Offset LED

[jsammarco]

Hi, I have an LED that is not part of my grid and I have offset it in my alternating-matrix.
ws281x.setIndexMapping(alt_matrix(75,8));
I have text scrolling great but my issue is my 601st led is just yellow and doesnt respond. I tried to manually adjust the array to fix it and just no luck. Here is my code.
-Thanks great work

const Image = require('canvas').Image;
var fs = require('fs'),
	getPixels = require("get-pixels"),
	led_lib = require('ledutils'),
	Gpio = require('pigpio').Gpio,
	ws281x = require('rpi-ws281x-native'),
	alt_matrix = require('./alternating-matrix.js'),
    canvas = require('rpi-ws281x-canvas').create(75,8),
    ctx = canvas.getContext('2d'),
	NUM_LEDS = 602,
	pixelData = new Uint32Array(NUM_LEDS),
	pixelDataArr = new Array(NUM_LEDS);
	moving = false;
	
ws281x.init(NUM_LEDS, {
  frequency: 800000, // 800kHz (some LEDs run at 400kHz)
  dmaNum: 10, // DMA-number to be used
  gpioPin: 12, // GPIO-PIN to use (this is the only one i tested)
  invert: 0,
  brightness: 100
});

ws281x.setIndexMapping(alt_matrix(75,8));

// ---- trap the SIGINT and reset before exit
process.on('SIGINT', function () { //CLEARS LEDS
  ws281x.reset();
  process.nextTick(function () { process.exit(0); });
});

console.log("Started");
var i = -55;
ctx.scale(-1, 1);
setInterval(function () {
	if(i > 75){ i = -55; }
	ctx.fillStyle = 'blue';
	ctx.translate(0, 8);
	ctx.scale(-1, -1);
	ctx.font = 'bold 11px times';
	ctx.fillText("Joe is Cool", i, 8, 75);
	ctx.fillStyle = '#000000';
	ctx.fillRect(-75, 0, 75, 75);
	ctx.fillStyle = '#ffffff';
	ctx.fillRect(0, 0, 1, 8);
	ws281x.render(canvas.toUint32Array());
	i++;
}, 24);

alternating-matrix.js

module.exports = function(width, height) {
    var map = new Uint16Array((width*height)+1);//One for extra LED offset
    for(var i = 0; i<map.length+1; i++) {
        var row = Math.floor(i/width), col = i % width ;

        if((row % 2) === 0) {
            map[i+1] = i;
        } else {
            map[i+1] = (row+1) * width - (col+1);
        }
    }
    //map[599] = 55;//(width*height)-width+1;
    //map[0] = 999;
    map[600] = 525;
    //map[(width*height)-width-1] = (row+1) * width - (col+1);
    console.log(map[524], map[525], map[526]);
    console.log(map[600]);
    return map;
};

[jsammarco]

I have tried these changes and still have an issue with the last LED in the strip.

module.exports = function(width, height) {
    var map = new Uint16Array((width*height)+2);//One for extra LED offset
    for(var i = 0; i<map.length+1; i++) {
        var row = Math.floor(i/width), col = i % width ;

        if((row % 2) === 0) {
            map[i+1] = i;
        } else {
            map[i+1] = (row+1) * width - (col+1);
        }
    }
    //map[599] = 55;//(width*height)-width+1;
    //map[0] = 999;
    //map[600] = 525;
    map[0] = 999;
    map[601] = 524;
    //map[(width*height)-width-1] = (row+1) * width - (col+1);
    console.log(map);
    return map;
};