Coding apparatus and method and storage medium storing program

A coding apparatus comprising:

• a color space conversion unit that converts input image data into image data which is represented in another color space;
• a gradation changing unit that changes gradation of the converted image data;
• a color space reconversion unit that converts the image data subjected to the gradation change into image data represented in the same color space as for the input image data;
• and
• a predictive coding unit that performs predictive coding on the image data converted by the color space reconversion unit.

A coding method comprising:

• converting input image data to image data which is represented in another color space;
• changing gradation of the input image data based on the converted image data;
• and
• performing predictive coding on the image data subjected to the gradation change.

A storage medium readable by a computer, the storage medium storing a program comprising instructions causing a computer to execute a function comprising:

• converting input image data to image data which is represented in another color space;
• changing gradation of the input image data based on the converted image data;
• and
• performing predictive coding on the image data subjected to the gradation change.