Method and apparatus of binarization and context-adaptive coding for syntax in video coding

A method of entropy decoding for source symbols generated in a video coding system, the method comprising:

• receiving an input coded bitstream including compressed data for a current source symbol;
• decoding the coded bitstream, using CABAC (context-adaptive binary arithmetic coding), to recover a first binary string and any second binary string;
• recovering a prefix part related to a MSB (most significant bit) index of a current symbol value of the current source symbol by decoding the first binary string using a unary code or a truncated unary code;
• recovering a suffix part corresponding to one or more refinement bits of the current symbol value by decoding the second binary string using a fixed-length code or a truncated binary code if any second binary string exists;
• and
• recovering the current source symbol based on the MSB index and any refinement bit of the current symbol value if any refinement bit exists,
• wherein when the current source symbol corresponds to syntax Index_Mode representing a second runlength minus one of a consecutive second index in a same row, a bin of the first binary string with a bin index equal to zero is coded in a regular CABAC mode with three contexts,
• wherein a first context is selected if the palette index is less than a first threshold T1, a second context is selected if the palette index is greater than or equal to the first threshold T1 and less than a second threshold T2, and a third context is selected if the palette index is greater than or equal to the second threshold T2, and
• wherein the second threshold T2 is greater than the first threshold T1.