How to read and write Png and jpeg with Alpha

Question

Hi
I have trouble reading and writeing PNG and JPEGs that have an alpha channel (for transparency).
Reading works, if i use Toolkit.getImage() method, but works NOT if i use ImageIO.read() method.
Writing does NOT work using ImageIO.write()method. Instead i got a "java.lang.UnsupportedOperationException: Unsupported write variant!"
See Test class and commandline output below:
/****************START*****************************/
package de.multivisual.bodo.test;
import java.awt.*;
import java.awt.image.*;
import java.io.File;
import java.net.URL;
import java.util.Iterator;
import javax.imageio.*;
import javax.imageio.metadata.IIOMetadata;
import javax.imageio.stream.ImageInputStream;
import javax.imageio.stream.ImageOutputStream;
public class AlphaChannelTest implements ImageObserver {
  Toolkit toolkit;
  Image img;
  public AlphaChannelTest() {
    super();
    toolkit = Toolkit.getDefaultToolkit();
    URL url =
      AlphaChannelTest.class.getResource(
        "/de/multivisual/bodo/test/" + "alphatest.png");
    img = toolkit.getImage(url);
    try {
      ImageInputStream imageInput =
        ImageIO.createImageInputStream(url.openStream());
      Iterator it = ImageIO.getImageReaders(imageInput);
      ImageReader reader = null;
      while (it.hasNext()) {
        reader = (ImageReader) it.next();
        System.out.println(reader.toString());
      reader.setInput(imageInput);
      ImageReadParam param = reader.getDefaultReadParam();
      BufferedImage bimg = reader.read(0, param);
      SampleModel samMod = bimg.getSampleModel();
      ColorModel colMod =       bimg.getColorModel();
      String[] propNames = bimg.getPropertyNames();
      IIOMetadata meta = reader.getImageMetadata(0);
      System.err.println("\n*****test image that was read using new Jdk 1.4 ImageIO.read() method");
      alphaTest(bimg);
    } catch (Exception e) {
      //e.printStackTrace();
    if (img != null)
      toolkit.prepareImage(img, -1, -1, this);
    try {
      synchronized (this) {
        System.out.println("wait");
        this.wait();
    } catch (Exception e) {
      e.printStackTrace();
    System.out.println("end");
  public void alphaTest(BufferedImage bi) {
    Raster raster = bi.getData();
    float[] sample = null;
    System.out.println("raster :");
    for (int y = 0; y < raster.getHeight(); y++) {
      for (int x = 0; x < raster.getWidth(); x++) {
        sample = raster.getPixel(x, y, sample);
        System.out.print("(");
        for (int i = 0; i < sample.length; i++) {
          System.out.print(":" + sample);
System.out.print(")");
System.out.println();
Raster araster = bi.getAlphaRaster();
if (araster == null){
     System.err.println("there is no Alpha channel!!!!!!!!!");
     return ;
} else {
     System.out.println("Alpha channel found !");
float[] asample = null;
System.out.println("raster alpha:");
for (int y = 0; y < araster.getHeight(); y++) {
for (int x = 0; x < araster.getWidth(); x++) {
asample = araster.getPixel(x, y, asample);
for (int i = 0; i < asample.length; i++) {
System.out.print(" " + asample[i]);
System.out.println();
String format ="PNG";
System.out.println("##########Test Writing using new JDK1.4.1 ImageIO:");
Iterator writers = ImageIO.getImageWritersByFormatName(format);
ImageWriter writer = (ImageWriter) writers.next();
ImageWriteParam param = writer.getDefaultWriteParam();
ImageTypeSpecifier imTy = param.getDestinationType();
ImageTypeSpecifier imTySp =
ImageTypeSpecifier.createFromRenderedImage(bi);
param.setDestinationType(imTySp);
File f = new File("c:/tmp/myimage."+format);
try {
ImageOutputStream ios = ImageIO.createImageOutputStream(f);
writer.setOutput(ios);
writer.writeInsert(0, new IIOImage(bi, null, null), param);
} catch (Exception e) {
     System.err.println("could not write "+format+" file with alpha channel !");
e.printStackTrace();
public boolean imageUpdate(
Image img,
int infoflags,
int x,
int y,
int width,
int height) {
if ((toolkit.checkImage(img, -1, -1, null)
& (ImageObserver.HEIGHT | ImageObserver.WIDTH | ImageObserver.ALLBITS))
== 35) {
int iw = img.getWidth(this);
int ih = img.getHeight(this);
BufferedImage bi = new BufferedImage(iw, ih, BufferedImage.TYPE_4BYTE_ABGR);
Graphics2D big = bi.createGraphics();
big.drawImage(img, 0, 0, this);
System.err.println("+++++test image that was read using old Toolkti.getImage method");
alphaTest(bi);
synchronized (this) {
this.notifyAll();
return false;
return true; // image is not yet completely loaded into memory
public static void main(String[] args) {
//     BufferedImage image = new
// BufferedImage();
new AlphaChannelTest();
/*************************END********************/
The commandline looks like this:
[i]
com.sun.imageio.plugins.png.PNGImageReader@d1fa5
*****test image that was read using new Jdk 1.4 ImageIO.read() method
raster :
there is no Alpha channel!!!!!!!!!
wait
+++++test image that was read using old Toolkti.getImage method
raster :
Alpha channel found !
raster alpha:
##########Test Writing using new JDK1.4.1 ImageIO:
could not write PNG file with alpha channel !
java.lang.UnsupportedOperationException: Unsupported write variant!
     at javax.imageio.ImageWriter.unsupported(ImageWriter.java:600)
     at javax.imageio.ImageWriter.writeInsert(ImageWriter.java:973)
     at de.multivisual.bodo.test.AlphaChannelTest.alphaTest(AlphaChannelTest.java:113)
     at de.multivisual.bodo.test.AlphaChannelTest.imageUpdate(AlphaChannelTest.java:135)
     at sun.awt.image.ImageWatched.newInfo(ImageWatched.java:55)
     at sun.awt.image.ImageRepresentation.imageComplete(ImageRepresentation.java:636)
     at sun.awt.image.ImageDecoder.imageComplete(ImageDecoder.java:135)
     at sun.awt.image.PNGImageDecoder.produceImage(PNGImageDecoder.java:511)
     at sun.awt.image.InputStreamImageSource.doFetch(InputStreamImageSource.java:257)
     at sun.awt.image.ImageFetcher.fetchloop(ImageFetcher.java:168)
     at sun.awt.image.ImageFetcher.run(ImageFetcher.java:136)
end

Answer 1

in between i found out that the my png and jpeg test images did not have an alpha channel, since the tool i used to create them, did not write the alpha channel to disk.
if i use png with alpha channel, then the read works correktly with ImageIO.read()
however the read problem still remains for gifs and the write does not work for gifs and neither for pngs.
whether jpegs can be read with alphachannel i don't know since i don't have a tool to create jpeg with alpha channel. (at least gimp and corel9 are not able to )
and it is not possible to write the previous read png with alpha channel back as and jpeg with alpha channel