Image coding device, image coding method, and image coding system

An image coding system comprising:

• an image coding device for coding block data each including a plurality of pixels by selecting, for each block data, one of a lossy mode and a lossless mode in which coding includes use of a dictionary with different colors each registered in a corresponding one of p dictionary indexes, p being a natural number, and an image decoding device configured to decode a code created by the image coding device, wherein
• the image coding device includes:
• an irreversible coding unit configured to perform coding in the lossy mode in a case where the number of unregistered colors is greater than a reference value, the number of unregistered colors being the number of colors included in block data of interest and not registered in the dictionary before coding of the block data of interest is started, the block data of interest being included in the block data to be coded and being of interest for coding;
• and
• a dictionary coding unit configured to perform coding in the lossless mode and register, in the dictionary, q (q is a number identical to the p) colors most recently coded in the lossless mode in a case where the number of unregistered colors is less than or equal to the reference value.