Fig. 2.

In the transformation chain of the RGBD-C-arm system for THA (a), the RGBD, x-ray, visual marker, and acetabular cup coordinate frames are denoted as RGBD, X, M, and C, respectively. In an offline calibration step, the extrinsic relation between the RGBD and x-ray (${{}^{\mathrm{X}}\mathbf{T}}_{\mathrm{RGBD}}$) is estimated. Once this constant relation is known, the pose of the x-ray source can be estimated for every C-arm repositioning (b) by identifying displacements in the RGBD camera coordinate frame.