Ouput different patterns on different channels (Help!)

Apologies if I posted this under the wrong catagory.

Hardware I got covered, coding is not my forte. My question is how do I apply different patterns to different channels on the output expander? The end goal is to apply it to a music sequencer where each frequency range outputs its control to a specific channel or channels on the output expander. ie Lows output to a pattern on channels 1 and 3, mids to a pattern on channels 0 and 2, highs to channel 5 etc.

For right now I am trying to just write some code that shows a different color in each section so I can then learn and apply that info. The problem is I can’t find a working example. If I could, I could figure it out by looking and breaking it down.

Here is my setup

0 = 150 pixels (Start Index 0)
1 = 80 (Index 150)
2 = 150 (Index 230)
3 = 80 (Index 380)
4 = 235 (Index 460)
5 = 300 (Index 695)
995 Pixels Total

Below is the most basic code showing solid red. However, it lights up all 995 of my pixels. How would I have it only light up the pixels attached to channel 0 of the output expander (Index 0 - 150). Then if I replaced that code with a more complicated pattern is there a way to define that entire pattern to only be executed within the confines of channel 0 on the output expander?

export function beforeRender(delta) {

export function render(index) {
  hsv(1, 1, 1)

Here is a photo of my project. The leds have not been fully installed yet to make troubleshooting easier. To give everyone a full picture of what I am trying to accomplish. Channels (1,3) are the strips that will be mounted underneath each speaker that I want to react to low frequencies (bass). Channels (0,2) will be mounted behind each speaker dedicated to mid frequencies (Midrange). Channel 4 would preferably be dedicated to the lower midrange mounted under the entertainment center and finally, channel 5 will be mounted to the upper part of the entertainment center and should react to high frequencies (Higher Midrange and above).

Thank you in advance to anyone who can help me out! My deadline is in a week for this project before hosting an event!!!

Hi @Cyptic_Wolfe!

We can certainly get you going.

I’ll write you something here that shows very basic control of different channels, then you’re going to want to look at the following patterns that ship with your Pixelblaze (or if you deleted them, find them on the pattern library).

  • Spectrum Analyzer 1D/2D shows one way to use different frequencies to control different sections of your LEDs
  • Multisegment shows how to run different patterns (very different - like, independent) on different sections

Different Colors on Different Channels

Each pattern tells every individual LED what color to be for the current frame. Each pixel, even the ones across all your various output expander channels, should have a unique index. Even if you had three channels, all the pixels can be thought of as strung together in one long chain.

For example, if you had:

Channel Pixels
0 3
1 5
2 2

This implies your Settings page should have a section you set up like this for the Output Expander Configuration:

Monosnap Desk 2 2023-09-29 01-25-04

Notice the Start Index column. That means your pixels are indexed like this:

Pixel index Is the __ pixel Of channel:
0 1st 0
1 2nd 0
2 3rd 0
3 1st 1
4 2nd 1
5 3rd 1
6 4th 1
7 5th 1
8 1st 2
9 2nd 2

To set a different color for the ones on the first channel (pixel index 0 and 1), you could branch your code with an if statement. Inside the export function render(index):

if (index <= 2) {
  hsv(0, 1, 1) // Channel 0's three pixels will be red
} else {
  hsv(0.33, 1, 1) // All other pixels will be green

To color the third segment, use an else if

if (index <= 2) {
  hsv(0, 1, 1) // Channel 0's three pixels will be red
} else if (index <= 7) {
  hsv(0.33, 1, 1) // Channel 1's five pixels will be green
} else {
  hsv(0.66, 1, 1) // // Channel 2's two pixels will be blue

This is worth understanding, but then you might start to write code that’s slightly more convenient to work with. This does the same thing but scales up a little better:

function getChannelFromIndex(index) {
  if (index <= 1) {
    return 0
  } else if (index <= 7) {
    return 1
  } else {
    return 2

function red() { hsv(0, 1, 1) }
function green() { hsv(0.33, 1, 1) }
function blue() { hsv(0.66, 1, 1) }

export function render(index) {
  var channel = getChannelFromIndex(index)

  if (channel == 0) {
  } else if (channel == 1) {
  } else {

Or you might feel fancy and want to optimize for typing less:

export function render(index) {
  hsv(0.33 * ((index > 2) + (index > 7)), 1, 1)

Monosnap Desk 2 2023-09-29 01-48-34

There are lots of different ways, the important thing is you keep messing with the code yourself to grow your skills.

You might have good luck using ChatGPT. I fed it the Pixelblaze code editor docs in chunks and then prompted it:

I want to code different patterns on different sections of the output indices. I call each segment a channel. For example, I might have channel 0 with the first 3 LEDs, channel 1 with indices 3 through 7, and channel 2 with two LEDs that have index 8 and 9. Can you write me a pixelblaze function that will take the channels, expressed as an array of their LED counts like var channelLEDs = [3, 5, 2] in this example, and render a different color on each channel?

The first answer included objects, stuff like:

var channelColors = [
  {h: 0.0, s: 1.0, v: 1.0},  // Red
  {h: 0.33, s: 1.0, v: 1.0}, // Green
  {h: 0.66, s: 1.0, v: 1.0}  // Blue
hsv(color.h, color.s, color.v);

So I said:

Oops - you might have forgotten that pixelblaze doesn’t have objects. Can it be done without using objects?

And then it produced this working Pixelblaze code that does the same as the examples above, in a more extensible way:

var channelLEDs = [3, 5, 2];
var channelHues = [0.0, 0.33, 0.66];
var channelSats = [1.0, 1.0, 1.0];
var channelVals = [1.0, 1.0, 1.0];

export function render(index) {
  var channelIndex = 0;
  var channelStart = 0;

  // Determine which channel this LED belongs to
  for (var i = 0; i < channelLEDs.length; i++) {
    if (index >= channelStart && index < channelStart + channelLEDs[i]) {
      channelIndex = i;
    channelStart += channelLEDs[i];

  var hue = channelHues[channelIndex];
  var sat = channelSats[channelIndex];
  var val = channelVals[channelIndex];
  hsv(hue, sat, val);

So start from these examples, then dive into the two patterns I linked above. You might even try feeding them to ChatGPT to get some hints. It’ll be easier if you use it as a tutor for each sub-problem on your way there, because then you’ll learn from trying small parts how to recognize code it produces that isn’t valid or working.

Good luck!


Thanks for the starting point!!! doing my best to learn what I need!