Method for run-length encoding of a bitmap data stream
View Patent ↗Subtitling aims at the presentation of text information and graphical data, encoded as pixel bitmaps. The size of subtitle bitmaps may exceed video frame dimensions, so that only portions are displayed at a time. The bitmaps are a separate layer lying above the video, e.g. for synchronized video subtitles, animations and navigation menus, and therefore contain many transparent pixels. An advanced adaptation for bitmap encoding for HDTV, e.g. 1920.times.1280 pixels per frame as defined for the Blu-ray Disc Prerecorded format, providing optimized compression results for such subtitling bitmaps, is achieved by a four-stage run length encoding. Shorter or longer sequences of pixels of a preferred color, e.g. transparent, are encoded using the second or third shortest code words, while single pixels of different color are encoded using the shortest code words, and sequences of pixels of equal color use the third or fourth shortest code words.
1. A method for run-length decoding of an encoded data stream for a video presentation on a display, wherein the encoded data stream is distributed to multiple data packets, and wherein the data packets comprise a first flag indicating whether they are the first of said multiple data packets, and a second flag indicating whether they are the last of said multiple data packets, and wherein the encoded data stream for a video presentation is a separate layer overlaying other video data on the display, the method comprising the steps of:
reconstructing bitmap formatted data, wherein multiple data packets are concatenated according to said first flag and said second flag and wherein the bitmap formatted data are extracted from the concatenated packets;
determining the first byte of a code word in the reconstructed bitmap formatted data;
if said first byte has not a defined first value, decoding said first byte to a single pixel having individual color defined by the value of said first byte, the color being other than a defined first color;
if said first byte has the defined first value, determining the first and second bit of the following byte being the second byte;
if the first and second bit of the second byte have a first value, decoding the remaining bits of the second byte to a sequence of pixels of the first color, wherein said remaining bits of the second byte define the sequence length;
if the first and second bit of the second byte have a second value, decoding said remaining bits of the second byte together with the following third byte to a sequence of pixels of the defined first color, wherein said remaining bits of the second byte and said third byte define the sequence length, and wherein said sequence length may exceed the display width;
if the first and second bit of the second byte have a third value, decoding said remaining bits of the second byte together with the third byte to a sequence of pixels, wherein said remaining bits of the second byte define the sequence length and the third byte defines the pixels color;
if the first and second bit of the second byte have a fourth value, decoding said remaining bits of the second byte together with the third and a following fourth byte, wherein said remaining bits of the second byte and the third byte define the sequence length and the fourth byte defines the pixel color, and wherein said sequence length may exceed the display width; and
selecting a portion of said separate layer for being displayed.