Setting Background Color for the Remaining Part of the Last Page in a PDF

   

Setting Background Color for the Remaining Part of the Last Page in a PDF

When creating a PDF from an HTML page using a headless browser like Chrome, ensuring that the background color fills the remaining portion of the last page can be challenging. This is due to the fact that HTML and CSS were not originally designed for paginated layouts, which PDFs rely on. Here’s a detailed explanation and approach to solve your problem.

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Challenges

1. Dynamic Content Height:

   • The height of the content is unpredictable because it depends on the number of rendered items.

2. Paginated Rendering:

   • HTML is continuous by nature, but PDFs divide content into pages. Styling specific sections of a page requires understanding where page breaks occur.

3. CSS Limitations in Paginated Context:

   • CSS properties like position: absolute or bottom: 0 might not behave as expected when rendering to a paginated PDF.

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Approach to Solve the Problem

1. Use the @page CSS Rule

The @page rule is specifically designed for paginated content. You can set the background color of pages and adjust margins or padding to control the appearance of the last page.

• Implementation:

  @page {
      size: A4; /* Or any desired size */
      margin: 20mm;
  }
  
  @page:last {
      background: #f0f0f0; /* Background color for the last page */
  }
  

• Limitations:
  • This applies the background to the entire last page. If your goal is to color only the portion after the content, additional steps are required.

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2. Add a Fixed-Sized Background Div

You can add a div at the end of your content, which serves as a placeholder for the background color.

• Implementation:

  <div class="content">
      <!-- Your dynamic content here -->
  </div>
  <div class="background-fill"></div>
  

  .background-fill {
      height: 100vh; /* Adjust as necessary */
      background: #f0f0f0;
      position: absolute;
      bottom: 0;
      left: 0;
      right: 0;
  }
  

• How It Works:

  • The background-fill div ensures any remaining space is visually filled.

• Drawback:

  • This approach assumes you can predict the height of the remaining space. It may not work well if the content height varies significantly.

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3. Calculate Remaining Space Dynamically

If your content spans multiple pages and you need to fill the last page dynamically:

• Use JavaScript before rendering the PDF:

  • Measure the height of the content using offsetHeight or similar properties.
  • Compare it with the total height of the page (e.g., A4 height at 96 DPI is 1122px).

• Implementation:

  <script>
      const pageHeight = 1122; // Height for A4 in pixels
      const contentHeight = document.querySelector('.content').offsetHeight;
  
      if (contentHeight % pageHeight !== 0) {
          const remainingHeight = pageHeight - (contentHeight % pageHeight);
          const backgroundDiv = document.createElement('div');
          backgroundDiv.style.height = `${remainingHeight}px`;
          backgroundDiv.style.background = '#f0f0f0';
          document.body.appendChild(backgroundDiv);
      }
  </script>
  

• How It Works:

  • Dynamically calculates the space left on the last page and fills it with a background-colored div.

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4. Use CSS Breaks and Pseudo-Elements

CSS pseudo-elements like ::after can create the effect of a background on the last page.

• Implementation:

  .content::after {
      content: "";
      display: block;
      height: calc(100vh - var(--content-height));
      background: #f0f0f0;
  }
  

• Limitations:
  • Relies on accurately calculating the height of the content.

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5. Use a PDF Library for Advanced Control

If headless Chrome’s rendering is insufficient, consider using a PDF library like jsPDF or PDFKit. These libraries give you programmatic control over page rendering and background settings.

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Best Practices

1. Use Consistent Units:

   • Use pixels or millimeters consistently when designing for print layouts.

2. Test with Sample Content:

   • Test your solution with varying amounts of content to ensure the background adapts dynamically.

3. Debug Using Chrome DevTools:

   • Use DevTools to inspect the rendered content and ensure the background behaves as expected.

4. Fallback Styling:

   • Provide fallback styles for non-paginated contexts like screen view.

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Conclusion

Setting a background color for the remaining part of the last page in a PDF requires a mix of CSS and JavaScript. The best approach depends on your specific requirements. If you’re generating PDFs dynamically, combining @page CSS rules with JavaScript-based height calculations ensures the most reliable results.