When is grey not grey?

[mikemeister_admin]

I've just been doing some experiments and found the following oddity..

If I create a black/white gradient in rgb, then as the mouse wanders over the screen the channel values agree.

If however I do the same in Lab mode, then the rgb channel values can differ by as much as 3 units.

Can anybody tell me why and even better how this happens and what it means when setting a neutral in lab mode?

Thanks
Chris

[-default]

You're very likely seeing the results of the gradient tool's "dither" option.  When I un-checked that option in the tool settings, the RGB values matched in Lab mode.  For dark areas, a small amount of dither can cause a large hue shift, and corresponding large shifts in the a and b values.

Curvemeister sets a neutral in Lab mode by converting the sample values to Lab, averaging them, and then and creating a curve control point that on the a and b curves that maps each value to zero.