Image encoder and encoding method

A transmitter for an encoded bitstream, the transmitter comprising:

• circuitry;
• and
• a memory coupled to the circuitry,
• wherein the circuitry, in operation:
• writes a first parameter and a second parameter to a bitstream;
• derives a partition mode based on the first and second parameters;
• and
• responsive to the derived partition mode being a first partition mode, executes the first partition mode including:
• splitting a block of a picture, in a vertical direction or in a horizontal direction, into a plurality of first blocks, the plurality of first blocks including a N×2N block sized N pixels by 2N pixels;
• splitting the N×2N block, wherein a ternary split is allowed to split the N×2N block in the vertical direction, which is a direction along the 2N pixels, into a plurality of sub blocks including at least one sub block sized N/4×2N, while a binary split is not allowed to split the N×2N block in the vertical direction into two sub blocks that are equally sized N/2×2N;
• encoding the plurality of sub blocks to the bitstream;
• and
• transmitting the encoded bitstream.