我遇到图像压缩问题。我使用了这个问题中描述的答案 compress pdf with large images via java 如果我将FACTOR变量设置为0.9f或1f（原始大小），则生成的pdf文件开始变得比ORIGINAL大。但并非所有文件都是如此。我自己创建的一些文件变得像计划的那样小，但有些文件变得更大，比如+ 1/3，我在它上面的一些图像上得到黑色背景。当我使用正常的图像压缩而不调整图像大小时，情况会变得更糟 This是我的测试文件。
Lowagies方法:(调整图像大小）
// TODO Auto-generated method stub
PdfName key = new PdfName("ITXT_SpecialId");
PdfName value = new PdfName("123456789");
// Read the file
PdfReader reader = new PdfReader(args[0]);
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 (value.equals(stream.get(key))) {
PdfObject pdfsubtype = stream.get(PdfName.SUBTYPE);
System.out.println(stream.type());
if (pdfsubtype != null && pdfsubtype.toString().equals(PdfName.IMAGE.toString())) {
PdfImageObject image = new PdfImageObject(stream);
BufferedImage bi = image.getBufferedImage();
if (bi == null) continue;
int width = (int)(bi.getWidth() * 1f);
int height = (int)(bi.getHeight() * 1f);
BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
AffineTransform at = AffineTransform.getScaleInstance(1f, 1f);
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.BEST_COMPRESSION);
stream.put(PdfName.TYPE, PdfName.XOBJECT);
stream.put(PdfName.SUBTYPE, PdfName.IMAGE);
stream.put(key, value);
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);
}
}
// Save altered PDF
PdfStamper stamper = new PdfStamper(reader, new FileOutputStream("/Applications/XAMPP/xamppfiles/htdocs/pdf_compress/download/"+args[2]));
stamper.close();
reader.close();
我的方法（通过设置图像的质量而不是调整图像的质量来使用实际压缩）
PdfReader reader = new PdfReader(args[0]);
// Read the file
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;
PdfObject pdfsubtype = stream.get(PdfName.SUBTYPE);
if (pdfsubtype != null && pdfsubtype.toString().equals(PdfName.IMAGE.toString())) {
System.out.println(pdfsubtype.length());
PdfImageObject image = new PdfImageObject(stream);
BufferedImage bi = image.getBufferedImage();
if (bi == null) continue;
int width = (int)(bi.getWidth());
int height = (int)(bi.getHeight());
if(width <=30 || height <=30){
continue;
}
BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
AffineTransform at = null;
Graphics2D g = img.createGraphics();
g.drawRenderedImage(bi, at );
ByteArrayOutputStream imgBytes = new ByteArrayOutputStream();
Iterator iter = ImageIO.getImageWritersByFormatName("JPG");
ImageWriter writer = (ImageWriter)iter.next();
ImageWriteParam iwp = writer.getDefaultWriteParam();
iwp.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
// here goes the compression
iwp.setCompressionQuality(Float.valueOf(args[1]));
ImageOutputStream imageos = ImageIO.createImageOutputStream(imgBytes);
writer.setOutput(imageos);
IIOImage images = new IIOImage(img, null, null);
writer.write(null,images , iwp);
imageos.close();
writer.dispose();
stream.clear();
stream.setData(imgBytes.toByteArray(), false, PRStream.BEST_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);
}
}
PdfStamper stamper = new PdfStamper(reader, new FileOutputStream("/Applications/XAMPP/xamppfiles/htdocs/pdf_compress/download/"+args[2]));
stamper.setFullCompression();
stamper.close();
reader.close();
System.out.println("Done");
代码有什么问题？我应该使用不同的图像压缩方法吗？还有其他人吗？
答案 0 :(得分：0)
当我只替换JPEG时，我已经获得了较低的文件大小。删除未使用的对象也有帮助：
public class ReduceSize {
public static final String SRC = "resources/pdfs/annual_report_2009.pdf";
public static final String DEST = "results/images/annual_report_2009.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();
}
}
这会将10,510 KB文件减少到9,159 KB。当然：字体也占用了相当大的空间。