Does the flow3d output have metric (physical) scale?

[SummerWXK]

Hi, thanks for the great work!

I noticed that Track4World uses monocular depth estimators (DepthAnythingV3 / MoGe / Pi3) as its geometry backbone. Since monocular depth estimation is inherently scale-ambiguous, I'm wondering:

Does the output flow3d carry metric (physically meaningful) scale, or is it up-to-scale only?

Specifically:

• Is the unit of the 3D coordinates (e.g., meters) meaningful across different videos?

[jiah-cloud]

Thank you for your interest! In principle, when using Depth Anything V3 as the backbone, our method has the capacity to produce metrically scaled 3D flow. However, the current implementation has not yet been adapted to preserve or calibrate metric scale in practice. If you’re interested, you’re very welcome to explore and implement this aspect. We also plan to look into adding proper metric-scale calibration in future updates.

[jiah-cloud]

Hi @SummerWXK, We've recently added a --metric_scale option that recovers physical (meter-level) scale when using the DA3 backbone:

python demo.py --use_model depthanythingv3 --metric_scale ...

This leverages the DA3 Nested backbone's metric depth capability to restore all geometric outputs (points, flow3d, world_points, camera_poses) to meter-level coordinates.

Note: metric scale recovery is currently only supported with the DA3 backbone. MoGe and Pi3 backbones remain in relative scale.