Image coding apparatus, image coding method, and image coding program for coding at least one still frame with still frame coding having a higher quality than normal frame coding of other frames

1. An image coding method performed by an apparatus comprising:

a still frame judgment step of judging whether or not a target frame to be coded is a still frame;

a still frame coding judgment step of judging whether a frame after the target frame is to be subjected to still frame coding or normal frame coding, the still frame coding having a higher image quality than the normal frame coding, on the basis of coding information of the target frame that is determined in coding a frame previous to the target frame;

a quantization parameter value control step of controlling a quantization scale of an externally inputted quantization parameter, on the basis of an output from the still frame judgment step;

a skip macroblock judgment step of judging whether each macroblock in the target frame is a skip macroblock or not, by using the quantization parameter controlled in the quantization parameter value control step, and a difference value between the target frame and a reference frame; and

a coding step of determining a coding method of the frame after the target frame on the basis of the judgment by the still frame coding judgment step, and coding the target frame on the basis of the coding information that is determined in coding the previous frame,

wherein the coding step includes a high image quality coding step of subjecting any macroblock among a plurality of macroblocks located at a same position in a plurality of continuous frames which are judged as still frames in the still frame judgment step, to at least one-time still frame coding having a higher image quality relative to coding for other macroblocks.

2. An image coding method as defined in claim 1 , wherein the high image quality coding step does not perform the high image quality coding on an I frame to be subjected to only intra-frame predictive coding.

3. An image coding method as defined in claim 1 , wherein the high image quality coding step does not perform the high image quality coding on a macroblock to be subjected to intra-frame predictive coding.

4. An image coding apparatus comprising:

a still frame judgment unit for judging whether or not a target frame to be coded is a still frame;

a still frame coding judgment unit for judging whether a frame after the target frame is to be subjected to still frame coding or normal frame coding, the still frame coding having a higher image quality than the normal frame coding, on the basis of coding information of the target frame that is determined in coding a frame previous to the target frame;

a quantization parameter value control unit for controlling a quantization scale of an externally inputted quantization parameter, for each macroblock, on the basis of an output from the still frame judgment unit;

a skip macroblock judgment unit for judging whether each macroblock in the target frame is a skip macroblock or not, by using the quantization parameter controlled by the quantization parameter value control unit, and a difference value between the target frame and a reference frame; and

a coding unit for performing coding of the target frame while determining a coding method for each macroblock in a frame next to the target frame on the basis of the judgment by the still frame judgment unit and the judgment by the skip macroblock judgment unit, wherein any macroblock among a plurality of macroblocks located at a same position in a plurality of continuous frames which are judged as still frames in the still frame coding judgment unit, is subjected to at least one-time still frame coding having a higher image quality relative to coding for other macroblocks.

5. An image coding apparatus as defined in claim 4 , wherein the coding unit performs the coding by using the quantization parameter that is controlled by the quantization parameter value control unit.

6. An image coding apparatus as defined in claim 4 , wherein

the skip macroblock judgment unit starts the judging upon receipt of a frame coding signal indicating a coding method of the target frame, the frame coding signal being supplied from outside, and

the coding unit performs coding of each macroblock in the target frame when the frame coding signal indicates intra-frame predictive coding, and performs coding of the difference value when the frame coding signal does not indicate intra-frame predictive coding.

7. An image coding apparatus comprising:

a skip macroblock judgment unit for judging whether each macroblock in a target frame is a skip macroblock or not, by using a difference value between the target frame and a reference frame and an externally inputted quantization parameter;

a still frame coding judgment unit for judging whether a frame after the target frame is to be subjected to still frame coding or normal frame coding, the still frame coding having a higher image quality than the normal frame coding, on the basis of coding information of the target frame that is determined in coding a frame previous to the target frame; and

a coding unit for performing coding of the target frame while determining a coding method for each macroblock in a frame next to the target frame on the basis of the judgment by the still frame coding judgment unit and the judgment by the skip macroblock judgment unit, wherein any macroblock among a plurality of macroblocks located at a same position in a plurality of continuous frames which are judged as still frames in the still frame coding judgment unit, is subjected to at least one-time still frame coding having a higher image quality relative to coding for other macroblocks.

8. An image coding apparatus as defined in claim 7 , further comprising

a quantization parameter value control unit for controlling a quantization scale of an externally inputted quantization parameter, on the basis of the coding method determined in the still frame coding judgment unit, wherein

said coding unit performs the coding by using the quantization parameter that is controlled by the quantization parameter value control unit.

9. An image coding apparatus as defined in claim 7 , wherein

the skip macroblock judgment unit starts the judging upon receipt of a frame coding signal indicating a coding method of the target frame, the frame coding signal being supplied from outside, and

the coding unit performs coding of each macroblock in the target frame when the frame coding signal indicates intra-frame predictive coding, and performs coding of the difference value when the frame coding signal does not indicate intra-frame predictive coding.