Approximation of calculations for forward discrete cosine transform

This paper presents new schemes to reduce the computation of Discrete Cosine Transform (DCT) with negligible peak signal-to-noise ratio (PSNR) degradation. The methods can be used in the software implementation of current video standard encoders; for example, H.26x and MPEG. We investigated the relationship between the quantization parameters and the position of the last nonzero DCT coefficient after quantization. That information is used to adaptively make the decision of calculating all 8x8 DCT coefficients or only part of the coefficients. To further reduce the computation, instead of using the exact DCT coefficients, we propose a method to approximate the DCT coefficients which leads to significant computation savings. The results show that for practical situations, significant computation reductions can be achieved while causing negligible PSNR degradation. The proposed method also results in computation savings in the quantization calculations.