Patent ID: 7180948

Claim:
An image decoder which applies a decoding process to image data, said image decoder comprising: means for applying a variable length decoding process to a first image data; means for applying an inverse quantizing process to said first image data; means for determining a mode of discrete cosine transform of said first image data; and inverse discrete cosine transform means for obtaining a second image, wherein: said inverse discrete cosine transform means transforms said first image data into said second image having a lower degree than said first image data when said mode comprises a field mode which performs said discrete cosine transform by each field, using a first mathematical expression as a first basis function, said inverse discrete cosine transform means transforms said first image data into said second image having said lower degree with keeping field information when said mode comprises a frame mode which performs said discrete cosine transform by each frame, using a second mathematical expression as a second basis function, such that a number of calculations included in said inverse discrete cosine transform using said second basis function is substantially equal to a number of calculations included in said inverse discrete cosine transform using said first basis function, and said first basis function comprises cos [(2x +1)uπ/2N] and said second basis function comprises cos [(2x+3)uπ/2N] cos[uπ/N].