public void setAlpha(float alpha) {
        letters.setBaseAlpha(alpha);
        needRefresh = true;
    }
Sets an overall alpha that will be applied to all
 letters.  If the alpha passed is -1 then alpha reverts
 to default... which will be 1 for anything unspecified
 and color tags will be reset to 1 or their encoded
 alpha.

@param alpha the desired alpha, or -1 to revert to the default
jme3-core/src/main/java/com/jme3/font/BitmapText.java