renderer.renderImageWithDPI(i, 144);

public class PdfToImageUtil {

    /**
     * 將 PDF 逐頁轉(zhuǎn)為壓縮后的 PNG 圖片
     */
    public static List<BizPptImage> convertPdfToImages(File pdfFile,
                                                       String outputDir,
                                                       String courseDetailId) throws IOException {

        List<BizPptImage> list = new ArrayList<>();

        try (PDDocument doc = PDDocument.load(pdfFile)) {

            PDFRenderer renderer = new PDFRenderer(doc);
            int pageCount = doc.getNumberOfPages();

            for (int i = 0; i < pageCount; i++) {

                // ① 降低 DPI（原 144 → 96）
                BufferedImage sourceImage = renderer.renderImageWithDPI(i, 96);

                // ② 限制最大寬度，等比例縮放
                BufferedImage scaledImage = scaleIfNecessary(sourceImage, 1280);

                String fileName = UUID.randomUUID() + "_" + (i + 1) + ".png";
                File outFile = new File(outputDir, fileName);

                ImageIO.write(scaledImage, "PNG", outFile);

                BizPptImage vo = new BizPptImage();
                vo.setImagePath(outFile.getAbsolutePath());
                vo.setFileName(fileName);
                vo.setCourseDetailId(courseDetailId);

                list.add(vo);
            }
        }

        return list;
    }

    /**
     * 按最大寬度等比例縮放
     */
    private static BufferedImage scaleIfNecessary(BufferedImage source, int maxWidth) {

        int width = source.getWidth();
        int height = source.getHeight();

        if (width <= maxWidth) {
            return source;
        }

        double scale = (double) maxWidth / width;
        int targetWidth = maxWidth;
        int targetHeight = (int) Math.round(height * scale);

        BufferedImage target = new BufferedImage(
                targetWidth,
                targetHeight,
                BufferedImage.TYPE_INT_RGB
        );

        Graphics2D g2d = target.createGraphics();
        g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                RenderingHints.VALUE_INTERPOLATION_BILINEAR);
        g2d.setRenderingHint(RenderingHints.KEY_RENDERING,
                RenderingHints.VALUE_RENDER_QUALITY);
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);

        g2d.drawImage(source, 0, 0, targetWidth, targetHeight, null);
        g2d.dispose();

        return target;
    }
}