MicroSim Screen Capture

Overview

This skill automates the process of capturing high-quality screenshots of MicroSim visualizations using Chrome headless mode. It properly handles dynamic JavaScript content, external CDN libraries (like vis-network.js, p5.js, Chart.js), and ensures the visualization has time to fully render before capturing.

When to Use This Skill

Use this skill when:

• Creating preview images for MicroSims that need social media metadata (og:image)
• Generating screenshots for MicroSim documentation
• Capturing visualizations for quality assessment or archival purposes
• Working with the microsim-standardization skill to achieve a perfect 100/100 quality score

Typical user requests:

• "Create a screenshot of this MicroSim"
• "I need a preview image for the org-chart MicroSim"
• "Generate a social media preview for this visualization"
• "Capture a screenshot of the main.html file"

Workflow

Step 1: Validate the MicroSim Directory

Before capturing a screenshot, verify:

1. The MicroSim directory exists at the provided path (typically docs/sims/{microsim-name}/)
2. The directory contains a main.html file
3. The MicroSim name follows kebab-case convention (lowercase letters and dashes only)

Step 2: Run the Screenshot Capture Script

Execute the provided shell script with the MicroSim directory path:

bash scripts/capture-screenshot.sh <microsim-directory-path>

Example:

bash scripts/capture-screenshot.sh $HOME/Documents/ws/intro-to-graph/docs/sims/org-chart

The script will:

1. Extract the MicroSim name from the directory path (e.g., org-chart from .../sims/org-chart/)
2. Locate the main.html file in the directory
3. Use Chrome headless mode with optimal flags for JavaScript visualization rendering
4. Save the screenshot as {microsim-name}.png in the MicroSim directory (e.g., org-chart.png)
5. Display the output file path and size upon success

Important Chrome flags used:

• --headless=new: Uses the modern headless mode
• --disable-web-security + --allow-file-access-from-files: Allows loading external CDN resources (critical for vis-network, p5.js, etc.)
• --timeout=5000: Gives JavaScript 5 seconds to load and render the visualization
• --window-size=1200,800: Sets a standard viewport size suitable for social media previews
• --hide-scrollbars: Ensures clean screenshots without scrollbar artifacts

Step 3: Verify the Screenshot

After the script completes:

1. Check that the image file was created: {microsim-name}.png
2. Verify the file size is reasonable (typically 20-100KB for rendered visualizations)
3. Use the Read tool to view the screenshot and confirm the visualization rendered properly
4. If the visualization area appears blank/white, the JavaScript may need more time to render - try increasing the --timeout value in the script

Step 4: Update MicroSim Metadata (Optional)

If capturing the screenshot as part of MicroSim standardization, update the index.md YAML frontmatter:

---
title: MicroSim Title
description: Brief description
image: microsim-name.png
og:image: microsim-name.png
quality_score: 100
---

This adds the social media preview metadata and contributes 10 points toward the quality score (5 points for metadata fields + 5 points for the image file existing).

Troubleshooting

Screenshot captures but visualization is blank

Problem: The screenshot shows the page header/controls but the main visualization area is white/empty.

Solutions:

1. Increase the timeout value in the script (change --timeout=5000 to --timeout=10000)
2. Check browser console for JavaScript errors (the script filters them out but they may indicate issues)
3. Verify the visualization works when opening main.html directly in a browser
4. For very complex visualizations, consider using --virtual-time-budget=10000 instead of --timeout

Chrome not found error

Problem: Script reports "Chrome/Chromium not found"

Solutions:

1. Install Google Chrome if not present
2. Update the CHROME_PATHS array in the script to include your Chrome installation path
3. On macOS, Chrome is typically at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

External resources not loading

Problem: Visualizations that use CDN libraries (vis-network, p5.js, Chart.js) don't render

Solution: The script already includes --disable-web-security and --allow-file-access-from-files flags which should allow CDN resources. If still not working:

1. Verify internet connectivity (CDNs need to be accessible)
2. Check if the library CDN URL is valid in main.html
3. Try using a localhost server instead of file:// URLs (see Advanced Usage below)

Advanced Usage: Using localhost Instead of file:// URLs

For MicroSims that have issues with file:// URLs, serve the content via HTTP:

# Start a local server in the project root
cd /path/to/project-root
python -m http.server 8000 &

# Modify the script to use localhost URL
# Replace: "file://$ABSOLUTE_PATH"
# With: "http://localhost:8000/docs/sims/microsim-name/main.html"

# Capture screenshot
bash scripts/capture-screenshot.sh /path/to/microsim

# Stop the server
pkill -f "http.server"

Technical Details

Why Chrome Headless?

Chrome headless mode is used because:

1. JavaScript Support: Full Chrome rendering engine handles complex JavaScript visualizations
2. CDN Loading: Can fetch external resources from CDNs (with proper flags)
3. Timing Control: Can wait for async content to load before capturing
4. Cross-platform: Works on macOS, Linux, and Windows
5. No GUI Required: Can run in CI/CD environments

Screenshot Naming Convention

The script names screenshots using the MicroSim directory name to maintain consistency:

• MicroSim: docs/sims/org-chart/ → Screenshot: org-chart.png
• MicroSim: docs/sims/learning-graph-viewer/ → Screenshot: learning-graph-viewer.png

This differs from using a generic name like preview.png because:

1. Makes the file purpose immediately clear when viewing the directory
2. Easier to identify which screenshot belongs to which MicroSim in bulk operations
3. Follows naming conventions used elsewhere in the project

Default Screenshot Dimensions

The default viewport size is 1200x800 pixels because:

• Width (1200px): Standard desktop viewport, suitable for most visualizations
• Height (800px): Captures header + controls + visualization without excessive whitespace
• Aspect Ratio (3:2): Works well for social media og:image tags
• File Size: Produces reasonably sized PNG files (typically 20-100KB)

To customize dimensions, modify the --window-size flag in the script.

Resources

scripts/capture-screenshot.sh

Bash script that automates the entire screenshot capture process. The script:

• Validates input and checks for required files
• Locates Chrome/Chromium across different platforms
• Constructs proper file:// URLs with absolute paths
• Runs Chrome headless with optimized flags for JavaScript visualizations
• Reports success/failure with file size information

The script is designed to be:

• Self-contained: No external dependencies beyond Chrome
• Cross-platform: Works on macOS, Linux, and Windows (with minor path adjustments)
• Error-tolerant: Filters out common Chrome headless warnings that don't affect functionality
• User-friendly: Clear error messages and success indicators