Moving image decoding/encoding

A moving image decoding method for deriving a prediction image of a chrominance image by using a luminance image, the moving image decoding method comprising:

• deriving a first luminance value for the luminance image, the first luminance value corresponding to a first position;
• deriving a first chrominance value for the chrominance image, the first chrominance value corresponding to the first position;
• deriving a second luminance value on-for the luminance image, the second luminance value corresponding to a second position;
• deriving a second chrominance value for the chrominance image, the second chrominance value corresponding to the second position;
• deriving a first difference value that indicates a difference between the first luminance value and the second luminance value;
• deriving a second difference value that indicates a difference between the first chrominance value and the second chrominance value;
• deriving a shift value for a shift operation by using the first difference value and the second difference value;
• deriving a first parameter and a second parameter by using the first difference value and the second difference value;
• setting the shift value to a first threshold based on a comparison between the first threshold and a third value, the third value being calculated by adding a first specified value to a first value, derived by using the first difference value, and then subtracting a second value, derived by using the second difference value, from a result of the addition, wherein:
• the first parameter is set to a value resulting from multiplying a second specified value by a sign of the first parameter in a case that the third value is less than the first threshold,
• the second specified value is 15,
• the shift value is set to the third value in a case that the third value is greater than or equal to the first threshold, and
• the shift value is set to the first threshold in a case that the third value is less than the first threshold;
• deriving the second parameter by using the second luminance value, the second chrominance value, the first parameter, and the shift value, according to a formula:
• b
• =
• C_Y
• ⁢
• _MIN
• -
• (
• (
• a
• *
• Y_MIN
• )
• ≫
• shiftA
• )
• ,
• wherein:
• C_Y_MIN is the second chrominance value, a is the first parameter, b is the second parameter, Y_MIN is the second luminance value, and shiftA is the shift value;
• and
• deriving the prediction image by using the first parameter, the second parameter, and the shift value.