb[0]=0x00; // red
  rgb[1]=0xff; // green
  rgb[2]=0x00; // blue
  
  val=1;
  x=2;
  top=255;
  
  while(1)
  {
      rgb[x]=rgb[x]+val;
      if (rgb[x]==top)
      {
          //if counting up, now count down.
          if (val==1) { val=-1; top=0; }
          else        { val=1;  top=255;}

          // and modify the next primary color
          if (x==0) x=2;
          else x=x-1;
          
      }
      strip.setPixelColor(0,strip.Color(rgb[0],rgb[1],rgb[2]));
      // change the RGB order to get an out of phase hue.
      strip.setPixelColor(1,strip.Color(rgb[1],rgb[2],rgb[0]));
      strip.show();
      delay(10);
  }

void loop() {

}

the code replicates the following RGB values.

the acrylic from the last post was hand filed to sit on the leds and that led to:

onto the next acrylic revision.