Techniques and tools for coding / decoding of video images, and in particular, B-frames, are described. In one aspect, a video
encoder / decoder determines a fraction for a current image in a sequence. The fraction represents an estimated temporal distance position for the current image relative to an interval between a reference images for the current image. The video
encoder / decoder processes the fraction along with a
motion vector for a first
reference image, resulting in a representation of motion (e.g., constant or variable velocity motion) in the current image. Other aspects are also described, including intra B-frames, forward and backward buffers for
motion vector prediction, bitplane encoding of
direct mode prediction information, multiple
motion vector resolutions / interpolation filters for B-frames, proactive dropping of B-frames, and signaling of dropped predicted frames.