Of itu-t sg16 wp3 and iso/iec jtc1/SC29/WG11

JCTVC-M0348 (m28812) AHG8: Quantitative measurement for subjectively lossless screen content coding [Tao Lin, Shuhui Wang, Kailun Zhou, Qiang Shen (Tongji Univ.)]

One of the key issues in subjectively lossless screen content coding is asserted to be to develop a quantitative measurement for assessing subjective losslessness. It is suggested that the assessment should guarantee absolutely subjective losslessness. Considering the well-known human visual masking effect, this contribution proposes considering a local area (such as 4x4 block) based and number of distinct colours related PSNR measurement for subjective losslessness. Moreover, this contribution discusses theoretical and experimental supports for the proposed quantitative measurement. Experimental results were also reported that were suggested to show that the measurement gives a more reasonable and correlated assessment of subjective losslessness than single PSNR measurement, resulting in a much lower bit rate than what would be needed for mathematically lossless coding. The measurement is also intended to help improving subjectively lossless coding technology.

The contribution suggested, as a starting point, a measure that considers the number of distinct (8b-ti tri-stimulus) colours and PSNR in 4×4 regions to determine whether the coding in each region is considered visually lossless or not.

In experiments, the contributor indicated that QP around 30 (all-intra) was approximately where distortions seemed to start to become visible on screen content. Another contributor said that contribution M0320 reported that QP around 27 was observed to be such a value.

A participant commented on the well-known concept of "just-noticeable differences".

Further study was encouraged, as it would be desirable to have improved objective quality measurement techniques to assist in our work.

Consulting with others such as V. Baroncini and VQEG might provide useful guidance on finding practical objective quality metrics for use in our work.

M0320 was noted to be related.