public void unsetBackground() {
        throw new RuntimeException(PropertyUtil.getString("PNGEncodeParam23"));
    }
Suppresses the 'bKGD' chunk from being output.
For API compatibility with JAI 1.0, the superclass
defines this method to throw a <code>RuntimeException</code>;
accordingly, subclasses must provide their own implementations.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java