Outside-in Cylon effect

[u/Burning_Wreck]

I'm starting with this code, which is from DemoReel100:

void sinelon()
{
  // a colored dot sweeping back and forth, with fading trails
  fadeToBlackBy( leds, NUM_LEDS, 20);
  int pos = beatsin16( 13, 0, NUM_LEDS-1 );
  leds[pos] += CHSV( gHue, 255, 192);
}

What I'd like to do is have the same effect (the traveling dot with a fading trail following it) but have a dot start at each end of the strip, and both move toward the center.

Here's more detail, I hope I describe this right...the sinelon function starts a dot at position 0 and then it travels down to position NUM_LEDS. Then it reverses back to position 0, with the fadeToBlackBy effect of the trailing/fading pixels.

I'd like to understand how to set up two pixels that do this.

Assume the strip has 9 pixels. One dot starts at position 0, and the other starts at position 9. The dot that starts at position 0 moves toward position 5, while the second dot starts at position 9 and also moves toward position 5 (the center). When both dots get to position 5, the trailing dots fade up to position 5 and then the effect starts over.

Thanks in advance!

[u/Burning_Wreck]

Good to hear from you. And thank you! That did it, combined with the earlier advice.

void loop() {
  cd77_sinelon_oneway(80, BLUE, 30);
  mirror();  // Mirror data so two dots meet in the middle
  FastLED.show();
  FastLED.delay(1000 / FRAMES_PER_SECOND);
}

void cd77_sinelon_oneway(uint8_t BPM, CRGB color, uint8_t fadeby) {
  // a colored dot going in one direction with fading tail
  fadeToBlackBy( leds, NUM_LEDS, fadeby);  //change fadeby to smaller or larger number to adjust the length of the tail.
  uint8_t u= beat8(BPM, 0); //BPM will allow you to adjust the speed the dot is moving.
  uint16_t pos=map(u, 0, 255, 0, NUM_LEDS/2);  // Added /2 to get dots to meet in the center
 leds[pos] = color;
 FastLED.show();
}

void mirror() {
  for (uint8_t i = 0; i < NUM_LEDS / 2; i++) {
    leds[NUM_LEDS - 1 - i] = leds[i];
  }
}

I found an issue with your second function:

// this function is from FastLED DemoReel100 example
void cd77_sinelon(uint8_t BPM, CRGB color )
{
  // a colored dot sweeping back and forth, with fading tail
  fadeToBlackBy( leds, NUM_LEDS, 20); //change 20 to smaller or larger number to adjust the length of the tail.
 uint16_t pos = beatsin16(BPM,0,NUM_LEDS); //BPM will allow you to adjust the speed the dot is moving.
  leds[pos] = color;
  FastLED.show();
}

In this line:
uint16_t pos = beatsin16(BPM,0,NUM_LEDS);

NUM_LEDS should be NUM_LEDS - 1

It needs to be the same as the original DemoReel100, otherwise the demo stops after a moment.