Luminosity Blend Mode - Help!

[smadell]

I feel like I should get this by now. But it's confusing me, and I can't help but think I'm not alone.

If I put a "Blend Layer" on top of a "Base Layer," my understanding of the Luminosity Blend Mode is that the Hue and Saturation of the base layer are unchanged, but the Luminosity of the blend layer is used. This is what I've always assumed. (Is this incorrect?)

Here is a "base layer" composed of two colors - one red and one green. They are different Hues, but both are 90% Saturated and 50% Luminant. The pointers on the Info panel are placed to indicate the values.

Here I have added a "blend layer" composed of an area of grey. It has 30% Luminance. Again, check the Info panel readout.

If I change the blend mode of the top layer, everything goes screwy. Or, at least, I think it's going screwy. My expectation was that both Info pointers would show me unchanged values for Hue, 90%
Saturation, and 30% Luminance. But, as the picture shows, while the Hue stays the same both the Saturation and the Luminance values have changed, and neither of them is as expected.

Are my expectations out of line, or is this something wrong? Am I misinterpreting what this Blend Mode actually does?

[R C-R]

The Wikipedia article for HSL and HSV, particularly the section about lightness, may help you understand why this is confusing ... but it probably won't do much to reduce the confusion. :(

 

The article is not the easiest to read, but in a nutshell, lightness/brightness/intensity/luminosity are related but do not necessarily represent the same thing. Also of some relevance, note this comment at the end of the Use in end-user software section:

Quote

Since version 4.0, Adobe Photoshop’s "Luminosity", "Hue", "Saturation", and "Color" blend modes composite layers using a luma/chroma/hue color geometry. These have been copied widely, but several imitators use the HSL (e.g. PhotoImpact, Paint Shop Pro) or HSV (e.g. GIMP) geometries instead.

 

[smadell]

Thank you for your input, R C-R...

One of the most confusing parts of photo editing has always been color management. Just when I thought I was even close to understanding some of it, other considerations made it even more obtuse. You were certainly correct in your observation that the Wikipedia article helps me understand why it's confusing, but certainly doesn't lessen the confusion (possibly, just the opposite!)

If I read the section on "end user software" correctly, the Luminosity (and other) blend modes don't work precisely the way they are said to work, and cannot be expected to give the admittedly more simplistic results that I thought I would get. If that's the case, though, how can I ever predict what those blend modes will produce? Is it all just trial and error?

And I thought this was going to be easy...

[R C-R]

For me at least, it is partially trial & error & partially application of what I know about the mathematical geometry of the color spaces & their transformations, tempered by my incomplete understanding of how the latter relates to human visual perception. IOW, I have some idea of how to get sort of close to what I want, but I know I am going to have to experiment to get closer to that & I may not ever get the exact results I want.

[anon1]

Looked at Wikipedia as well but still seems somehow weird to me 

[R C-R]

52 minutes ago, MBd said:

Looked at Wikipedia as well but still seems somehow weird to me 

I think it probably seems at least a little weird to everyone. A big part of it is human visual perception does't match very well the still relatively simple mathematical models we use to describe it. As it is, it is too easy to overlook things like that typical color gamut charts are just 2D cross sections of a 3D gamut object that exists in some artificially contrived color space.

[anon1]

just had a second thought 

 

think I know what goes wrong now 

 

change your document to LAB and monitor your info panel in LAB mode as well 

LAB is just a beautiful color space 

 

the issue is that affinity does not calculate in HSL such that the Luminance blend mode color model is not HSL and thus it does not match up

 

if you setup everything in LAB everything is beautiful 

 

more info on LAB

 

cheers 

[jenkm]

The HSL in the Info panel (Color panel, HSL Adjustment) and the "HSL" blend modes use different color models. The "Luminosity" for blend mode it’s just a name, actually it's a kind of luma:

Lum = 0.3R + 0.59G + 0.11B

The values of the Saturation are also different, despite the same names. So you can't check the HSL blending modes focusing on the Info panel.