presenter: fixed slide overlay detection for some slide sets #889

lect-core/src/main/java/org/lecturestudio/core/model/Document.java

@@ -28,7 +28,6 @@ import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
 import java.util.UUID;
-import java.util.stream.Stream;
 
 import org.lecturestudio.core.geometry.Dimension2D;
 import org.lecturestudio.core.geometry.Rectangle2D;
@@ -758,8 +757,8 @@ public class Document {
 		pages.clear();
 
 		int pageCount = pdfDocument.getPageCount();
-		String[] prevSplitPageText = new String[2];
-		String[] splitPageText;
+
+		List<String> lastPageTextLines = List.of();
 
 		for (int number = 0; number < pageCount; number++) {
 			Page page = new Page(this, number);
@@ -772,26 +771,24 @@ public class Document {
 				}
 			}
 
-			splitPageText = page.getPageText().split("\n");
+			List<String> pageTextLines = pdfDocument.getPageTextLines(number, 2);
 
-			if (splitPageText.length >= 2 && prevSplitPageText.length >= 2 &&
-					Stream.of(prevSplitPageText[0], prevSplitPageText[1], splitPageText[0], splitPageText[1])
-							.allMatch(Objects::nonNull)) {
-				if (prevSplitPageText[0].equals(splitPageText[0]) && prevSplitPageText[1].equals(splitPageText[1])) {
-					page.setOverlay(true);
+			List<String> finalLastPageTextLines = lastPageTextLines;
+			List<String> differences = pageTextLines.stream()
+					.filter(element -> !finalLastPageTextLines.contains(element)).toList();
 
-					if (number > 0) {
-						Page prevPage = pages.get(number - 1);
-						prevPage.setOverlay(true);
-					}
-				}
-				else {
-					prevSplitPageText = splitPageText;
+			// The pages have equal content, thus mark them as overlay pages.
+			if (differences.isEmpty()) {
+				page.setOverlay(true);
+
+				if (number > 0) {
+					Page prevPage = pages.get(number - 1);
+					prevPage.setOverlay(true);
 				}
 			}
-			else {
-				prevSplitPageText = splitPageText;
-			}
+
+			lastPageTextLines = pageTextLines;
+
 			pages.add(page);
 		}
 

lect-core/src/main/java/org/lecturestudio/core/pdf/DocumentAdapter.java

@@ -130,6 +130,16 @@ public interface DocumentAdapter {
 	 */
 	String getPageText(int pageNumber) throws IOException;
 
+	/**
+	 * Get the text of the page as a list where each list entry represents a text line in the page.
+	 *
+	 * @param pageNumber The page number.
+	 * @param maxLines   The maximum number of lines to retrieve.
+	 *
+	 * @return the text of the page as a list of text lines in the page.
+	 */
+	List<String> getPageTextLines(int pageNumber, int maxLines);
+
 	/**
 	 * Get the word bounds of the page that has the specified page number.
 	 *

lect-core/src/main/java/org/lecturestudio/core/pdf/PdfDocument.java

@@ -317,6 +317,18 @@ public class PdfDocument {
 		return muPDFDocument.getPageText(pageNumber);
 	}
 
+	/**
+	 * Get the text of the page as a list where each list entry represents a text line in the page.
+	 *
+	 * @param pageNumber The page number.
+	 * @param maxLines   The maximum number of lines to retrieve.
+	 *
+	 * @return the text of the page as a list of text lines in the page.
+	 */
+	public List<String> getPageTextLines(int pageNumber, int maxLines) {
+		return muPDFDocument.getPageTextLines(pageNumber, maxLines);
+	}
+
 	/**
 	 * Get the word bounds of the page that has the specified page number.
 	 *

lect-core/src/main/java/org/lecturestudio/core/pdf/mupdf/MuPDFDocument.java

@@ -41,14 +41,9 @@ import java.io.IOException;
 import java.io.OutputStream;
 import java.lang.reflect.Constructor;
 import java.net.URI;
-import java.util.AbstractMap;
-import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Map;
-import java.util.Set;
+import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
 
 import org.lecturestudio.core.geometry.Rectangle2D;
 import org.lecturestudio.core.io.BitConverter;
@@ -201,7 +196,6 @@ public class MuPDFDocument implements DocumentAdapter {
 			Page page = getPage(pageNumber);
 
