public int[] getTransparentRGB() {
            if (!transparencySet) {
                throw new IllegalStateException(PropertyUtil.getString("PNGEncodeParam10"));
            }
            return (int[])(transparency.clone());
        }
Returns the RGB value to be used to denote transparency.

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

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