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In this paper, we propose a generalized discrete cosine transform (DCT) decimation scheme for DCT-domain spatial downscaling which performs two-fold decimation on subframes of a flexible size larger than the traditional 8 times 8 block size to improve the visual quality. Efficient sparse-matrix representations are then derived to reduce the computation of the proposed DCT decimation method. We compare the antialiasing filtering performances and computational complexities of the proposed downscaling scheme with the existing DCT-domain and pixel-domain downscaling schemes. Our analysis shows that the proposed scheme can reduce the aliasing artifact compared to the pixel-domain downscaling schemes, whereas the computational complexity may be increased. Experimental results are reported to show the efficacy of the proposed approach.