Posted by hernan on
URL: https://forum.jogamp.org/Able-to-disable-JOGL-s-PNG-decoder-tp4027701p4027739.html

"It appears that the alpha channel is rendered instead of the indexed color channel."

There is no "alpha channel" in a PNG indexed image. What happens here is that the palette is being ignored by JOGL, and the pixels values (indexes into the palette) are interpreted as grayscale values - or, equivalently, a grayscale palette is being assumed. The current result (screenshot in the issue attachment) can be reproduced by this PNGJ-only code:

        public static void main(String[] args) {
                indexedToG("C:/temp/indexed.png","C:/temp/indexed2.png");
        }

        private static void indexedToG(String src, String tgt) {
                PngReader pngr =FileHelper.createPngReader(new File(src));
                ImageInfo imi1 = pngr.imgInfo;
                ImageInfo imi2 = new ImageInfo(imi1.cols,imi1.rows,8,false,true,false); // grayscale
                PngWriter pngw = FileHelper.createPngWriter(new File(tgt),imi2,false);
                for(int i=0;i<imi1.rows;i++) {
                        ImageLine line = pngr.readRow(i);
                        pngw.writeRow(line,i);
                }
                pngr.end();
                pngw.end();
        }


Hernan
https://code.google.com/p/pngj/