Question

使用{{3}}新创建的缩小PDF文件中JPEG图像大小的最佳做法是什么？（我的目标是在图像质量和文件大小之间进行权衡。）

图像创建如下：

Image image = new Image(ImageDataFactory.create(imagePath))

我想提供一个比例因子，例如0.5，它将一行中的像素数减半。

假设我使用单个3 MB图像生成PDF。我尝试了image.scale(0.5f, 0.5f)，但生成的PDF文件仍大约为3 MB。我预计它会变小。

因此我猜想嵌入在PDF文件中的源图像不会被触及。但这就是我需要的：应该减少存储在磁盘上的整个PDF文件中的像素总数。

实现这一目标的最简单/推荐方法是什么？

Answer 1

首先缩放图像，然后使用iText打开缩放图像。

ImageDataFactory中有一个接受AWT图像的create方法。首先使用AWT工具缩放图像，然后像这样打开它：

String imagePath = "C:\\path\\to\\image.jpg";
java.awt.Image awtImage = ImageIO.read(new File(imagePath));

// scale image here
int scaledWidth = awtImage.getWidth(null) / 2;
int scaledHeight = awtImage.getHeight(null) / 2;
BufferedImage scaledAwtImage = new BufferedImage(scaledWidth, scaledHeight, BufferedImage.TYPE_INT_RGB);
Graphics2D g = scaledAwtImage.createGraphics();
g.drawImage(awtImage, 0, 0, scaledWidth, scaledHeight, null); 
g.dispose();

/* 
Optionally pick a color to replace with transparency.
Any pixels that match this color will be replaced by tansparency.
*/
Color bgColor = Color.WHITE;

Image itextImage = new Image(ImageDataFactory.create(scaledAwtImage, bgColor));

有关如何缩放图像的更好提示，请参阅How can I resize an image using Java?

如果在添加到PDF时仍需要原始尺寸，请再次将其缩放。

itextImage.scale(2f, 2f);

注意：此代码未经测试。

编辑以回应有关赏金的评论

你让我思考和寻找。似乎iText将AWT图像导入为原始图像。我认为它将它视为与BMP相同，只是writes the pixel data using /FlateDecode，这可能远远不是最优的。我能想到实现您的要求的唯一方法是使用ImageIO将缩放后的图像写入文件系统或将ByteArrayOutputStream写为jpeg，然后使用生成的文件/字节打开iText。

这是使用字节数组的更新示例。如果你想更加了解压缩等级，refer here。

String imagePath = "C:\\path\\to\\image.jpg";
java.awt.Image awtImage = ImageIO.read(new File(imagePath));

// scale image here
int scaledWidth = awtImage.getWidth(null) / 2;
int scaledHeight = awtImage.getHeight(null) / 2;
BufferedImage scaledAwtImage = new BufferedImage(scaledWidth, scaledHeight, BufferedImage.TYPE_INT_RGB);
Graphics2D g = scaledAwtImage.createGraphics();
g.drawImage(awtImage, 0, 0, scaledWidth, scaledHeight, null); 
g.dispose();

ByteArrayOutputStream bout = new ByteArrayOutputStream()
ImageIO.write(scaledAwtImage, "jpeg", bout);
byte[] imageBytes = bout.toByteArray();

Image itextImage = new Image(ImageDataFactory.create(imageBytes));

Answer 2

您可以随意设置编写器对象的压缩级别。

PdfWriter writer = new PdfWriter("MyPdf.pdf");
writer.setCompressionLevel(0);
PdfDocument pdf = new PdfDocument(writer);
Document document = new Document(pdf, PageSize.A4);

如果压缩级别为0，那么以pdf呈现的图像将不会占用任何大小。具有图像且没有图像的Pdf尺寸将是相同的。无论您想要什么样的高度和宽度，都可以进行缩放。

Answer 3

此documentations中列出的方法是，您可以访问压缩图像并减少存储在磁盘上的整个PDF文件。希望它有所帮助。

下面是代码示例：

/*
 * This example was written by Bruno Lowagie in answer to the following question:
 * http://stackoverflow.com/questions/30483622/compressing-images-in-existing-pdfs-makes-the-resulting-pdf-file-bigger-lowagie
 */
package sandbox.images;

import com.itextpdf.text.DocumentException;
import com.itextpdf.text.pdf.PRStream;
import com.itextpdf.text.pdf.PdfName;
import com.itextpdf.text.pdf.PdfNumber;
import com.itextpdf.text.pdf.PdfObject;
import com.itextpdf.text.pdf.PdfReader;
import com.itextpdf.text.pdf.PdfStamper;
import com.itextpdf.text.pdf.parser.PdfImageObject;

import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

import javax.imageio.ImageIO;
import sandbox.WrapToTest;

/**
 * @author Bruno Lowagie (iText Software)
 */
@WrapToTest
public class ReduceSize {

    public static final String SRC = "resources/pdfs/single_image.pdf";
    public static final String DEST = "results/images/single_image_reduced.pdf";
    public static final float FACTOR = 0.5f;

    public static void main(String[] args) throws DocumentException, IOException {
        File file = new File(DEST);
        file.getParentFile().mkdirs();
        new ReduceSize().manipulatePdf(SRC, DEST);
    }
    public void manipulatePdf(String src, String dest) throws DocumentException, IOException {
        PdfReader reader = new PdfReader(src);
        int n = reader.getXrefSize();
        PdfObject object;
        PRStream stream;
        // Look for image and manipulate image stream
        for (int i = 0; i < n; i++) {
            object = reader.getPdfObject(i);
            if (object == null || !object.isStream())
                continue;
            stream = (PRStream)object;
            if (!PdfName.IMAGE.equals(stream.getAsName(PdfName.SUBTYPE)))
                continue;
            if (!PdfName.DCTDECODE.equals(stream.getAsName(PdfName.FILTER)))
                continue;
            PdfImageObject image = new PdfImageObject(stream);
            BufferedImage bi = image.getBufferedImage();
            if (bi == null)
                continue;
            int width = (int)(bi.getWidth() * FACTOR);
            int height = (int)(bi.getHeight() * FACTOR);
            if (width <= 0 || height <= 0)
                continue;
            BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
            AffineTransform at = AffineTransform.getScaleInstance(FACTOR, FACTOR);
            Graphics2D g = img.createGraphics();
            g.drawRenderedImage(bi, at);
            ByteArrayOutputStream imgBytes = new ByteArrayOutputStream();
            ImageIO.write(img, "JPG", imgBytes);
            stream.clear();
            stream.setData(imgBytes.toByteArray(), false, PRStream.NO_COMPRESSION);
            stream.put(PdfName.TYPE, PdfName.XOBJECT);
            stream.put(PdfName.SUBTYPE, PdfName.IMAGE);
            stream.put(PdfName.FILTER, PdfName.DCTDECODE);
            stream.put(PdfName.WIDTH, new PdfNumber(width));
            stream.put(PdfName.HEIGHT, new PdfNumber(height));
            stream.put(PdfName.BITSPERCOMPONENT, new PdfNumber(8));
            stream.put(PdfName.COLORSPACE, PdfName.DEVICERGB);
        }
        reader.removeUnusedObjects();
        // Save altered PDF
        PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
        stamper.setFullCompression();
        stamper.close();
        reader.close();
    }
}

iText：降低图像质量（减少生成的PDF大小）

3 个答案: