public void setBlurThreshold( float f ) {
        this.blurThreshold = f;
        if (material != null) {
            material.setFloat("BlurThreshold", blurThreshold);
        }
    }
Sets the minimum blur factor before the convolution filter is
 calculated.  The default is 0.2 which means if the "unfocus"
 amount is less than 0.2 (where 0 is no blur and 1.0 is full blurScale) 
 then no blur will be applied at all.  Depending on the GPU implementation,
 this may be an optimization since it uses branching to skip the expensive
 convolution filter.

 <p>In scenes where the focus distance is close (like 0) and the focus range
 is relatively large, this threshold will remove some subtlety in
 the near-camera blurring and should be set smaller than the default
 or to 0 to disable completely.  Sometimes that cut-off is desired if
 mid-to-far field unfocusing is all that is desired.</p>

@param f the desired blur factor (default=0.2)
jme3-effects/src/main/java/com/jme3/post/filters/DepthOfFieldFilter.java