add ChangeTextColor example to modify text color in PDF documents

examples/src/main/java/org/apache/pdfbox/examples/pdmodel/ChangeTextColor.java

@@ -0,0 +1,123 @@
+package org.apache.pdfbox.examples.pdmodel;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.apache.pdfbox.Loader;
+import org.apache.pdfbox.cos.COSFloat;
+import org.apache.pdfbox.pdfparser.PDFStreamParser;
+import org.apache.pdfbox.pdfwriter.ContentStreamWriter;
+import org.apache.pdfbox.pdmodel.PDDocument;
+import org.apache.pdfbox.pdmodel.PDPage;
+import org.apache.pdfbox.pdmodel.PDPageTree;
+import org.apache.pdfbox.pdmodel.common.PDStream;
+import org.apache.pdfbox.contentstream.operator.Operator;
+import java.io.File;
+import java.nio.file.Path;
+import java.util.Iterator;
+
+/**
+ * This is an example that takes an existing PDF document and changes the text color
+ *
+ * I came up with this example while working on my own command-line tool
+ *
+ * @author Krish Vij
+ */
+public class ChangeTextColor {
+
+    /**
+     * Change the current Text color to a desired one by the use.
+     *
+     * @param Document The input PDF Document to change the text color of.
+     * @param r,g,b    The color values for the new text to be put in the PDF.
+     * @param outFile  The file to write to the pdf to.
+     *
+     * @throws IOException If there is an error writing the data.
+     */
+
+    	public void setTextColor(PDDocument document, String r, String g, String b, Path outFile) throws IOException {
+
+		float	RED	  = Float.parseFloat(r)/ 255;
+		float	GREEN = Float.parseFloat(g)/ 255;
+		float	BLUE  = Float.parseFloat(b)/ 255;
+
+		PDPageTree pages = document.getPages();
+
+
+		for (PDPage page : pages) {
+
+			//For every page we get the content stream
+			//and a parser to parse the content stream
+			Iterator<PDStream> streams = page.getContentStreams();
+
+			PDFStreamParser parser = new PDFStreamParser(page);
+
+			//A new list of streams to hold the modified content streams
+			List<PDStream> newStreams =  new ArrayList<>();
+			while (streams.hasNext()) {
+                //We get all the tokens from the content stream
+				List<Object> tokens = parser.parse();
+				//A new list of tokens to add to the new content stream
+				List<Object> newTokens =  new ArrayList<>();
+
+				for (Object token : tokens) {
+
+					if (token instanceof Operator) {
+
+						Operator op = (Operator)token;
+						String name = op.getName();
+						//We check for the "BT" operator which marks the begining of a text content
+						//and we add the new set of RGB values right before it
+						if ("BT".equals(name)) {
+
+							newTokens.add(new COSFloat(RED));
+							newTokens.add(new COSFloat(GREEN));
+							newTokens.add(new COSFloat(BLUE));
+							newTokens.add(Operator.getOperator("rg"));
+							newTokens.add(new COSFloat(RED));
+							newTokens.add(new COSFloat(GREEN));
+							newTokens.add(new COSFloat(BLUE));
+							newTokens.add(Operator.getOperator("RG"));
+						}
+					}
+					//We add all the old tokens to the new list
+					newTokens.add(token);
+				}
+
+				//We create a new content stream to rewrite the modified tokens
+				PDStream newStream = new PDStream(document);
+
+				try (OutputStream out = newStream.createOutputStream()) {
+
+					ContentStreamWriter writer = new ContentStreamWriter(out);
+					writer.writeTokens(newTokens);
+				}
+                //We add the modified content stream to the list of new streams
+				//as a page can have multiple content streams
+				newStreams.add(newStream);
+				streams.next();
+
+			}
+			//we set the modified content streams to the page
+			page.setContents(newStreams);
+		}
+		//Finally we save the document with the changed text color
+		document.save(outFile.toFile());
+	}
+
+    void main() {
+
+		//Input PDF file to be changed
+        File pdf = new File("someInputfile");
+        //Loading the document
+        try (PDDocument document = Loader.loadPDF(pdf) ) {
+            //Calling the method to change the text color
+            setTextColor(document, "255", "0", "0", Path.of("someOutputFile"));
+        } catch (IOException e) {
+
+            e.printStackTrace();
+        }
+    }
+}