SOURCE

var w=20000,h=6000,size=256,l=(w>h)?w:h
console.log('l',l)
var a = Math.log(l / size) / Math.log(2)
var z = Math.ceil(a)

console.log('zoom:', z)

cut()
for (var i = z-1; i > 0; i--) {
    console.log(i)
    resize(i)
}

function cut() {
    console.log('cut')
}

function resize(i) {
    var nw=w/(z-i)
    console.log('resize',nw)
    cut()
}