An entropy encoder includes an entropy encoding circuit and a size determining circuit. The entropy encoding circuit receives symbols of a pixel group, and entropy encodes data derived from the symbols of the pixel group to generate a bitstream segment which is composed of a first bitstream portion and a second bitstream portion. The first bitstream portion contains encoded magnitude data of the symbols of the pixel group, and the second bitstream portion contains encoded sign data of at least a portion of the symbols of the pixel group. The size determining circuit determines a size of a bitstream portion, wherein the bitstream portion comprises at least one of the first bitstream portion and the second bitstream portion.