Cleaning up unused images in PDF page resources - pdfbox

Question

Please forgive me if this has been asked but I have not found any matches yet.
I have some PDF files where images are duplicated on each page's resources but never used in its content stream. I think this is causing the PDFSplit command to create very bloated pages. Is there any utility code or examples to clean up unused resources like this? Maybe a starting point for me to get going?

Answer 1

I was able to clean up the resources for each page by gathering a list of the images used inside the page's content stream. With the list of images, I then check the resources for the page and remove any that weren't used. See the PageExtractor.stripUnusedImages below for implementation details.
The resource object was shared between pages so I also had to make sure each page had its own copy of the resource object before removing images. See PageExtractor.copyResources below for implementation details.
The page splitter:
package org.apache.pdfbox.examples;
import org.apache.pdfbox.contentstream.operator.Operator;
import org.apache.pdfbox.cos.COSBase;
import org.apache.pdfbox.cos.COSDictionary;
import org.apache.pdfbox.cos.COSName;
import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.pdmodel.PDPage;
import org.apache.pdfbox.pdmodel.PDResources;
import org.apache.pdfbox.pdmodel.interactive.action.PDAction;
import org.apache.pdfbox.pdmodel.interactive.action.PDActionGoTo;
import org.apache.pdfbox.pdmodel.interactive.annotation.PDAnnotation;
import org.apache.pdfbox.pdmodel.interactive.annotation.PDAnnotationLink;
import org.apache.pdfbox.pdmodel.interactive.documentnavigation.destination.PDDestination;
import org.apache.pdfbox.pdmodel.interactive.documentnavigation.destination.PDPageDestination;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.io.IOException;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class PageExtractor {
private final Logger log = LoggerFactory.getLogger(this.getClass());
public PDDocument extractPage(PDDocument source, Integer pageNumber) throws IOException {
PDDocument targetPdf = new PDDocument();
targetPdf.getDocument().setVersion(source.getVersion());
targetPdf.setDocumentInformation(source.getDocumentInformation());
targetPdf.getDocumentCatalog().setViewerPreferences(source.getDocumentCatalog().getViewerPreferences());
PDPage sourcePage = source.getPage(pageNumber);
PDPage targetPage = targetPdf.importPage(sourcePage);
targetPage.setResources(sourcePage.getResources());
stripUnusedImages(targetPage);
stripPageLinks(targetPage);
return targetPdf;
}
/**
* Collect the images used from a custom PDFStreamEngine (BI and DO operators)
* Create an empty COSDictionary
* Loop through the page's XObjects that are images and add them to the new COSDictionary if they were found in the PDFStreamEngine
* Assign the newly filled COSDictionary to the page's resource as COSName.XOBJECT
*/
protected void stripUnusedImages(PDPage page) throws IOException {
PDResources resources = copyResources(page);
COSDictionary pageObjects = (COSDictionary) resources.getCOSObject().getDictionaryObject(COSName.XOBJECT);
COSDictionary newObjects = new COSDictionary();
Set<String> imageNames = findImageNames(page);
Iterable<COSName> xObjectNames = resources.getXObjectNames();
for (COSName xObjectName : xObjectNames) {
if (resources.isImageXObject(xObjectName)) {
Boolean used = imageNames.contains(xObjectName.getName());
if (used) {
newObjects.setItem(xObjectName, pageObjects.getItem(xObjectName));
} else {
log.info("Found unused image: name={}", xObjectName.getName());
}
} else {
newObjects.setItem(xObjectName, pageObjects.getItem(xObjectName));
}
}
resources.getCOSObject().setItem(COSName.XOBJECT, newObjects);
page.setResources(resources);
}
/**
* It is necessary to copy the page's resources since it can be shared with other pages. We must ensure changes
* to the resources are scoped to the current page.
*/
protected PDResources copyResources(PDPage page) {
return new PDResources(new COSDictionary(page.getResources().getCOSObject()));
}
protected Set<String> findImageNames(PDPage page) throws IOException {
Set<String> imageNames = new HashSet<>();
PdfImageStreamEngine engine = new PdfImageStreamEngine() {
#Override
void handleImage(Operator operator, List<COSBase> operands) {
COSName name = (COSName) operands.get(0);
imageNames.add(name.getName());
}
};
engine.processPage(page);
return imageNames;
}
/**
* Borrowed from PDFBox page splitter
*
* #see org.apache.pdfbox.multipdf.Splitter#processAnnotations(org.apache.pdfbox.pdmodel.PDPage)
*/
protected void stripPageLinks(PDPage imported) throws IOException {
List<PDAnnotation> annotations = imported.getAnnotations();
for (PDAnnotation annotation : annotations) {
if (annotation instanceof PDAnnotationLink) {
PDAnnotationLink link = (PDAnnotationLink) annotation;
PDDestination destination = link.getDestination();
if (destination == null && link.getAction() != null) {
PDAction action = link.getAction();
if (action instanceof PDActionGoTo) {
destination = ((PDActionGoTo) action).getDestination();
}
}
if (destination instanceof PDPageDestination) {
// TODO preserve links to pages within the splitted result
((PDPageDestination) destination).setPage(null);
}
}
// TODO preserve links to pages within the splitted result
annotation.setPage(null);
}
}
}
The stream reader used to analyze the page's images:
package org.apache.pdfbox.examples;
import org.apache.pdfbox.contentstream.PDFStreamEngine;
import org.apache.pdfbox.contentstream.operator.Operator;
import org.apache.pdfbox.contentstream.operator.OperatorProcessor;
import org.apache.pdfbox.cos.COSBase;
import org.apache.pdfbox.cos.COSName;
import org.apache.pdfbox.pdmodel.graphics.PDXObject;
import org.apache.pdfbox.pdmodel.graphics.form.PDFormXObject;
import org.apache.pdfbox.pdmodel.graphics.form.PDTransparencyGroup;
import java.io.IOException;
import java.util.List;
abstract public class PdfImageStreamEngine extends PDFStreamEngine {
PdfImageStreamEngine() {
addOperator(new DrawObjectCounter());
}
abstract void handleImage(Operator operator, List<COSBase> operands);
protected class DrawObjectCounter extends OperatorProcessor {
#Override
public void process(Operator operator, List<COSBase> operands) throws IOException {
if (operands != null && isImage(operands.get(0))) {
handleImage(operator, operands);
}
}
protected Boolean isImage(COSBase base) throws IOException {
if (!(base instanceof COSName)) {
return false;
}
COSName name = (COSName)base;
if (context.getResources().isImageXObject(name)) {
return true;
}
PDXObject xObject = context.getResources().getXObject(name);
if (xObject instanceof PDTransparencyGroup) {
context.showTransparencyGroup((PDTransparencyGroup)xObject);
} else if (xObject instanceof PDFormXObject) {
context.showForm((PDFormXObject)xObject);
}
return false;
}
#Override
public String getName() {
return "Do";
}
}
}