Fast thumbnail generation for MPEG video by using a multiple-symbol lookup table

A novel method using a multiple-symbol lookup table (mLUT) is proposed to fast-skip the ac coefficients (codewords) not needed to construct a dc image from MPEG-1/2 video streams, resulting in fast thumbnail generation. For MPEG-1/2 video streams, thumbnail generation schemes usually extract dc images directly in a compressed domain where a dc image is constructed using a dc coefficient and a few ac coefficients from among the discrete cosine transform (DCT) coefficients. However, it is required that all codewords for DCT coefficients should be fully decoded whether they are needed or not in generating a dc image, since the bit length of a codeword coded with variable-length coding (VLC) cannot be determined until the previous VLC codeword has been decoded. Thus, a method using a mLUT designed for fast-skipping unnecessary DCT coefficients to construct a dc image is proposed, resulting in a significantly reduced number of table lookups (LUT count) for variable-length decoding of codewords. Experimental results show that the proposed method significantly improves the performance by reducing the LUT count by 50%.