GroundTruth (Blender Add-on)

Imports and exports RealityCapture/RealityScan-style XMP camera priors (xcr: namespace), either from a Blender camera animation or as one XMP per scene camera.

Features

• Export one .xmp per frame (RC-style <xcr:Rotation>, <xcr:Position>, intrinsics).
• Import or export one .xmp per camera in Multi-Camera mode.
• Export a single Metashape/Agisoft XML from an animation range or from many scene cameras.
• Render Frame + XMP and Render Animation + XMP:
  • Internal backend uses Blender’s normal render UI/preview.
  • External backend runs a background Blender render (reliable cancel).
• Export XMP (Range) writes XMPs without rendering.
• Import XMP Cameras creates or updates scene cameras from selected XMP files.
• Export XMP Cameras writes one XMP per visible, selected, or all scene cameras.
• Imported XMP intrinsics, distortion, groups, and double-precision pose values are stored on the camera for safer round trips.
• Export Metashape XML (Range) writes one XML camera per animation frame without rendering.
• Export Metashape XML Cameras writes one XML camera per visible, selected, or all scene cameras.
• Principal point is exported as zero (xcr:PrincipalPointU/V = 0).
• Distortion model:
  • Perspective: coefficients exported as zeros.
  • Brown: manual coefficient entry in the UI (k1 k2 k3 k4 t1 t2), and the add-on auto-selects the RC model:
    • brown3 / brown4 (if k4 != 0)
    • brown3t2 / brown4t2 (if t1 != 0 or t2 != 0)

Install

1. Build the install zip:
   • python3 build_ground_truth_addon_zip.py
2. In Blender: Edit → Preferences → Add-ons → Install… and pick ground_truth_blender.zip.
3. Enable the add-on: GroundTruth.

Usage

Open a .blend with an animated camera and use:

• View3D → Sidebar → GroundTruth panel (main controls)
• Image Editor → Sidebar → GroundTruth (render-time status + cancel)

Key options:

• Camera: defaults to the scene active camera.
• Camera Mode: use Animation for one camera over frames, or Multi-Camera for one scene camera per XMP/XML camera.
• Name Matching: controls whether Multi-Camera import updates existing cameras by object name, camera data name, case, and extension.
• Preserve Imported XMP: reuses imported XMP metadata during export while the camera pose still matches the imported pose.
• Use Scene Render Output: names XMP sidecars next to Blender’s render output (scene.render.filepath / frame_path).
• Use Scene Frame Range: uses scene.frame_start/end/step for animation operations.
• Write Metashape XML: after render, also writes Metashape XML into the output directory.

Output naming

XMP is written as a sidecar next to the rendered image:

• .../frame_0001.png → .../frame_0001.xmp

For still renders, Blender may write directly to scene.render.filepath; the add-on writes the matching .xmp next to that file.

In Multi-Camera mode, camera XMP files are written to Output Dir and named from the camera object names. Metashape XML camera labels also use camera object names; rename cameras to match photo filenames when you want Metashape to attach transforms to already loaded photos.

Use Clear Stored XMP on a selected camera if you want export to ignore imported metadata and use the current Blender camera settings instead.

Cancel behavior

• Internal renders: Blender does not reliably expose a cancellable render operator via bpy. Cancel usually requires Esc in the render window. If Blender is launched with --enable-event-simulate, the add-on can attempt to send an Esc event.
• External renders: cancel terminates/kills the background Blender process.

Notes (External Backend)

• External renders require a .blend on disk. If the file is unsaved/dirty, a temporary copy is saved into the output directory.
• The external process runs with isolated BLENDER_USER_* paths (to avoid broken user add-ons affecting the render).