title: openlayers additive opacity does not render equally
body: <p>I recently integrated the <a href="http://madrobby.github.com/scriptaculous/slider/" rel="nofollow">scriptaculous slider</a> with a few similar layers to acheive a 'tween' effect between the layers.</p>
  
  <p>As the slider is dragged, the layers opacity is altered. There are never more than 2 layers visible at any one time and at the intersecting points, only one layer is visible.</p>
  
  <p>The total opacity for the two (or one) visible layers is always 0.63, so the layers will be allocated a portion of that for their individual opacity level ie at one point, layerA=0.1 and layerB=0.53.</p>
  
  <p>The layers are all solid colours and in many cases, the same colour covers the same area for all of layers.</p>
  
  <p>As far as the math is concerned, my implementation is perfect.</p>
  
  <p>The <strong>problem</strong> is that while the total opacity for the 2 layers remains at 0.63, the visual transparency for the combination fluctuates. As the midpoint between 2 layers is reached, the combined visual transparency increases.</p>
  
  <p>This gives a flickering effect while the slider is dragged.</p>
  
  <p>Essentially, if <code>a</code> and <code>b</code> are both solid red visible layers over a baselayer, my observation is that:</p>
  
  <pre><code>a.opacity=0.31 + b.opacity=0.32 &lt; a.opacity=0.01 + b.opacity=0.62
  </code></pre>
  
  <p>I have tested this on chrome and firefox with identical results.</p>
  
  <p>Is this an openlayers, javascript or renderer issue and is there a alternative approach?</p>