A computer-implemented method of tracking a target, the method comprising: receiving an observed depth image of the target from a source (20, 48), the observed depth image including a plurality of observed depth pixels, one or more observed pixels including an observed depth value indicating a distance between the source (20,48) and a portion of the target corresponding to that observed pixel; obtaining a model (70, 80, 82, 180) of the target, the model (70, 80, 82, 180) having a pose; rasterizing the model (70, 80, 82, 180) into a synthesized depth image, the synthesized depth image including a plurality of synthesized pixels (162, 166), one or more synthesized pixels including a synthesized depth value indicating a distance between the source (20, 48) and a portion of the mode represented by that synthesized pixel; comparing the synthesized depth image with the observed depth image; classifying individual pixels of the synthesized depth image into a refine-z pixel case or no pixel case, the refine-z pixel case indicating a force vector to be applied in a direction along a Z axis perpendicular to an image plane of the synthesized depth image, a magnitude of the force vector being based on a difference in the observed depth image and synthesized depth image, with greater differences corresponding to larger forces; wherein the classifying is done based on the results of the comparison comprising a difference between an observed depth value and a synthesized depth value for a synthesized pixel, the refine-z pixel case being selected for that pixel if said difference is within a predetermined range; computing a force vector at the individual pixels of the synthesized depth image based on the classification; and adjusting the pose of the model (70, 80, 82, 180) by applying the computed force vectors to predetermined force-receiving locations of the model (70, 80, 82, 180) and allowing the model (70, 80, 82, 180) to move responsive to the computed force vectors, wherein the force-receiving locations are selected among the predetermined force-receiving locations to be nearest to a respective pixel of said individual pixels of the synthesized depth image classified as refine-z pixel case; and wherein no force vectors are computed at individual pixels of the synthesized depth image classified as a no pixel case. receiving an observed depth image of the target from a source (20, 48), the observed depth image including a plurality of observed depth pixels, one or more observed pixels including an observed depth value indicating a distance between the source (20,48) and a portion of the target corresponding to that observed pixel; obtaining a model (70, 80, 82, 180) of the target, the model (70, 80, 82, 180) having a pose; rasterizing the model (70, 80, 82, 180) into a synthesized depth image, the synthesized depth image including a plurality of synthesized pixels (162, 166), one or more synthesized pixels including a synthesized depth value indicating a distance between the source (20, 48) and a portion of the mode represented by that synthesized pixel; comparing the synthesized depth image with the observed depth image; classifying individual pixels of the synthesized depth image into a refine-z pixel case or no pixel case, the refine-z pixel case indicating a force vector to be applied in a direction along a Z axis perpendicular to an image plane of the synthesized depth image, a magnitude of the force vector being based on a difference in the observed depth image and synthesized depth image, with greater differences corresponding to larger forces; wherein the classifying is done based on the results of the comparison comprising a difference between an observed depth value and a synthesized depth value for a synthesized pixel, the refine-z pixel case being selected for that pixel if said difference is within a predetermined range; computing a force vector at the individual pixels of the synthesized depth image based on the classification; and adjusting the pose of the model (70, 80, 82, 180) by applying the computed force vectors to predetermined force-receiving locations of the model (70, 80, 82, 180) and allowing the model (70, 80, 82, 180) to move responsive to the computed force vectors, wherein the force-receiving locations are selected among the predetermined force-receiving locations to be nearest to a respective pixel of said individual pixels of the synthesized depth image classified as refine-z pixel case; and wherein no force vectors are computed at individual pixels of the synthesized depth image classified as a no pixel case.