public int getBackgroundPaletteIndex() {
            if (!backgroundSet) {
                throw new IllegalStateException(PropertyUtil.getString("PNGEncodeParam4"));
            }
            return backgroundPaletteIndex;
        }
Returns the palette index of the suggested background color.

<p> If the background palette index has not previously been
set, or has been unset, an
<code>IllegalStateException</code> will be thrown.

@throws IllegalStateException if the palette index is not set.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java