public void setPaletteTransparency(byte[] alpha) {
            transparency = new int[alpha.length];
            for (int i = 0; i < alpha.length; i++) {
                transparency[i] = alpha[i] & 0xff;
            }
            transparencySet = true;
        }
Sets the alpha values associated with each palette entry.
The <code>alpha</code> parameter should have as many entries
as there are RGB triples in the palette.

<p> The 'tRNS' chunk will encode this information.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java