Setting image DPI in Java AWT Graphics2D

When using Java AWT Graphics2D for creating images in memory, the default image DPI (Dots Per Inch) value is 72. And it has no direct way of choosing or changing this.

The scaling of the default transform is set to identity for those devices that are close to 72 dpi, such as screen devices. ¹

To set an image DPI other than 72, you need to find the relation between the desired DPI and 72 and use it combined with Graphics2D scale method. ²

Apache PDFBox does the first part on method renderImageWithDPI of PDFRenderer class.

public BufferedImage renderImageWithDPI(int pageIndex, float dpi) throws IOException
{
    return renderImage(pageIndex, dpi / 72f, ImageType.RGB);
}

public BufferedImage renderImageWithDPI(int pageIndex, float dpi, ImageType imageType) throws IOException
{
    return renderImage(pageIndex, dpi / 72f, imageType);
}

The second part is on graphics.scale(scaleX, scaleY); line of the transform method.

private void transform(Graphics2D graphics, int rotationAngle, PDRectangle cropBox, float scaleX, float scaleY)
{
    graphics.scale(scaleX, scaleY);

    if (rotationAngle != 0)
    {
        float translateX = 0;
        float translateY = 0;
        switch (rotationAngle)
        {
            case 90:
                translateX = cropBox.getHeight();
                break;
            case 270:
                translateY = cropBox.getWidth();
                break;
            case 180:
                translateX = cropBox.getWidth();
                translateY = cropBox.getHeight();
                break;
            default:
                break;
        }
        graphics.translate(translateX, translateY);
        graphics.rotate(Math.toRadians(rotationAngle));
    }
}