  A new methodology is proposed to estimate 3D displacement fields from pairs
of images obtained from X-Ray Computed Micro Tomography (XCMT). Contrary to
local approaches, a global approach is followed herein that evaluates {\em
continuous} displacement fields. Although any displacement basis could be
considered, the procedure is specialized to finite element shape functions. The
method is illustrated with the analysis of a compression test on a
polypropylene solid foam (independently studied in a companion paper). A good
stability of the measured displacement field is obtained for cubic element
sizes ranging from 16 voxels to 6 voxels.
