Patent ID: 7088775

Claim:
An image data converting apparatus for converting first compressed image data to second compressed image data being more compressed than the first compressed image data, said first compressed image data being interlaced-scan data compressed by orthogonal transform and motion compensation, and said second compressed data being serial-scan data, said apparatus comprising: image data decoding means for decoding the first compressed image data by using only lower mth-order orthogonal transform coefficients included in nth-order orthogonal transform coefficients (where m<n), in both a vertical direction and a horizontal direction in the first compressed image data; scan-converting means for converting interlaced-scan data output from the image data decoding means to serial-scan data; image data encoding means for encoding the serial-scan data, thereby generating the second compressed image data; and wherein the image data decoding means comprises compression inverse discrete-cosine transform means of a frame-discrete cosine transform mode, wherein the compression inverse discrete-cosine transform means of frame-discrete cosine transform mode performs the inverse discrete cosine transform by using a part of coefficients included in (4×8)th-order discrete cosine transform coefficients input to achieve the field-discrete compression inverse discrete cosine transform, while replacing remaining coefficients by 0s, thus discarding the remaining coefficients, and wherein the part of the coefficients included in the (4×8)th-order discrete cosine transform coefficients is only (4×4) and (4×2)th-order coefficients included in (4×8)th-order discrete cosine transform coefficients.