When I connect chain of LED strips to power from both ends, do I need to make the power line disconnect in the middle?

Hardware
41

And that does nothing else then combining the RGB components into on 32bit number and placing that integer in a pixel buffer at position px.
Just like this

#define LEDCOUNT 300
uint32_t pxBuffer[LEDCOUNT];
// actually for the APA102 protocoll you need some extra bytes for leadin/leadout
//uint32_t pxBuffer[1 + LEDCOUNT + LEDCOUNT/16 + 1]; // element 0 (leadin) ... LEDs ... 1 leadout clock per 2 LEDs (ceil)

bool needRefresh;
volatile bool canRefresh = true;

uint32_t setAPA102Color(int px, uint8_t r, uint8_t g, uint8_t b, uint8_t brightness = 31) {
  uint32_t color = 0;
  
  // due to big endianness in revers order 
  color |= (brighntess <<   0) | 0xE0;  //top 3 bits must be 111
  color |= (r          <<   8);
  color |= (g          <<  16);
  color |= (b          <<  24);

  pxBuffer[px] = color;

  return color;
}

void refreshDone() {
  canRefresh = true;
}

void setup() {
  SPI.begin();
  SPI.setClockSpeed(8, MHZ);
  //SPI.setDataMode(0);
  SPI.setBitOrder(MSBFIRST);
}

void loop() {
  // setup your pixels 

  if (needRefresh && canRefresh) {
    canRefresh = 
    needRefresh = false;
    SPI.transfer(pxBuffer, NULL, sizeof(pxBuffer), refreshDone);
  }
}

This is all you need to work with APA102 strips - actually it's already a bit more to also illustrate use of DMA and guard against messing up data while still being sent out in the background.

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