Resizing images with JAI

If you haven't guessed already, the category "GoogleFood" is just a place for me to put stuff and find it later, and make it easier to find using Google.

Anyways...

When you want to resize an image using Java Advanced Imaging (JAI), most tutorials on the web say to use JAI "resize" operator. Unfortunately, if you do that, your images will look like crap because it doesn't do any antialiaing. It doesn't matter what RenderHints you pass to it, it will always look bad. I tried applying low-pass filters both before and after the scaling, and it still looked bad.

The trick is to convert your RenderedImage to a BufferedImage and then use the .getScaledInstance() method on it. That will properly antialias the scaling.

Here's some code to convert from a RenderedImage to a BufferedImage...

    public BufferedImage fromRenderedToBuffered(RenderedImage img) {
        if (img instanceof BufferedImage) {
            return (BufferedImage) img;
        }

        ColorModel     cm = img.getColorModel();
        int            w  = img.getWidth();
        int            h  = img.getHeight();
        WritableRaster raster = cm.createCompatibleWritableRaster(w,h);
        boolean        isAlphaPremultiplied = cm.isAlphaPremultiplied();
        Hashtable      props = new Hashtable();
        String []      keys = img.getPropertyNames();

        if (keys != null) {
            for (int i = 0 ; i < keys.length ; i++) {
                props.put(keys[i], img.getProperty(keys[i]));
            }
        }
        BufferedImage ret = new BufferedImage(cm, raster,
                                              isAlphaPremultiplied,
                                              props);
        img.copyData(raster);
        return ret;
    }

Thanks to "Thomas M." who posted this method as a comment on this web page.

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