Feature/convert to grayscale (#3003)

# Description of Changes

Please provide a summary of the changes, including:

- What was changed
  -Grayscale Image Compression:
Modified the compressImagesInPDF method to optionally convert images to
grayscale during compression. This is
achieved by checking if the grayScale flag is true (or if the image is
already in grayscale) and then processing the image
   accordingly.

   UI Update:
Updated the compress-pdf.html file by adding a checkbox for grayscale
compression. Additionally, a new translation
key compress.grayscale.label with the text "Apply Grayscale for
Compression" has been added across all supported
   languages. 
- Why the change was made
  -Enhanced Compression Options:
This feature provides users with an option to compress images in PDFs
more effectively by reducing the color complexity, which can lead to
smaller file sizes.

Improved Flexibility:
It allows users to decide whether they want to maintain the original
color images or opt for a grayscale version.
- Any challenges encountered
- The translation for compress.grayscale.label was generated using an
automated translator, so it might not be completely accurate and could
require further review.

Closes #2603

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

src/main/java/stirling/software/SPDF/controller/api/misc/CompressController.java

@@ -31,6 +31,7 @@ import io.swagger.v3.oas.annotations.Operation;
 import io.swagger.v3.oas.annotations.tags.Tag;
 
 import lombok.extern.slf4j.Slf4j;
+
 import stirling.software.SPDF.model.api.misc.OptimizePdfRequest;
 import stirling.software.SPDF.service.CustomPDDocumentFactory;
 import stirling.software.SPDF.utils.GeneralUtils;
@@ -51,7 +52,7 @@ public class CompressController {
         this.pdfDocumentFactory = pdfDocumentFactory;
     }
 
-    private void compressImagesInPDF(Path pdfFile, double initialScaleFactor) throws Exception {
+    private void compressImagesInPDF(Path pdfFile, double initialScaleFactor, boolean grayScale) throws Exception {
         byte[] fileBytes = Files.readAllBytes(pdfFile);
         try (PDDocument doc = Loader.loadPDF(fileBytes)) {
             double scaleFactor = initialScaleFactor;
@@ -76,11 +77,23 @@ public class CompressController {
                                     bufferedImage.getScaledInstance(
                                             newWidth, newHeight, Image.SCALE_SMOOTH);
 
-                            BufferedImage scaledBufferedImage =
-                                    new BufferedImage(
-                                            newWidth, newHeight, BufferedImage.TYPE_INT_RGB);
-                            scaledBufferedImage.getGraphics().drawImage(scaledImage, 0, 0, null);
-
+                            BufferedImage scaledBufferedImage;
+                            if (grayScale
+                                    || bufferedImage.getType() == BufferedImage.TYPE_BYTE_GRAY) {
+                                scaledBufferedImage =
+                                        new BufferedImage(
+                                                newWidth, newHeight, BufferedImage.TYPE_BYTE_GRAY);
+                                scaledBufferedImage
+                                        .getGraphics()
+                                        .drawImage(scaledImage, 0, 0, null);
+                            } else {
+                                scaledBufferedImage =
+                                        new BufferedImage(
+                                                newWidth, newHeight, BufferedImage.TYPE_INT_RGB);
+                                scaledBufferedImage
+                                        .getGraphics()
+                                        .drawImage(scaledImage, 0, 0, null);
+                            }
                             ByteArrayOutputStream compressedImageStream =
                                     new ByteArrayOutputStream();
                             ImageIO.write(scaledBufferedImage, "jpeg", compressedImageStream);
@@ -139,6 +152,7 @@ public class CompressController {
             }
 
             boolean sizeMet = false;
+            boolean grayscaleEnabled = Boolean.TRUE.equals(request.getGrayscale());
             while (!sizeMet && optimizeLevel <= 9) {
 
                 // Apply additional image compression for levels 6-9
@@ -152,7 +166,7 @@ public class CompressController {
                                 case 9 -> 0.5; // 60% of original size
                                 default -> 1.0;
                             };
-                    compressImagesInPDF(tempInputFile, scaleFactor);
+                    compressImagesInPDF(tempInputFile, scaleFactor, grayscaleEnabled);
                 }
 
                 // Run QPDF optimization
@@ -169,6 +183,7 @@ public class CompressController {
                 command.add("--compression-level=" + optimizeLevel);
                 command.add("--compress-streams=y");
                 command.add("--object-streams=generate");
+                command.add("--no-warn");
                 command.add(tempInputFile.toString());
                 command.add(tempOutputFile.toString());
 

src/main/java/stirling/software/SPDF/model/api/misc/OptimizePdfRequest.java

@@ -29,4 +29,13 @@ public class OptimizePdfRequest extends PDFFile {
                     "Whether to normalize the PDF content for better compatibility. Default is false.",
             defaultValue = "false")
     private Boolean normalize = false;
+
+    @Schema(
+            description = "Whether to convert the PDF to grayscale. Default is false.",
+            defaultValue = "false")
+    private Boolean grayscale = false;
+
+    public Boolean getGrayscale() {
+        return grayscale;
+    }
 }