 			SimpleTextWalker textWalker = new SimpleTextWalker(page.getBounds());
-
 			StructuredText structuredText = displayList.toStructuredText();
 			structuredText.walk(textWalker);
 
@@ -496,6 +490,62 @@ public class MuPDFDocument implements DocumentAdapter {
 		}
 	}
 
+	@Override
+	public List<String> getPageTextLines(int pageNumber, int maxLines) {
+		synchronized (mutex) {
+			List<String> lines = new ArrayList<>();
+			DisplayList displayList = getDisplayList(pageNumber);
+			StructuredText structuredText = displayList.toStructuredText();
+
+			// Sort page text blocks, so that the page text is chronologically ordered (top-down).
+			var sorted = Arrays.stream(structuredText.getBlocks())
+					.sorted((o1, o2) -> o1.bbox.y0 < o2.bbox.y0 ? -1 : 1)
+					.toList();
+
+			// Last text block y-position.
+			double lastY0 = Double.MAX_VALUE;
+			double lastY1 = Double.MAX_VALUE;
+
+			// Read text lines from each block.
+			for (var block : sorted) {
+				// Check if the blocks intersect each other.
+				boolean intersects = (block.bbox.y0 >= lastY0 && block.bbox.y0 <= lastY1)
+						|| (block.bbox.y1 >= lastY0 && block.bbox.y1 <= lastY1);
+
+				if (block.lines.length > 0) {
+					StructuredText.TextLine textLine = block.lines[0];
+
+					if (textLine.chars.length > 0) {
+						// Convert individual chars into lines of strings.
+						var text = Arrays.stream(textLine.chars)
+								.map(textChar -> Character.toString(textChar.c))
+								.collect(Collectors.joining(""));
+
+						// If the blocks intersect, merge the text into a single line.
+						if (intersects) {
+							// Get last line text.
+							int index = lines.size() - 1;
+							String lastLine = lines.get(index) + text;
+							lines.set(index, lastLine);
+						}
+						else {
+							lines.add(text);
+						}
+
+						if (lines.size() >= maxLines) {
+							break;
+						}
+					}
+				}
+
+				lastY0 = block.bbox.y0;
+				lastY1 = block.bbox.y1;
+			}
+
+			return lines;
+		}
+	}
+
 	/**
 	 * Get the display list of the {@link PageEntry} that is mapped to the specified page number.
 	 *

lect-core/src/main/java/org/lecturestudio/core/pdf/pdfbox/PDFBoxDocument.java

@@ -605,6 +605,11 @@ public class PDFBoxDocument implements DocumentAdapter {
 		return shapes;
 	}
 
+	@Override
+	public List<String> getPageTextLines(int pageNumber, int maxLines) {
+		return List.of();
+	}
+
 	private void loadOutline(PDDocumentCatalog catalog, PDOutlineItem item,
 			DocumentOutlineItem outline) {
 		if (isNull(item)) {

lect-swing/src/main/java/org/lecturestudio/swing/components/ThumbPanel.java

@@ -455,7 +455,8 @@ public class ThumbPanel extends JPanel {
 			if (selected) {
 				g2d.setColor(Color.BLUE);
 				g2d.fillRect(0, 0, getWidth(), getHeight());
-			}else if(page.isOverlay()){
+			}
+			else if (page.isOverlay()) {
 				g2d.setColor(Color.GRAY);
 				g2d.fillRect(0, 0, getWidth(), getHeight());
 			}