Java BufferedImage to PNG format Base64 String

Question

I'm trying to get a screenshot output as a base64 encoded string but not getting very far. The code I have so far uses a Base64 library ( http://iharder.sourceforge.net/current/java/base64/ ):

    Robot robot = new Robot();
    Rectangle r = new Rectangle( Toolkit.getDefaultToolkit().getScreenSize() );
    BufferedImage bi = robot.createScreenCapture(r);
    ByteArrayOutputStream os = new ByteArrayOutputStream();
    OutputStream b64 = new Base64.OutputStream(os);
    ImageIO.write(bi, "png", os);
    ByteArrayOutputStream out = new ByteArrayOutputStream();
    out.writeTo(b64);
    String result = out.toString("UTF-8");

Each time I run this, "result" is always an empty string but I don't understand why. Any ideas?

Note: I don't want to have to write the png to a file on disk.

Answer 1

I followed xehpuk's answer but had issues with certain images having the last few rows of pixels missing when rendered in certain browsers via a data url (Chrome and Firefox, Safari seemed to render them fine). I suspect this is because the browser is doing it's best to interpret the data but the last few bytes of data was missing so it shows what it can.

The wrapping of the output stream seems to be the cause of this problem. The documentation for Base64.wrap(OutputStream os) explains:

It is recommended to promptly close the returned output stream after use, during which it will flush all possible leftover bytes to the underlying output stream.

So depending on the length of the data, it's possible the last few bytes are not flushed from the stream because close() isn't called on it. My solution to this was to not bother wrapping the stream and just encode the stream directly:

public static String imgToBase64String(final RenderedImage img, final String formatName)
{
  final ByteArrayOutputStream os = new ByteArrayOutputStream();

  try
  {
    ImageIO.write(img, formatName, os);
    return Base64.getEncoder().encodeToString(os.toByteArray());
  }
  catch (final IOException ioe)
  {
    throw new UncheckedIOException(ioe);
  }
}

This resolved the issues with the missing rows of pixels when rendered in a browser.

Answer 2

The following statement works in the wrong direction:

out.writeTo(b64);

It overwrites the Base 64 data with the empty byte array of out.

What's the purpose of out anyway? I don't think you need it.

Update:

And you write the image directly to os instead of writing through the Base 64 encoder.

The following code should work:

...
ByteArrayOutputStream os = new ByteArrayOutputStream();
OutputStream b64 = new Base64.OutputStream(os);
ImageIO.write(bi, "png", b64);
String result = os.toString("UTF-8");

Answer 3

Base64 encoding and decoding of images using Java 8:

public static String imgToBase64String(final RenderedImage img, final String formatName) {
    final ByteArrayOutputStream os = new ByteArrayOutputStream();
    try (final OutputStream b64os = Base64.getEncoder().wrap(os)) {
        ImageIO.write(img, formatName, b64os);
    } catch (final IOException ioe) {
        throw new UncheckedIOException(ioe);
    }
    return os.toString();
}

public static BufferedImage base64StringToImg(final String base64String) {
    try {
        return ImageIO.read(new ByteArrayInputStream(Base64.getDecoder().decode(base64String)));
    } catch (final IOException ioe) {
        throw new UncheckedIOException(ioe);
    }
}

Use it like this for your screenshot scenario:

final Robot robot = new Robot();
final Rectangle r = new Rectangle(Toolkit.getDefaultToolkit().getScreenSize());
final BufferedImage bi = robot.createScreenCapture(r);
final String base64String = imgToBase64String(bi, "png");