title: How to compute viewport from scrollbar?
body: <p>This is probably straight forward enough but for some reason I just can't crack it (well actually its because I'm terrible at maths sadly!).</p>
  
  <p>I have a viewport that shows 800 pixels at a time and I want to move it left and right over a virtual number of pixels say 2400 pixels wide.</p>
  
  <p>I use the relative position of a scrollbar (0-1) to determine the virtual pixel that the viewport should have moved to. </p>
  
  <p>I have a loop n = 800 that I run through. 
  I want to start reading an array of plottin points from the position the slider tells me and iterate through the next 800 points from that point.</p>
  
  <p>The problem I have is when my scrollbar is all the way to the right and its relative position = 1 my readpoint = 2400..... which in a way is right. But that  makes my read position of plotting points go to the last point and when I begin my iteration I have no points left to read.</p>
  
  <p>Based on my moving an 800 pixel wide sliding box analagy the left point of the box should be 1600 in this case and its right point should be 2400</p>
  
  <p>How can I achieve this?</p>
  
  <p>Do I need some sort of value mapping formula?</p>
  
  <pre><code>        private function calculateShuttleRelativePosition():void
          {
  
              var _x:Number=_scrollthumb.x
              _sliderCenter=_x + (_scrollthumb.width / 2)
              _sliderRange=_scrollbar.width - _scrollthumb.width;
              _slider_rel_pos=(_sliderCenter - _sliderCenterMinLeft) / _sliderRange
  
          }
  
  
  
  pixel = slider relative position * 2400
  readpoint= pixel
  </code></pre>