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The Discrete Cosine Transform (DCT) is widely used in lossy image and videocompression schemes, e.g., JPEG and MPEG. In this paper, we show that thecompression efficiency of the DCT is dependent on the edge directions within ablock. In particular, higher compression ratios are achieved when edges arealigned with the image axes. To maximize compression for general images, wepropose a rotated block DCT method. It consists of rotating each block, beforeapplying the DCT, by an angle that aligns the edges, and rotating back theblock in the decompression stage. We show how to compute the rotation angle andanalyze two alternative block rotation approaches. Our experiments show thatour method enables both a perceptual improvement and a PSNR increase of up to2dB, compared with the standard DCT, for low and medium bit rates.

Maximizing compression efficiency through block rotation