Patent ID: 7499592

Claim:
An image decoding apparatus having a function of decoding Extended sequential DCT based JPEG encoded data, comprising: determination means for determining by analyzing header information of encoding data if the encoded data is baseline JPEG encoded data or Extended sequential DCT based JPEG encoded data; entropy decoding means for entropy decoding the encoded data; calculation means for, when said determination means determines that the encoded data is baseline JPEG encoded data, calculating inverse quantization data X by: X=D×Q ×2 n (n is an integer not less than 1 and not more than 4) where D is a decoded value decoded by said entropy decoding means, Q is a quantization step value of quantization table information included in the header information of the encoded data, and X is inverse quantization data; inverse DCT transformation means for performing inverse DCT transformation of the inverse quantization data X obtained by said calculation means; and output means for, when said determination means determines that the input encoded data is baseline JPEG encoded data, applying a rounding process to image data after the inverse DCT transformation and outputting the processed image data, and for, when said determination means determines that the input encoded data is Extended sequential DCT based JPEG encoded data, outputting image data after the inverse DCT transformation as a decoded result.