[AP] BW adjustment luminocity

[shojtsy]

Hi,

I use AP 1.5.1.54 on Win10/64 on RGBA/16 image. I am trying to use a technique I have seen in one of the tutorial videos (I forgot which): creating a BW adjustment layer, and setting it's blend mode to luminosity. This should allow me to adjust the luminosity of each color range separately. However I have noticed that the default BW adjustment layer, with all sliders on 100, have quite different luminosity than the original image. I have expected the above described layer to be of no visible effect, but it significantly brightens the image.

I am doing something wrong? Wouldn't it be a reasonable expectation for the default BW adjustment to have the same luminosity on all pixels as the original image?

[Bart_van_der_Wolf]

Hi,

I use AP 1.5.1.54 on Win10/64 on RGBA/16 image. I am trying to use a technique I have seen in one of the tutorial videos (I forgot which): creating a BW adjustment layer, and setting it's blend mode to luminosity. This should allow me to adjust the luminosity of each color range separately. However I have noticed that the default BW adjustment layer, with all sliders on 100, have quite different luminosity than the original image. I have expected the above described layer to be of no visible effect, but it significantly brightens the image.

I am doing something wrong? Wouldn't it be a reasonable expectation for the default BW adjustment to have the same luminosity on all pixels as the original image?

Hi,

 

A Black&White adjustment is not necessarily the same as Luminance, in fact, it rarely is, as you've found out. A B&W adjustment uses a certain proportional mix of RGB and CMY channels, presumably in the document's gamma (e.g. gamma 1/2.2). Luminance can be approximated by using a specific mix of R/G/B in linear gamma space.

 

So it depends on how Luminance is calculated from the R/G/B source data, and if you can approximate that with the B&W adjustment dialog (in isolation or together with a Gamma adjustment).

 

Cheers,

Bart

[shojtsy]

Thanks Bart,

 

Wouldn't such a BW adjustment be more useful/natural which defaults to the desaturated image, and applies changes relative to that? At least as an alternative setting? It would be more in line to how most of the adjustment layers work, the default setting making no change. Because the current one seems like a quite arbitrary mix of those channels with a result not resembling the original image.

 

Cheers

[Bart_van_der_Wolf]

Thanks Bart,

 

Wouldn't such a BW adjustment be more useful/natural which defaults to the desaturated image, and applies changes relative to that? At least as an alternative setting? It would be more in line to how most of the adjustment layers work, the default setting making no change. Because the current one seems like a quite arbitrary mix of those channels with a result not resembling the original image.

 

Cheers

Well, there are multiple formulas in use to calculate the R/G/B weights. I don't know which one AP uses.

One of them is : 

Y = 0.2126 R + 0.7152 G + 0.0722 B

So you could use those (21% Red, 72% Green, 7% Blue) as a starting point for the R/G/B ratios in the Black&White Adjustment tool, and save that as a Preset. It's not perfect (it doesn't adjust for gamma) but it does go in the right direction. I'd have to do a better analysis of the B&W tool to find better settings.

 

Using that preset makes it fast enough to set a personal starting point with one mouse click.

Do note that the ratios used like this are dependent on which color profile is used for the document when editing.

 

Cheers,

Bart