Coding block-level perceptual video coding for 4:4:4 data in HEVC

Abstract

There is an increasing consumer demand for high bit-depth 4:4:4 HD video data playback due to its superior perceptual visual quality compared with standard 8-bit subsampled 4:2:0 video data. Due to vast file sizes and associated bitrates, it is desirable to compress raw high bit-depth 4:4:4 HD video sequences as much as possible without incurring a discernible decrease in visual quality. In this paper, we propose a Coding Block (CB)-level perceptual video coding technique for HEVC named Full Color Perceptual Quantization (FCPQ). FCPQ is designed to adjust the Quantization Parameter (QP) at the CB level -i.e., the luma CB and the chroma Cb and Cr CBs -according to the variances of pixel data in each CB. FCPQ is based on the default perceptual quantization method in HEVC called AdaptiveQP. AdaptiveQP adjusts the QP of an entire 2N×2N CU based only on the spatial activity of the constituent luma CB. As demonstrated in this paper, by not accounting for the spatial activity of the constituent chroma CBs, as is the case with AdaptiveQP, coding performance can be significantly affected; this is because the variance of pixel data in a luma CB is notably different from the variances of pixel data in chroma Cb and Cr CBs. FCPQ, therefore, addresses this problem. In terms of coding performance, FCPQ achieves BD-Rate improvements of up to 39.5% (Y), 16% (Cb) and 29.9% (Cr) compared with AdaptiveQP.

Visually lossless coding in HEVC: A high bit depth and 4:4:4 capable JND-based perceptual quantisation technique for HEVC

“…The 4:4:4 and 4:2:2 versions of these sequences contain a higher dynamic range (i.e., 10-bits per pixel per channel, which equates to 30-bits per pixel), whereas the 4:2:0 versions comprise 8-bits per pixel per channel. In our previously published work in [37], we provide empirical evidence that an absence of chroma subsampling in addition to a higher dynamic range for each colour channel are significantly advantageous for the perceptual quantisation of YCbCr data; this is particularly pertinent to 4:4:4 data. Therefore, this is the primary reason for employing a similar experimental setup to the one conducted in [37].…”

“…As shown in Figure 7, the structural reconstruction errors are concentrated mostly in the high variance regions in the Y, Cb and Cr channels. This is primarily because the HVS is less capable of detecting quantisation-induced compression artifacts in high spatial variance regions of compressed luma and chroma data [37]. Therefore, in spite of the reconstruction errors shown in Figure 7, visually lossless coding is attained by Pixel-PAQ in both the AI QP = 22 test -and also RA QP = 22 test on the DuckAndLegs 4:4:4 sequence.…”

“…These QP and QStep offsets, denoted as OQP Cb , OStep Cb , OQP Cr and OStep Cr , respectively, are quantified in (19)- (22). In relation to the aforementioned CB-level chroma Cb and Cr QP offset signalling technique present in the latest versions of JCT-VC HEVC HM [35,36], this method is also exploited in our previously published perceptual quantisation contribution named FCPQ [37]. That is, we exploit the flexibility provided by JCT-VC in terms of signalling to the decoder -in the Picture Parameter Set (PPS) -chroma QP offsets at the CB level.…”

“…In addition, the decreased reconstruction quality (per channel) for sequences coded by Pixel-PAQ, as quantified by PSNR percentage decreases, are tabulated. The bitrate reductions are averaged over four QP data points (i.e., initialQPs 22,27,37 and 37). The AI results are shown on the left; the RA results are shown on the right.…”

JND-Based Perceptual Video Coding for 4:4:4 Screen Content Data in HEVC

“…Variables μCb and μCr are computed in (7) and (8), respectively: (8) where variables zCb and sCr refer to the m th raw sample and the i th raw sample in a Cb CB and a Cr CB, respectively. Similar to our work in [33], which is closely related to our work in [34,35], we exploit the CU-level chroma Cb and Cr CB QP offset signaling mechanism in the Picture Parameter Set (PPS) [36]. This facilitates a straightforward encoder side implementation of SC-PAQ and therefore ensures that the resulting bitstream conforms to the HEVC standard (i.e., ITU-T with Rec.…”