public int[] getSignificantBits() {
        if (!significantBitsSet) {
            throw new IllegalStateException(PropertyUtil.getString("PNGEncodeParam18"));
        }
        return (int[])significantBits.clone();
    }
Returns the number of significant bits for each band of the image.

<p> If the significant bits values have not previously been
set, or have been unset, an <code>IllegalStateException</code>
will be thrown.

@throws IllegalStateException if the significant bits values are
       not set.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java