problem to save an image in JPG format on linux

Hello

I used two libraries (ImageIO and com.sun.image.codec.jpeg)
to save in JPG format an image of my application

in both cases it works on windows but it doesn't on linux (I checked
that on
SUSE and REDHAT) : the result is a black image!

has anyone an explanation ?

thanks in advance

Jean-Marie

PS : below is my code

// creating the image of my application
Dimension dim = parentframe.getSize();
BufferedImage bi = new BufferedImage
(dim.width,dim.height,BufferedImage.TYPE_INT_BGR);
Graphics bg = bi.getGraphics();
parentframe.paintAll(bg);

// looking for the best quality
Iterator iter = ImageIO.getImageWritersByFormatName("JPG");
if (iter.hasNext()) {
            ImageWriter writer = (ImageWriter) iter.next();
            ImageWriteParam iwp = writer.getDefaultWriteParam();
            iwp.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
            float values[] = iwp.getCompressionQualityValues();
            // Write one for each compression values
            float maxvalue=Float.MIN_VALUE;
            for (int i = 0, n = values.length; i < n; i++) {
                maxvalue=Math.max(maxvalue, values[i]);
}
iwp.setCompressionQuality(maxvalue);

// writing into a file
File outFile = new File(_filename);
FileImageOutputStream output = new FileImageOutputStream(outFile);
writer.setOutput(output);
IIOImage image = new IIOImage(bi, null, null);
writer.write(null, image, iwp);
}