Logarithmic slider

I have a slider with values ranging from 0 to 100.

I want to map them to a range from 100 to 10,000,000.

I've seen some functions scattered around the net but they're all in C++. I need it in Javascript.

Any ideas?

Solution

You can use a function like this:

function logslider(position) {
  // position will be between 0 and 100
  var minp = 0;
  var maxp = 100;

  // The result should be between 100 an 10000000
  var minv = Math.log(100);
  var maxv = Math.log(10000000);

  // calculate adjustment factor
  var scale = (maxv-minv) / (maxp-minp);

  return Math.exp(minv + scale*(position-minp));
}

The resulting values match a logarithmic scale:

js> logslider(0);
100.00000000000004
js> logslider(10);
316.22776601683825
js> logslider(20);
1000.0000000000007
js> logslider(40);
10000.00000000001
js> logslider(60);
100000.0000000002
js> logslider(100);
10000000.000000006

The reverse function would, with the same definitions for minp, maxp, minv, maxv and scale, calculate a slider position from a value like this:

function logposition(value) {
   // set minv, ... like above
   // ...
   return (Math.log(value)-minv) / scale + minp;
}

All together, wrapped in a class and as a functional code snippet, it would look like this:

// Generic class:

function LogSlider(options) {
   options = options || {};
   this.minpos = options.minpos || 0;
   this.maxpos = options.maxpos || 100;
   this.minlval = Math.log(options.minval || 1);
   this.maxlval = Math.log(options.maxval || 100000);

   this.scale = (this.maxlval - this.minlval) / (this.maxpos - this.minpos);
}

LogSlider.prototype = {
   // Calculate value from a slider position
   value: function(position) {
      return Math.exp((position - this.minpos) * this.scale + this.minlval);
   },
   // Calculate slider position from a value
   position: function(value) {
      return this.minpos + (Math.log(value) - this.minlval) / this.scale;
   }
};


// Usage:

var logsl = new LogSlider({maxpos: 20, minval: 100, maxval: 10000000});

$('#slider').on('change', function() {
   var val = logsl.value(+$(this).val());
   $('#value').val(val.toFixed(0));
});

$('#value').on('keyup', function() {
   var pos = logsl.position(+$(this).val());
   $('#slider').val(pos);
});

$('#value').val("1000").trigger("keyup");

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Input value or use slider:
<input id="value" />
<input id="slider" type="range" min="0" max="20" />