[daala] Maximizing compression efficiency through block rotation

[David ledin]

The Discrete Cosine Transform (DCT) is widely used in lossy image and
video compression schemes, e.g., JPEG and MPEG. In this paper, we show
that the compression efficiency of the DCT is dependent on the edge
directions within a block. In particular, higher compression ratios
are achieved when edges are aligned with the image axes. To maximize
compression for general images, we propose a rotated block DCT method.
It consists of rotating each block, before applying the DCT, by an
angle that aligns the edges, and rotating back the block in the
decompression stage. We show how to compute the rotation angle and
analyze two alternative block rotation approaches. Our experiments
show that our method enables both a perceptual improvement and a PSNR
increase of up to 2dB, compared with the standard DCT, for low and
medium bit rates.

http://arxiv.org/pdf/1411.4290v1