Perceptual Axis Alignment Outperforms Statistical Decorrelation for Colour Transform Design in Image Compression

"We compare the NTSC YIQ transform (1953) and the YCoCg transform (Malvar et al., 2008) for JPEG image compression on the 24-image Kodak benchmark — the same dataset from which YCoCg was derived. Using four metrics from PSNR to S-CIELAB, we show that the ranking between transforms reverses as metrics become more perceptually accurate. Under equalised quantisation at Q75, PSNR favours YCoCg; S-CIELAB favours YIQ by 14% on all 24 images. Under standard JPEG chroma quantisation with 4:2:0 subsampling, S-CIELAB favours YIQ by 30% on 23/24 images. The more accurately a metric models human colour vision, the larger the advantage of perceptual axis alignment over statistical decorrelation."