Method for adaptive entropy coding of tree structures

Method for decoding a bitstream, the bitstream representing a traversed octree representation of a 3D mesh model, comprising:

• decoding from the beginning of the bitstream control information that comprises lengths of the first, second and third portions;
• determining, according to said control information, within the bitstream at least two or more first portions, each having consecutive symbols of a first symbol group, zero or more second portions, each having consecutive symbols of a second symbol group, and two or more third portions having symbols of a third symbol group, wherein at least one of said third portions and said zero or more second portions are in a consecutive first part of the bitstream, and at least one of said first portions and said at least two second portions are in a consecutive second part of the bitstream;
• decoding said consecutive symbols of the at least two or more first portions to symbols of a fixed length using a first symbol decoder based on a first symbol group;
• decoding said consecutive symbols of the zero or more second portions to symbols of said fixed length using a first symbol decoder based on a second symbol group, wherein the second symbol group comprises said symbols of the first symbol group and additional symbols if at least one second portion was determined;
• decoding the symbols of said two or more third portions to symbols of said fixed length using a third symbol decoder based on a third symbol group, wherein the third symbol group comprises only symbols that are not comprised in said second symbol group;
• and
• providing the decoded symbols of said first, second and third portions of the bitstream in a sequential order according to said control information.

An apparatus for encoding a bitstream, comprising at least a memory and one or more processors, wherein the one or more processors are configured to:

• define at least a first and a second symbol group of binary symbols, wherein the first symbol group is a subset of the second symbol group;
• determine within the bitstream two or more first portions, zero or more second portions and two or more third portions, wherein each first portion has at least a first pre-defined number of consecutive symbols of the first symbol group, each second portion has at least a second pre-defined number of consecutive symbols of the second symbol group, and wherein remaining parts of the bitstream are third portions;
• determine in the bitstream a part that comprises only first and third portions;
• encode the bitstream, wherein said first portions are encoded in a first sub-encoder using a first code, said second portions are encoded in a second sub-encoder using a second code and said third portions are encoded in a third sub-encoder using a third code;
• and
• encode values indicating the boundary positions between the first, second and third portions in the bitstream, wherein in said part that comprises only first and third portions the encoding of a boundary position between a first portion and an adjacent third portion is skipped and implicitely encoded if the third portion has a pre-defined length.

An apparatus for decoding a bitstream, comprising at least a memory and one or more processors, wherein the one or more processors are configured to:

• decode from the beginning of the bitstream control information;
• determine, according to said control information, within the bitstream at least two or more first portions, each having consecutive symbols of a first symbol group, zero or more second portions, each having consecutive symbols of a second symbol group, and two or more third portions having symbols of a third symbol group, wherein at least one of said third portions and said zero or more second portions are in a consecutive first part of the bitstream, and at least one of said first portions and said at least two second portions are in a consecutive second part of the bitstream;
• decode said consecutive symbols of the at least two or more first portions to symbols of a fixed length using a first symbol decoder, based on a first symbol group;
• decode said consecutive symbols of the zero or more second portions to symbols of said fixed length using a first symbol decoder if at least one second portion was determined;
• and
• decode the symbols of said two or more third portions to symbols of said fixed length using a third symbol decoder based on a third symbol group, wherein the third symbol group comprises only symbols that are not comprised in said second symbol group;
• and
• provide the decoded symbols of said first, second and third portions of the bitstream in a sequential order according to said control information.

A non-transitory computer readable medium having stored thereon an encoded bitstream representing a 3D mesh model and being encoded using at least a first and a second symbol group of binary symbols, wherein the first symbol group is a subset of the second symbol group, the encoded bitstream comprising:

• two or more first portions, zero or more second portions and two or more third portions, wherein each first portion has at least a first pre-defined number of consecutive symbols of the first symbol group, each second portion has at least a second pre-defined number of consecutive symbols of the second symbol group, and remaining parts of the bitstream are third portions, and wherein said first portions are encoded using a first code, said second portions are encoded using a different second code and said third portions are encoded using a third code different from the first and second codes;
• at least one part that comprises only first and third portions;
• and
• values indicating boundary positions between the first, second and third portions in the bitstream, wherein in a part that comprises only first and third portions the encoding of a boundary position between a first portion and an adjacent third portion is skipped and implicitely encoded if the third portion has a pre-defined length.