Controlling Image Quality in Qt Rich Text Documents with QTextImageFormat::setQuality()

Purpose

• Higher quality results in sharper images but larger file sizes, while lower quality leads to smaller files but potentially blurry images.
• Controls the compression quality of images inserted into rich text documents using Qt's rich text formatting features.

Functionality

• Qt uses the PNG format for image storage when quality is set to 100 or higher. For lower values, the format might differ depending on Qt's internal implementation.
• Valid values range from 0 (worst quality, smallest file) to 100 (best quality, largest file).
• Takes an integer argument quality (default: 100).

Usage in Qt GUI Applications

1. #include <QTextDocument>
   #include <QTextImageFormat>
   

2. Create a QTextImageFormat Object
   

   QTextImageFormat imageFormat;
   

3. Set the Image Quality
   

   imageFormat.setQuality(75); // Example: Set quality to 75 (between best and worst)
   

4. Apply the Format to an Image in a Rich Text Document
   

   There are several ways to achieve this, depending on your specific Qt GUI widget or approach:

   • Using QTextCursor
     

     QTextCursor cursor = textDocument->textCursor();
     cursor.insertImage(imagePath, imageFormat);
     

   • Using QTextCharFormat
     

     QTextCharFormat charFormat;
     charFormat.setImageFormat(imageFormat);
     cursor.insertText("Text with image", charFormat);
     cursor.insertImage(imagePath);
     

Key Points

• The specific format used when quality is below 100 might vary depending on Qt's internal logic.
• Adjust the quality value based on your desired balance between image fidelity and file size.

Example

#include <QApplication>
#include <QTextEdit>
#include <QTextImageFormat>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QTextEdit *textEdit = new QTextEdit;

    // Create image format with quality set to 80
    QTextImageFormat imageFormat;
    imageFormat.setQuality(80);

    // Insert image with the format
    QString imagePath = "path/to/your/image.png";
    textEdit->textCursor().insertImage(imagePath, imageFormat);

    textEdit->show();

    return app.exec();
}

---

Setting Quality for Multiple Images

This code iterates through a list of image paths and inserts them into a QTextEdit with a specific quality setting:

#include <QTextDocument>
#include <QTextImageFormat>
#include <QStringList>

void insertImagesWithQuality(QTextEdit* textEdit, const QStringList& imagePaths, int quality) {
  QTextCursor cursor = textEdit->textCursor();
  QTextImageFormat imageFormat;
  imageFormat.setQuality(quality);

  for (const QString& imagePath : imagePaths) {
    cursor.insertImage(imagePath, imageFormat);
  }
}

int main(int argc, char *argv[]) {
  // ... (your application setup)

  QStringList imagePaths = {"image1.jpg", "image2.png", "image3.bmp"};
  insertImagesWithQuality(textEdit, imagePaths, 70);

  // ... (show textEdit)

  return app.exec();
}

Setting Quality Based on User Input

This code allows the user to enter a desired quality level through a dialog box and then inserts an image with that quality:

#include <QTextDocument>
#include <QTextImageFormat>
#include <QInputDialog>

void insertImageWithCustomQuality(QTextEdit* textEdit) {
  bool ok;
  int quality = QInputDialog::getInt(textEdit, "Image Quality", "Enter desired quality (0-100):", 75, 0, 100, 1, &ok);
  if (!ok) {
    return;
  }

  QTextCursor cursor = textEdit->textCursor();
  QTextImageFormat imageFormat;
  imageFormat.setQuality(quality);

  QString imagePath = QFileDialog::getOpenFileName(textEdit, "Select Image", "", "Image Files (*.png *.jpg *.bmp);;All Files (*.*)");
  if (imagePath.isEmpty()) {
    return;
  }

  cursor.insertImage(imagePath, imageFormat);
}

int main(int argc, char *argv[]) {
  // ... (your application setup)

  insertImageWithCustomQuality(textEdit);

  // ... (show textEdit)

  return app.exec();
}

---

Manual Image Scaling

• This gives you finer control over the image size and potentially reduces file size more efficiently than relying solely on setQuality().
• If you have more control over the image data before inserting it into the rich text document, you can manually scale down the image using Qt's image manipulation classes like QImage before setting it as the source for the QTextCursor::insertImage() function.

Third-Party Image Compression Libraries

• These libraries can be integrated into your Qt application to pre-compress images before insertion.

Custom Image Format Handling

• This approach requires a deeper understanding of Qt's rich text format internals and potentially modifying the format to accommodate your custom image storage. It's generally recommended for advanced use cases only.
• If you have very specific requirements for image storage within the rich text document, you might consider implementing custom logic to handle image data in a format of your choice (e.g., compressing the image data yourself using a custom algorithm).

Choosing the Right Approach

• Custom image format handling is the least recommended option due to its complexity and should only be attempted for very specialized use cases.
• If you need more granular control over image size or have specific compression requirements, consider manual image scaling or third-party libraries.
• For most scenarios, QTextImageFormat::setQuality() provides a sufficient and convenient way to control image compression within Qt's rich text formatting.

• Evaluate your specific needs and choose the approach that best balances compression efficiency, image fidelity, and development effort.
• The suitability of each alternative depends on factors like your desired level of compression control, the trade-off between image quality and file size, and the complexity you're willing to introduce into your codebase.