Is there any way to measure and adjust the exact value of the focal length and FOV of Revit camera?

[z5521569]

How can I quantitatively adjust the focal length and FOV of a camera in Revit? It seems that I can only change the focal length by dragging in the plan view. In addition, what are the specific FOV and focal length values ​​of a Revit camera, and where do I get this information?

[jeremy_tammik]

I do not have a clear description at hand for you, so I asked an LLM to summarise and instruct, as described in my blog post yesterday. Here is the answer it provides; please take a look at let us know whether it helps:

Measuring and Adjusting Focal Length and FOV of Revit Camera

You're hitting on a common frustration with the Revit API: direct access to and manipulation of camera focal length and Field of View (FOV) isn't as straightforward as one might expect. Let's break down the situation and explore viable solutions.

Understanding the Challenge

• Indirect Control: The Revit API doesn't expose explicit properties for focal length or FOV. Instead, these are implicitly controlled through the CropBox property of the View3D.
• Geometric Relationship: The relationship between the CropBox, camera position, target point, and FOV is geometric. Changing the CropBox effectively alters the FOV, which in turn relates to the focal length.

Solutions and Workarounds

1. Geometric Calculation:

   • This is the most precise method. By understanding the geometry of the camera and CropBox, you can calculate the FOV and derive the equivalent focal length.

   • Formula:

     Focal Length = (Film Width / 2) / tan(FOV / 2)
     

     Where:

     • Film Width is the width of the CropBox at the camera's plane.
     • FOV is the Field of View in radians.

   • Code Example (Conceptual):

     C#

     // Get the CropBox of the 3D view
     CropBox cropBox = view3D.GetCropBox();
     
     // Get the camera position and target
     XYZ cameraPosition = camera.Position;
     XYZ cameraTarget = camera.Target;
     
     // Calculate the camera's view direction
     XYZ viewDirection = cameraTarget.Subtract(cameraPosition).Normalize();
     
     // Calculate the 'Film Width' (width of the CropBox at the camera plane)
     // ... (complex geometry calculations)
     
     // Calculate FOV based on CropBox dimensions and camera position
     double FOV = CalculateFOV(cropBox, cameraPosition, viewDirection);
     
     // Calculate focal length
     double focalLength = (filmWidth / 2) / Math.Tan(FOV / 2);
     

   • Note: The most challenging part here is accurately determining the Film Width, which requires projecting the CropBox onto the camera's image plane.

2. Dynamo for Visual Adjustment:

   • If absolute precision isn't paramount, Dynamo can provide a more visual and interactive way to adjust the FOV.
   • Nodes like "Camera.SetEyePosition," "Camera.SetTarget," and "View.SetCropBox" can be used to manipulate the camera and CropBox, allowing for visual feedback on the FOV.

3. Third-Party Libraries:

   • Some third-party Revit API libraries might offer helper functions or extensions that simplify camera manipulation, potentially including FOV and focal length adjustments. Explore available options and their documentation.

Key Considerations:

• Units: Ensure consistent units (e.g., millimeters for focal length, radians for FOV) throughout your calculations.
• Coordinate Systems: Pay close attention to Revit's coordinate system and any transformations needed when working with camera and CropBox geometry.
• Performance: Complex geometric calculations can impact performance, especially in large models. Optimize your code for efficiency.

Practical Demonstration (Conceptual):

Imagine a scenario where you want to set a specific FOV for a camera. Using the geometric approach, you would:

1. Get the desired FOV value.
2. Perform the inverse calculation to determine the required CropBox dimensions at the camera plane.
3. Transform the CropBox to match these calculated dimensions.

This would effectively set the camera's FOV to the desired value.

Additional Tips:

• The Building Coder: Explore The Building Coder blog for valuable insights into Revit API geometry and camera manipulation.
• Revit API Documentation: Refer to the official Revit API documentation for detailed information on the View3D, Camera, and CropBox classes.
• Community Forums: Engage with the Revit API community on forums and online platforms to learn from others' experiences and solutions.

By combining a strong understanding of the underlying geometry with the appropriate API tools and techniques, you can effectively measure and adjust the focal length and FOV of Revit cameras, even without direct API access.