Video coding of 8K UHDTV by HEVC/H.265 with spatio-gradational reduction and its restoration

In order to perform high compression coding of 8K UHDTV with a 12-bit depth, we propose a video coding method with spatio-gradational reduction before the HEVC/H.265 encoding procedure and its restoration after the decoding procedure. Original 8K UHDTV is reduced spatio-gradationally to a 4K image with an 8-bit depth by Lloyd-Max quantization with wavelet decomposition to minimize quantization errors and to prevent degradation for high compression coding. This spatio-gradationally reduced image is restored by wavelet image super-resolution with interpolation of gradation after the decoding procedure. In order to restore the original image faithfully, restoration parameters are selected so as to minimize the difference between the spatio-gradationally restored image and its original image on the encoder side and are then transmitted to the decoder side as side information. Experiments show that the proposed method has a better PSNR value and subjective quality than conventional HEVC/H.265 in cases of low bitrate. The proposed method delivers particularly high quality when an original image features high encoding difficulty and a biased histogram.