图片无法放大? #12

使用ScaleParameter对图片进行缩放,缩小是正常的..但是无法放大.请问下是什么原因

WriteParameter wp = new WriteParameter();
wp.setDefaultQuality(level);
ScaleParameter sp = new ScaleParameter(width, height, ScaleParameter.Algorithm.LANCZOS);
ImageRender ir = new ReadRender(bis, true);
ImageRender sc = new ScaleRender(ir, sp);
wr = new WriteRender(sc, bos, ImageFormat.getImageFormat(FilenameUtils.getExtension(outFile.getName())), wp);
wr.render();