Many quality metrics take as input gamma corrected images and assume that pixel code values are scaled perceptually uniform. Although this is a valid assumption for darker displays operating in the luminance range typical for CRT displays (from 0.1 to 80 cd/m²), it is no longer true for much brighter LCD displays (typically up to 500 cd/m²), plasma displays (small regions up to 1000 cd/m²) and HDR displays (up to 3000 cd/m²). The distortions that are barely visible on dark displays become clearly noticeable when shown on much brighter displays. To estimate quality of images shown on bright displays, we propose a straightforward extension to the popular quality metrics, such as PSNR and SSIM, that makes them capable of handling all luminance levels visible to the human eye without altering their results for typical CRT display luminance levels. Such extended quality metrics can be used to estimate quality of high dynamic range (HDR) images as well as account for display brightness.

Figure 1. Data flow diagram of the extended metrics for typical 8-bit images. Pixel values are converted to luminance and re-encoded with the Perceptually Uniform (PU) encoding before quality assessment.

Resources

• A set of simple quality metrics inluding the improved version of PU encoding
• Paper [bib]
• Matlab code for PU encoding including PU-PSNR and PU-SSIM

Links

• HDR Resources