Question on ROS frame convention

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Hello, I'm not sure if my question was confusing, but this is not what I meant. The question was more about the convention used for representing the data 'in a frame'. I used 'Camera' and 'map' in my question in the context of SLAM, but it could refer to the pose of any object in any frame.

Perhaps another example would be useful in the question edit, because I understood your question nearly the same way (frame and data associated with it)

Hmm. Do you understand what a frame is in this context? I think of them as origins for a coordinate system. The camera frame, then, is a coordinate system based on the camera. A transform is a matrix that moves frame A to frame B via matrix multiplication. The location of the object in that frame is well defined as a Pose (http://docs.ros.org/en/noetic/api/geo...), ie a 3D point and a 4D quaternion, and always in relation to the frame. You typically don't need to consider other representation unless you need anything specific, since tf2_ros takes care of any transforming between them.

"The location of the object in that frame is well defined as a Pose (http://docs.ros.org/en/noetic/api/geo...), ie a 3D point and a 4D quaternion, and always in relation to the frame." This 'relation' is defined depending on convention, which is what my question is. In a pose message, when frame_id is 'map', is the pose then the coordinates of the 'object' in the 'map' or vice versa (coordinates of the bases of 'map' in the frame of the object). In SLAM, 3D reconstruction,etc. literature a pose wrt a map is the latter.

EDIT: To clarify, the 'vice versa' is the coordinates of the map/world/whatever frame in the frame of the 'camera/object/..'

@pointsnadpixels

In SLAM, 3D reconstruction,etc. literature a pose wrt a map is the latter.

This statement shows a deep misunderstanding of the calculation being done. A Pose can be represented by a 4x4 matrix, and the object T used to transform PoseA from frameA to frameB can also be represented by a 4x4 matrix. This transform matrix is NOT a Pose. And if we take the inverse of the transform matrix, it's still not a Pose.

How so? A pose can very much be seen as a transform. While computing the Projection matrix for a camera, the extrinsics parameters (R,t) are quite literally the pose of the camera wrt a world , where R in this case is the coordinates of the bases of the world frame in the camera frame, and 't' the position of the origin of the world frame in the camera coordinate system.
i.e. X_c = R*X_w + t where X_w is a point in the world frame, and X_c is the same point in the camera frame . There is plenty of literature which you can very easily look up.

Hello @Mike Scheutzow can you please expand on why you think this interpretation is a misunderstanding?

@pointsnadpixels, wrt your earlier comment, if a message of type PoseStamped has a header with frame_id of 'map', then the pose described in that message is given in terms of the 'map' frame. The nomenclature is maybe a little unfortunate here: in ROS, a frame is essentially a coordinate system, defined in the world (through a 3D point and a 4D quaternion). A transform takes you from one frame to another, such that B = XA, where A and B are frames, and X is a transform. Poses, here, are a type of message, linked above. It is very possible to transfer the information contained in a frame (3d point + quat) into another medium. Typically, the camera 3x4 projection matrix is not used. They both contain the same information, but saying that transform is a pose is going to be confusing in this context.