Method for run-length encoding of a bitmap data stream

Method for run-length decoding of an encoded data stream for a video presentation on a display, comprising the steps of

• determining the first byte of a code word;
• 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 defined 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;
• and
• 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.

An apparatus for run-length encoding of a data stream comprising bitmap formatted subtitle or menu data for a visual presentation on a display, wherein the subtitle or menu data include graphics or text or both, comprising

• means for defining a first color;
• means for defining a range of run-lengths;
• means for encoding pixels of the first color to first code words with two or three bytes, wherein said first code words comprise a run-length value, and wherein the run-length value comprised in first code words having three bytes exceeds said defined range and may exceed the width of the display;
• means for encoding pixels of another than the first color to second code words with one, three or four bytes, wherein the second code words comprise a color value, and wherein second code words having three or four bytes comprise a run-length value, and wherein the run-length value comprised in second code words having four bytes exceeds said defined range and may exceed the width of the display.

An apparatus for run-length decoding of an encoded data stream containing compressed bitmap formatted subtitle or menu data for video application, comprising

• means for determining code word length, wherein the first byte of a code word is evaluated, and wherein if said first byte has another than a defined first value then said code word length is determined to be one byte, and otherwise the first and second bit of the following, second byte are evaluated, and depending on said first and second bit the code word length is determined to be two, three or four bytes respectively;
• means for decoding code words determined to be one byte long to single pixels having a color defined by said one byte, the color being different from a defined first color;
• means for decoding code words determined to be two bytes long to sequences of pixels of the defined first color, wherein the sequence length is defined by the remaining bits of the second byte of the code word;
• means for decoding code words determined to be three bytes long either to sequences of pixels of the defined first color, wherein the sequence length may exceed the width of the video display and is defined by the third byte and the remaining bits of the second byte, or to sequences of pixels of equal color other than the defined first color, wherein the sequence length is defined by the remaining bits of the second byte;
• and
• means for decoding code words determined to be four bytes long to sequences of pixels of equal color other than the defined first color, wherein the sequence length may exceed the width of the video display.