Luminance or Lightness masks?

[Ignacio]

Hi,

I notice that people use these terms loosely. But, what is best, say, when one wants to mask darker areas of an image to do a more aggressive denoising? Or to do an HDR or histogram equalization?

At first, I used to use lightness as the basis for a mask (also because is handy with the little button in the tools bar). But then I switched to luminance (Y component in the XYZ space), which I find more natural looking and less noisy, and it is a linear transformation as opposed to lightness (not sure if this matters at all, as to build a mask you do a histogram stretch anyway).

Is there a fundamental reason to use one over the other?

thanks,
Ignacio

[Phil Leigh]

I'm confused... the little button the toolbar extracts the luminance component ( CIE L* ).

[Ignacio]

My understanding is that CIE L* is lightness (a non-linear transformation of the RGB space) and CIE Y is luminance (a linear transfomation of the RGB space).

Ignacio

[pfile]

yah L* is lightness. this color theory stuff is pretty involved and quite confusing actually. the wikipedia pages dealing with this stuff is a start.

if you use RGBWorkingSpace to set the RGB weights to 1,1,1 before you extract L*, then the extracted image should be similar to L, luminance. but i think you then want to undo that change to the RGB since you still want the D50 coefficients in your RGB image.

rob

[Ignacio]

I typically do the 1:1:1 weighting to preserve linearity on luminance during deconvolution. Still, lightness and luminance look fairly different, and as I said, I find luminance more pleasing and less noisy (likely because lightness involves some nonlinear stretching).

Ignacio

[Phil Leigh]

oooh - very interesting... using CIE Y as a mask looks very useful! - thanks