Calibrate Reference Image Scale in FreeCAD

Calibrating an imported reference image in FreeCAD ensures that background blueprints or sketches match real-world dimensions for precise 3D modeling. This guide provides a straightforward, step-by-step process to import an image plane, use the calibration tool to define a known distance, and automatically resize the image to a 1:1 scale.

1. Import the Image Plane

  1. Open your project in FreeCAD.
  2. In the workbench dropdown menu, select the Image Workbench.
  3. Click the Create planar image tool (icon showing a picture with a green plus).
  4. Browse to and select your image file.
  5. In the pop-up dialog, choose the coordinate plane (XY, XZ, or YZ) that best matches your design intent and click OK.

2. Calibrate the Scale

FreeCAD includes a dedicated calibration feature in the Image Workbench that calculates the correct scale factor automatically.

  1. Select the imported ImagePlane object in the Tree View.
  2. Click the Scale an image plane tool (icon depicting a ruler over an image) from the toolbar.
  3. In the 3D viewport, click on the starting point of a feature with a known real-world dimension (e.g., an overall length dimension or a scale bar).
  4. Click on the end point of that same feature.
  5. A dialog box will appear asking for the real-world distance. Enter the exact dimension in millimeters (or your active unit of measurement) and confirm.

FreeCAD will automatically adjust the XSize and YSize properties of the image plane so the distance between those two selected points matches the input value.

3. Alternative: Manual Scaling via Properties

If you prefer manual adjustment or need to fine-tune the scale:

  1. Select the ImagePlane in the Tree View.
  2. Navigate to the Data tab in the Property View panel at the bottom left.
  3. Locate the XSize and YSize properties.
  4. Calculate the scale factor: divide your target dimension by the measured dimension of the image.
  5. Multiply the existing XSize and YSize values by that factor and press Enter.

4. Position the Calibrated Image

Once the image is scaled correctly:

  1. Select the ImagePlane in the Tree View.
  2. In the Data tab, expand the Placement property.
  3. Adjust the Position (X, Y, and Z offsets) to align the reference image with your global origin or existing sketches.
  4. Adjust the Transparency in the View tab if you need to see overlaid sketches clearly while modeling.