Moving image coding apparatus, code amount control method, and storage medium

A method for controlling a code amount to be generated in coding a moving image signal for forming a frame image including a plurality of macroblocks, the method comprising:

• setting a target code amount targetBit as a code amount to be allocated to 2×k1 macroblocks, where k1 is a natural number;
• obtaining a sum of absolute transformed differences calculated from a difference image indicating a difference between a current macroblock and a corresponding image region of a reference frame image in a motion estimate process;
• obtaining a sum of absolute transformed differences recent_SATD for the current macroblock and sums of absolute transformed differences for k1 previous macroblocks, where k1 is a natural number and k1≦N1, and calculating an average avrg_AllocatedBit of the sums of absolute transformed differences for the k1 previous macroblocks;
• obtaining code amounts for k1 previous macroblocks, where k1 is a natural number and k1≦N1, and calculating an average avrg_AllocatedBit of the code amounts for the k1 previous macroblocks, and calculating a total code amount usedBit allocated to the k1 previous macroblocks, and
• calculating a code amount current_AllocatedBit to be allocated to the current macroblock by using the formulas:
• allocatedBit=targetBit−usedBit,
• avrg_AllocatedBit=allocatedBit/k1,
• and
• current_AllocatedBit=avrg_AllocatedBit+coe×(recent_SATD−avrg_SATD),
• where coe is a coefficient;
• obtaining a code amount recent_AllocatedBit for a macroblock that is one-macroblock previous to the current macroblock, and calculating a transformed code amount bitQP26 by quantizing the obtained code amount recent_AllocatedBit using a median of possible quantization parameter values in a quantization performed in coding the moving image signal;
• obtaining a median-QP-transformed code amount predBitQP26 by performing IIR filtering with respect to the median-QP-transformed code amount bitQP26 calculated by the step of calculating the median-QP-transformed code amount using the code amount recent_AllocatedBit for the macroblock that is one-macroblock previous to the current macroblock;
• and
• calculating a quantization parameter value QP to be used in coding the current macroblock by using the formula:
• QP=C0−6×log 2(current_AllocatedBit/predBitQP26),
• where C0 is a median of possible quantization parameter values, and
• log 2 ( ) is a function that calculates a logarithm to the base 2.

A method for controlling a code amount to be generated in coding a moving image signal for forming a frame image including a plurality of macroblocks, the method comprising:

• setting a target code amount targetBit as a code amount to be allocated to 2×k1 macroblocks, where k1 is a natural number;
• obtaining a sum of absolute transformed differences calculated from a difference image indicating a difference between a current macroblock and a corresponding image region of a reference frame image in a motion estimate process;
• obtaining a sum of absolute transformed differences recent_SATD for the current macroblock and sums of absolute transformed differences for k1 previous macroblocks, where k1 is a natural number and k1≦N1, and calculating an average avrg_AllocatedBit of the sums of absolute transformed differences for the k1 previous macroblocks;
• obtaining code amounts for k1 previous macroblocks, where k1 is a natural number and k1≦N1, and calculating an average avrg_AllocatedBit of the code amounts for the k1 previous macroblocks, and calculating a total code amount usedBit allocated to the k1 previous macroblocks;
• calculating a code amount current_AllocatedBit to be allocated to the current macroblock by using the formulas:
• allocatedBit=targetBit−usedBit,
• avrg_AllocatedBit=allocatedBit/k1,
• and
• current_AllocatedBit=avrg_AllocatedBit+coe×(recent_SATD−avrg_SATD),
• where coe is a coefficient;
• transforming a code amount recent_AllocatedBit for a macroblock that is one-macroblock previous to the current macroblock to a transformed code amount by quantizing the code amount recent_AllocatedBit using a median of possible quantization parameter values in a quantization performed in coding the moving image signal;
• storing, for an intra macroblock, the transformed code amount obtained in the step of transforming the code amount recent_AllocatedBit when the macroblock that is one-macroblock previous to the current macroblock is an intra macroblock;
• storing, for an inter macroblock, the transformed code amount obtained in the step of transforming the code amount recent_AllocatedBit when the macroblock that is one-macroblock previous to the current macroblock is an inter macroblock;
• calculating a first average predBitQP26(1) of median-QP-transformed code amounts for intra macroblocks stored in the step of storing the transformed code amount for an intra macroblock;
• calculating a second average predBitQP26(0) of median-QP-transformed code amounts for inter macroblocks stored in the step of storing the transformed code amount for an inter macroblock;
• and
• (1) calculating a quantization parameter value to be used in coding the current macroblock based on the average predBitQP26(1) of median-QP-transformed code amounts for intra macroblocks when the current macroblock is an intra macroblock, and (2) calculating the quantization parameter value to be used in coding the current macroblock based on the average predBitQP26(0) of median-QP-transformed code amounts for inter macroblocks when the current macroblock is an inter macroblock.