Lab is not linear. L predicts perceptual brightness, just like sRGB, which is probably why your two gradients are almost the same. Gamma encoding is lin_to_srg

Lab is not linear. L predicts perceptual brightness, just like sRGB, which is probably why your two gradients are almost the same. Gamma encoding is lin_to_srgb(c) = c*12.92 if c<=0.0031308 else 1.055*c**(1/2.4)-0.055. Since lin_to_srgb(0.214)=0.5, in a linear ramp, the middle gray (808080) pixel should occur about 20% of the way through.