Fixing FreeCAD BIM Sun Study Errors: A Simple Guide

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Are you encountering issues with the Sun Ray Study in FreeCAD's BIM module? Specifically, are you seeing errors when enabling the Solar Diagram and Show Sun Position features? You're not alone! Many users have faced this problem, where the sun and sun arrow fail to appear unless date and time data are manually entered. This article dives deep into the issue and provides a comprehensive guide to troubleshooting and resolving it, ensuring your solar studies run smoothly in FreeCAD. Let's get started and fix this together, guys!

Understanding the Problem: FreeCAD Sun Study Failures

When working with FreeCAD for Building Information Modeling (BIM), accurately simulating solar conditions is crucial for design and analysis. The Sun Ray Study feature allows you to visualize the sun's path and its impact on your building model. However, a common issue arises when enabling the Solar Diagram and Show Sun Position options within the ArchSite object. Instead of the expected sun and arrow visualization, an error message pops up, and nothing is displayed. The error typically looks like this:

pyException: Traceback (most recent call last):
  File "C:\Users\semhu\Documents\tyo\BF\FreeCAD\FC_daily\FreeCAD_weekly-2025.09.10-Windows-x86_64-py311\Mod\BIM\ArchSite.py", line 1196, in onChanged
    self.updateSunPosition(vobj)
  File "C:\Users\semhu\Documents\tyo\BF\FreeCAD\FC_daily\FreeCAD_weekly-2025.09.10-Windows-x86_64-py311\Mod\BIM\ArchSite.py", line 1382, in updateSunPosition
    sun = sp.calculate_sun(month=vobj.SunDateMonth, day=vobj.SunDateDay, hour=hour_float)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\semhu\AppData\Roaming\Python\Python311\site-packages\ladybug\sunpath.py", line 202, in calculate_sun
    datetime = DateTime(month, day, *self._calculate_hour_and_minute(hour),
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\semhu\AppData\Roaming\Python\Python311\site-packages\ladybug\dt.py", line 47, in __new__
    raise ValueError("{}:\n\t({}/{}@{}:{})(m/d@h:m)".format(
<class 'ValueError'>: month must be in 1..12:
\t(0/0@0:0)(m/d@h:m)

This error trace indicates a ValueError related to the month, stating that it must be within the range of 1 to 12. The issue stems from the ArchSite object lacking default values for date and time, causing the ladybug library, used for sun path calculations, to fail. Specifically, the SunDateMonth and SunDateDay properties are initialized with zero values, which are invalid for month and day inputs. This is something we can easily resolve, and I'll show you how!

Step-by-Step Guide to Replicating the Error

Before we jump into the fix, let's make sure we all understand how to reproduce this error. This helps confirm that we're addressing the correct issue. Here’s a simple, step-by-step guide:

  1. Create a Site Object: Start by creating a new FreeCAD document and adding a site object. This is the foundation for your BIM model and where the sun study settings reside.
  2. Enable Solar Diagram: In the properties of the site object, find and enable the “Solar Diagram” option. This tells FreeCAD to display the sun's path.
  3. Enable Show Sun Position: Next, enable the “Show Sun Position” property. This should display the sun and the sun arrow in your 3D view.
  4. Observe the Error: If the issue persists, you'll see the error message we discussed earlier, and the sun and arrow won't appear. This confirms that you're facing the same problem.
  5. Manual Data Input (Temporary Fix): As a temporary workaround, manually enter the date and time values (month, day, and hour) in the site object properties. You’ll notice that the sun and arrow appear once valid values are provided. This confirms that the issue is indeed related to missing default date and time values. This manual step can be a bit tedious, so let's find a more permanent solution, shall we?

By following these steps, you can consistently reproduce the error and verify that our solution effectively addresses it. Now, let's dive into the fix!

The Solution: Setting Default Date and Time Values

The core of the problem lies in the absence of default date and time values for the ArchSite object. To resolve this, we need to ensure that the SunDateMonth, SunDateDay, and other related properties have default values assigned when a new ArchSite object is created. This prevents the ValueError from occurring and allows the sun study to function correctly right from the start. Here’s how we can tackle this:

1. Modifying the ArchSite.py File:

The most effective solution involves modifying the ArchSite.py file within the FreeCAD BIM module. This file contains the definition of the ArchSite object and its properties. By adding default values here, we ensure that all new ArchSite objects are initialized correctly. This is where the magic happens, guys!

  • Locate the File: First, you need to find the ArchSite.py file on your system. The file path will vary depending on your FreeCAD installation and operating system. Typically, it’s located in the Mod/BIM directory within your FreeCAD installation folder. For example, on Windows, it might be something like C:\Program Files\FreeCAD\Mod\BIM\ArchSite.py or, as in the error traceback, C:\Users\semhu\Documents\tyo\BF\FreeCAD\FC_daily\FreeCAD_weekly-2025.09.10-Windows-x86_64-py311\Mod\BIM\ArchSite.py. On Linux, it's often in /usr/share/freecad/Mod/BIM/. Finding the right file is the first step to fixing this!
  • Edit the File: Once you’ve located the file, open it in a text editor. You’ll need administrator privileges to save changes to this file, so keep that in mind. Be cautious when editing system files, and always create a backup before making any changes. Safety first!
  • Add Default Values: Within the ArchSite.py file, look for the ArchSite class definition. Inside this class, you'll find the property definitions. Add default values for SunDateMonth, SunDateDay, and potentially SunTimeHour. A good starting point is to set the default date to the current date. You can use the datetime module in Python to get the current date. Here’s an example of how you might modify the code:
import FreeCAD
import Arch
import datetime

