pycolmap.absolute_pose_estimation() - Does it matter if 3D points are in world or camera space ?

[colmap]

pycolmap.absolute_pose_estimation() - Does it matter if 3D points are in world or camera space ?

[mihaidusmanu]

I am not sure I understand the question. If the 3D points are in "camera coordinate space" then the pose of the camera is by definition identity (I_{4x4}) so there's nothing to estimate.

[alexs7]

@mihaidusmanu, For example in COLMAP, 3D points are given in world space coordinates. Camera poses are in camera space. What I mean is that if I multiply the 3D world points with a COLMAP camera pose, then I will get a list of transformed 3D points in camera space. Then in that space if I use the 2D - 3D (i.e the transformed ones) and run the pycolmap.absolute_pose_estimation() that should return the identity matrix ? (I was mistaken to think that I will get the camera pose here)

[mihaidusmanu]

Camera coordinate space is defined as the space where your camera is axis-aligned and placed at the origin. So yes, you will get identity if you try to estimate a pose in its own coordinate frame.

If there are multiple cameras in the scene and you transform the points to one of them, e.g., C_1 and try to estimate the pose for another camera C_2, then you will get the relative pose between them.

[alexs7]

@mihaidusmanu, Ok this helps a lot! Especially your second point. If I got it right, If I transform 3D points in two different camera spaces, C1, and C2, then running pycolmap.absolute_pose_estimation(), between 2D_c1 - 3D_c1 should give me the I matrix, but 2D_c1 - 3D_c2 (i.e 2D points from camera 1 to 3D points from camera 2 space) should return the relative pose between C1 and C2 ? (Just getting my head around coordinate systems)

[mihaidusmanu]

Yes, that's correct.

[alexs7]

Thanks!

[alexs7]

@mihaidusmanu I was just wondering what if the points are not in camera space but just in world coordinates. And I follow the same ? In that case I won't get a relative pose as a result but just an arbitrary camera world pose ?