The global alpha value to be used for every pixel.
<p>
If this value is set, the <code>alphaData</code> field
is ignored and when the image is rendered each pixel
will be blended with the background an amount
proportional to this value.
</p><p>
The default is -1 which means 'no global alpha value'
</p>
The global alpha value to be used for every pixel. <p> If this value is set, the <code>alphaData</code> field is ignored and when the image is rendered each pixel will be blended with the background an amount proportional to this value. </p><p> The default is -1 which means 'no global alpha value' </p>