class Site(Arch.ArchSite):
    def __init__(self, obj, autoUpdate=True):
        super(Site, self).__init__(obj)
        today = datetime.date.today()
        obj.addProperty("Integer", "SunDateMonth", "Sun", "Month for sun position calculation").SunDateMonth = today.month
        obj.addProperty("Integer", "SunDateDay", "Sun", "Day for sun position calculation").SunDateDay = today.day
        obj.addProperty("Float", "SunTimeHour", "Sun", "Hour for sun position calculation").SunTimeHour = 12.0 # Setting default to noon
        # Other properties...

This code snippet demonstrates how to import the datetime module, get the current date, and assign the month and day values to the SunDateMonth and SunDateDay properties, respectively. We also set a default value of 12.0 for SunTimeHour, representing noon. This is a great starting point for most solar studies. Adding these lines will prevent the dreaded error from popping up!

  • Save the File: After making the changes, save the ArchSite.py file. Make sure you save it in the same location and with the same filename. This is a crucial step, so double-check before closing the editor!

2. Restart FreeCAD:

For the changes to take effect, you need to restart FreeCAD. This ensures that the modified ArchSite.py file is loaded and the new default values are applied to ArchSite objects. A simple restart can work wonders!

3. Test the Solution:

After restarting FreeCAD, create a new ArchSite object and enable the Solar Diagram and Show Sun Position options. You should now see the sun and arrow displayed correctly without any errors. If everything works as expected, congratulations! You've successfully fixed the issue. Give yourself a pat on the back!

By implementing these steps, you ensure that FreeCAD initializes the necessary date and time values for sun studies, preventing the ValueError and allowing you to proceed with your BIM projects smoothly. This fix not only resolves the immediate error but also improves the usability of FreeCAD's solar analysis tools. It's a win-win!

Additional Tips and Considerations

While setting default date and time values resolves the primary issue, there are a few additional tips and considerations to keep in mind for a smoother experience with FreeCAD's BIM module and solar studies:

1. User Preferences:

Consider adding a user preference setting to allow users to customize the default date and time values. This provides flexibility for users who frequently work with specific dates or times for solar analysis. FreeCAD's preference system is quite robust, and adding this option can significantly enhance the user experience. Imagine being able to set your preferred default date and time – how cool is that?

2. Error Handling:

Improve error handling in the updateSunPosition function to provide more informative error messages. Instead of a generic ValueError, a message indicating that the date or time values are invalid would be more helpful for users. Clear error messages are a lifesaver when troubleshooting, guys!

3. Documentation:

Update the FreeCAD documentation to include information about this issue and the solution. This will help other users who encounter the same problem find a quick and easy fix. Good documentation is key to a thriving open-source community!

4. Community Contributions:

If you're comfortable with Git and GitHub, consider submitting a pull request with your changes to the FreeCAD repository. This helps contribute to the project and ensures that your fix is included in future releases. Contributing to open source is an awesome way to give back to the community!

5. Stay Updated:

Keep your FreeCAD installation updated to the latest version. Bug fixes and improvements are frequently added, and the issue you're experiencing might already be resolved in a newer release. Staying up-to-date ensures you have the latest and greatest features and fixes.

System Information

For reference, the original issue was reported with the following system information:

OS: Windows 10 build 19045
Architecture: x86_64
Version: 1.1.0dev.14555 (Git shallow)
Build date: 2025/09/09 06:43:51
Build type: Release
Branch: main
Hash: c1dd116e1e3304511d9e5e634e711e549b2b84e7
Python 3.11.13, Qt 6.8.3, Coin 4.0.3, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.2, OCC 7.8.1
Locale: English/United States (en_US)
Navigation Style/Orbit Style/Rotation Mode: Revit/Turntable/Drag at cursor
Stylesheet/Theme/QtStyle: FreeCAD Light.qss/FreeCAD Light/
Logical DPI/Physical DPI/Pixel Ratio: 96/126.557/1.25
Installed mods: 
  * Reporting
  * Telemetry 1.0.2

This information can be helpful for developers and other users when diagnosing similar issues. If you're reporting a bug, always include your system information to help others understand your environment.

By following these additional tips and considerations, you can further enhance your experience with FreeCAD and ensure that your BIM projects run smoothly. It's all about making the most of this awesome software!

Conclusion

Troubleshooting issues in software like FreeCAD can sometimes feel like a daunting task, but with a systematic approach, most problems can be resolved effectively. The BIM Sun Ray Study failure, caused by missing default date and time values in the ArchSite object, is a prime example of a fixable issue. By modifying the ArchSite.py file and adding default values, you can overcome this hurdle and ensure that your solar studies run seamlessly. Remember to restart FreeCAD after making changes and test the solution to confirm its effectiveness.

Furthermore, contributing to the FreeCAD community by reporting bugs, suggesting improvements, and even submitting code changes can help make the software even better for everyone. So, keep exploring, keep learning, and keep building amazing things with FreeCAD! You guys are doing great!