Abstract
When optimizing a wavelet image coder, the two main targets are to (1) improve its rate-distortion (R/D) performance and (2) reduce the coding times. In general, the encoding engine is mainly responsible for achieving R/D performance. It is usually more complex than the decoding part. A large number of works about R/D or complexity optimizations can be found, but only a few tackle the problem of increasing R/D performance while reducing the computational cost at the same time, like Kakadu, an optimized version of JPEG2000. In this work we propose an optimization of the E_LTW encoder with the aim to increase its R/D performance through perceptual encoding techniques and reduce the encoding time by means of a graphics processing unit-optimized version of the two-dimensional discrete wavelet transform. The results show that in both performance dimensions, our enhanced encoder achieves good results compared with Kakadu and SPIHT encoders, achieving speedups of 6 times with respect to the original E_LTW encoder.
Highlights
Wavelet transforms have been reported to have good performance for image compression; many state-of-the-art image codecs, including the JPEG2000 image coding standard, use the discrete wavelet transform (DWT) [1,2]
We propose the PE_LTW as an enhanced version of the E_LTW encoder by including perceptual coding based on the contrast sensitivity function (CSF) and the use of graphics processing unit (GPU)-optimized 2D-DWT algorithms based on the methods described in [4,8]
The 2D-DWT computation stage runs on a GPU and includes the perceptual weighting based on the CSF and implemented as an invariant scaling factor weighting (ISFW) [9] that weights the obtained coefficients depending on the importance that the frequency subband has for the human visual system (HVS) contrast sensitivity
Summary
Wavelet transforms have been reported to have good performance for image compression; many state-of-the-art image codecs, including the JPEG2000 image coding standard, use the discrete wavelet transform (DWT) [1,2]. Bitplane coding is implemented by the JPEG2000 encoding codeblocks with three passes per plane, so the most important information, from a rate-distortion (R/D) point of view, is first encoded The CSF relates the spatial frequency with the contrast sensitivity This perceptual coding will improve the perceptual quality of the reconstructed images, so that for a desired rate range, a better perceptual R/D behavior is achieved. We propose the PE_LTW (perceptually enhanced LTW) as an enhanced version of the E_LTW encoder by including perceptual coding based on the CSF and the use of graphics processing unit (GPU)-optimized 2D-DWT algorithms based on the methods described in [4,8]. As to test the behavior of our proposal, we have compared the performance of our PE_LTW encoder in terms of perceptual quality and encoding delays with the Kakadu implementation of the JPEG2000 standard, with and without enabling its perceptual weighting mode, and with the SPIHT image encoder
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