Google Tango - Getting pose data with IMU as base frame

Question

I am working with Google Project Tango and I tried a basic example with getting pose data:

TangoCoordinateFramePair pair;
pair.base = TANGO_COORDINATE_FRAME_START_OF_SERVICE;
pair.target = TANGO_COORDINATE_FRAME_CAMERA_COLOR;
base = TANGO_SUPPORT_ENGINE_OPENGL;
target = TANGO_SUPPORT_ENGINE_OPENGL;
error = TangoSupport_getPoseAtTime(poseTimestamp, pair.base, pair.target, base, target, ROTATION_0, &pose);

This gives TANGO_SUCCESS.

However, if I only change base to this

pair.base = TANGO_COORDINATE_FRAME_IMU;

...I keep getting TANGO_INVALID.

I tried using C API and Unity SDK, and both have a same invalid result.
Why is that? Why can't I use TANGO_COORDINATE_FRAME_IMU?

I am trying to fix Camera offset as mentioned here:
Camera-Offset | Project Tango
but without any success...

Answer 1

TangoSupport_getPoseAtTime only works for getting a pose between a fixed coordinate frame and a moving coordinate frame. The TANGO_INVALID error results from the fact that TANGO_COORDINATE_FRAME_CAMERA_COLOR and TANGO_COORDINATE_FRAME_IMU are both moving coordinate frames.

In order to find the offset between TANGO_COORDINATE_FRAME_IMU and TANGO_COORDINATE_FRAME_CAMERA_COLOR (or between any pair of moving coordinate frames), you need to use TangoService_getPoseAtTime instead.

This code snippet should give you the transform you're looking for:

TangoCoordinateFramePair pair;
pair.base = TANGO_COORDINATE_FRAME_IMU;
pair.target = TANGO_COORDINATE_FRAME_CAMERA_COLOR;
TangoPoseData pose;
TangoErrorType result = TangoService_getPoseAtTime(0.0, pair, &pose);

Note also that since both of these coordinate frames are moving (ie in a fixed position with respect to the device, and each other) the pose resulting from this call will not change as the device moves.