a) Source, coded, or reconstructed image data. A source or reconstructed picture consists of three rectangular matrices of 8-bit numbers representing the luminance and two chrominance signals. For progressive video, a picture is identical to a frame, while for interlaced video, a picture can refer to a frame, the top field, or the bottom field of the frame depending on the context. b) In general, the term “picture” covers a coded entity. A picture can either be a frame or a field. It is possible to change dynamically between frame coding and field coding from frame to frame. Frame coding is preferred when a lot of details, but little motion is present, and field coding is best for fast movements. It is also important to realize that when the coded material originates from film, the two fields cover the exact same time, but when the source material comes from a video camera, the two fields relate to different moments.