Method and apparatus for coding/decoding image signal providing accurate determination of an image contour and efficient compression coding

A method of coding/decoding an image signal, comprising coding and decoding steps, said coding steps comprising the steps of:

• dividing an image signal into an upper signal for coarsely designating a level of the image signal and a lower signal for finely designating the level of the image signal;
• coding the upper signal;
• detecting a contour direction of an image from said upper signal;
• selecting a reference signal for coding the lower signal in accordance with a direction code indicating the contour direction;
• coding the lower signal by referring to the selected reference signal;
• and
• synthesizing the coded upper signal and the coded lower signal and outputting a code corresponding to the image signal,
• said decoding steps comprising the steps of:
• dividing the synthesized code into a code corresponding to the upper signal and a code corresponding to the lower signal;
• decoding the code corresponding to the upper signal into an upper signal;
• detecting a contour direction of an image from said upper signal;
• selecting a reference signal for decoding the code corresponding to the lower signal in accordance with said direction code indicating the contour direction;
• decoding the code corresponding to the lower signal into a lower signal by referring to the selected signal;
• and
• synthesizing the decoded upper and lower signals to obtain the image signal.

An apparatus for coding an image signal, comprising:

• means for dividing an image signal into an upper signal for coarsely designating a level of the image signal and a lower signal for finely designating the level of the image signal;
• means for coding the upper signal;
• means for detecting a contour direction of an image signal from said upper signal;
• means for selecting a reference signal for coding the lower signal in accordance with a direction code indicating the contour direction;
• means for coding the lower signal by referring to the selected reference signal;
• and
• means for synthesizing the coded upper signal and the coded lower signal and outputting a code corresponding to the image signal.

An apparatus for decoding an image signal, comprising:

• means for dividing a synthesized coded into a code corresponding to an upper signal for coarsely designating a level of an image signal and a code corresponding to a lower signal for finely designating the level of the image signal;
• means for decoding the code corresponding to the upper signal into the upper signal;
• means for detecting a contour direction of an image from said upper signal;
• means for selecting a reference signal for decoding the code corresponding to the lower signal in accordance with a direction code indicating the contour direction;
• means for decoding the code corresponding to the lower signal into the lower signal by referring to the selected signal;
• and
• means for synthesizing the decoded upper and lower signals to obtain the image signal.