A method of processing image data, in which object data is associated with animation data. The animation data has a plurality of keyframes, and each keyframe has a time value, an animation value and two handles. Adjacent keyframes are interpolated to produce a curve that does not overshoot animation values of either of the adjacent keyframes ensuring a smooth transition between the adjacent keyframes. When the position of a selected keyframe is changed in response to an input command, a slope and a magnitude corresponding to each handle of the selected keyframe are recalculated. Furthermore, a slope and a magnitude corresponding to each handle of a keyframe immediately before and a keyframe immediately after the selected keyframe are recalculated.