Image data compression apparatus, decompression apparatus, compressing method, decompressing method, and storage medium

An image decompression apparatus, comprising:

• a line memory unit to hold pixel values for at least one immediately previous line in lines to be processed of an image to be decompressed;
• a division unit to divide pixels of the line to be processed into 2n-pixel blocks;
• a first estimation value calculation unit to extrapolation-estimate a 2n-1-th pixel and a 2n-th pixel in the blocks divided by the division unit using, as reference values, values of pixels immediately above the 2n-1-th pixel and the 2n-th pixel, respectively, and to obtain an estimated value in processing for a first hierarchical level;
• a second estimation value calculation unit to interpolation-estimate certain pixels using, as reference values, pixel values of one immediately previous line in the line memory and estimated values obtained up to processing for an immediately previous hierarchical level, and to obtain an estimated value in processing for a second hierarchical level through processing for an n-th hierarchical level;
• and
• a decompressed pixel value calculation unit to obtain a quantization number from a compression code, to obtain an estimation error quantization value from the quantization number, and to obtain a pixel value of a decompressed pixel using the estimation error quantization value and an estimated value obtained by the first estimation coding unit or an estimated value obtained by the second estimation value calculation unit.

A computer-readable, non-transitory storage medium storing a program used in an information processing device for performing the processes, comprising:

• holding pixel values for at least one immediately previous line in lines to be processed of an image to be compressed;
• dividing pixels of the line to be processed into 2n-pixel blocks;
• extrapolation-estimating a 2n-1-th pixel and a 2n-th pixel in the divided blocks using, as reference values, values of pixels immediately above the 2n-1-th pixel and the 2n-th pixel, and obtaining an estimated value in processing for a first hierarchical level;
• interpolation-estimating certain pixels using, as reference values, pixel values of one immediately previous line and estimated values obtained up to processing for an immediately previous hierarchical level, and obtaining an estimated value in processing for a second hierarchical level through processing for an n-th hierarchical level;
• and
• obtaining an estimation error from the estimated value obtained in the processes from the processing for the first hierarchical level to the processing for the n-th hierarchical level, converting the estimation error into a quantization number, converting the quantization number into a variable length code, and obtaining a compression code.

A computer-readable, non-transitory storage medium storing a program used in an information processing device for performing the processes, comprising:

• holding pixel values for at least one immediately previous line in lines to be processed of an image to be decompressed;
• dividing pixels of the line to be processed into 2n-pixel blocks;
• extrapolation-estimating a 2n-1-th pixel and a 2n-th pixel in the divided blocks using, as reference values, values of pixels immediately above the 2n-1-th pixel and the 2n-th pixel in the line memory, and obtaining an estimated value in processing for a first hierarchical level;
• interpolation-estimating certain pixels using, as reference values, pixel values of one immediately previous line and estimated values obtained up to processing for an immediately previous hierarchical level, and obtaining an estimated value in processing for a second hierarchical level through processing for an n-th hierarchical level;
• and
• obtaining a quantization number from a compression code, obtaining an estimation error quantization value from the quantization number, and obtaining a pixel value of a decompressed pixel using the estimation error quantization value and the estimated value.