Oklab and its cylindrical representation Oklch are widely adopted in interpolation and design workflows as perceptually motivated color spaces, but their color difference prediction accuracy falls short of CIEDE2000. We propose Oklch+, a three-parameter extension of Oklab comprising a power transformation on the L-axis and a Naka-Rushton compression on the C-axis, with Euclidean distance computed in the resulting transformed Oklab coordinates. The Naka-Rushton function is bounded in [0,1], reflecting the saturating nature of chroma sensitivity at high colorimetric values. Evaluated on COMBVD -- 3,813 suprathreshold color difference pairs spanning six independent experimental datasets -- Oklch+ achieves STRESS = 29.09, closely matching CIEDE2000 (29.13; difference = 0.04), using only three parameters optimized against color difference data compared to approximately 17 for CIEDE2000. Cross-validation on a held-out BFD-P D65 subset (2,028 pairs) confirms generalization (STRESS = 26.14), with Oklch+ substantially outperforming Oklab (51.45) and achieving STRESS comparable to CIEDE2000 (24.12) on the held-out set. Improvement over Oklab (47.35) is confirmed across all six COMBVD sub-datasets. Because Oklch+ defines a coordinate system in which Euclidean distance approximates perceptual distance, linear interpolation in the transformed space offers substantially improved perceptual uniformity relative to Oklab. Current evaluation is limited to the sRGB-centered COMBVD dataset; validation in high-chroma regions with empirical observer-rated discrimination data remains future work.

翻译：Oklab及其圆柱表示Oklch因其感知一致性而广泛应用于插值和设计工作流程中，但其色差预测精度不及CIEDE2000。我们提出Oklch+——一种基于Oklab的三参数扩展，包含对L轴的幂变换和对C轴的纳卡-拉什顿压缩，并在变换后的Oklab坐标中计算欧几里得距离。纳卡-拉什顿函数在[0,1]范围内有界，反映了色度灵敏度在高色度值时的饱和特性。在COMBVD数据集（包含来自六个独立实验数据集的3813对超阈值色差对）上的评估显示，Oklch+的STRESS值为29.09，与CIEDE2000（29.13；差异为0.04）高度接近，且仅使用针对色差数据优化的三个参数，而CIEDE2000约需17个参数。在保留的BFD-P D65子集（2028对）上的交叉验证证实了其泛化能力（STRESS=26.14），Oklch+在该保留集上显著优于Oklab（51.45），并获得与CIEDE2000（24.12）相当的STRESS值。在所有六个COMBVD子数据集上均确认了相对于Oklab（47.35）的改进。由于Oklch+定义了一个欧几里得距离近似于感知距离的坐标系，因此在变换空间中的线性插值相对于Oklab具有显著改善的感知均匀性。当前评估仅限于以sRGB为中心的COMBVD数据集；在高色度区域使用经验观察者判别数据的验证仍是未来工作。