Gamma slider feels inverted

[chip2]

In Affinity Photo 2.3.0 in the Levels dialog, the "Gamma" slider feels inverted to me:

Turning it up (right direction) makes the image darker. Somehow I feel it more intuitive that increasing it should result in a brighter image. I'm sure there's some mathematical reasoning behind it, but it feels odd to me.

Does anyone else feel that it would be more logic if it were the other way around?

[NotMyFault]

It is simple math: gamma is used as exponent applied to the current lightness value. Higher gamma values will lead to lower output, because lightness is normalized to a range from zero to 1. So your gut feeling is wrong. It is kind of same wrong as wanting a larger share of a cake, and choosing 1/3 instead of 1/2. larger denominator results in lower share.

[smadell]

Although I understand the math, my “gut” understanding of the Gamma slider has always been to see it as the “grey point.” I visualize this as if I had created a black to white gradient map with a 50% grey value dead center. If I shift that central point to the right, the underlying image gets darker because more of the midtones are being mapped to darker grey values. Slide the midpoint to the left, and the image gets lighter, because the underlying luminance values in the midtones are being mapped to even lighter values.

I don’t know if the Gamma slider is precisely equivalent to a Grey Point slider mathematically. But, conceptually, it has always worked for me as a way to understand what’s going on.

[walt.farrell]

33 minutes ago, NotMyFault said:

It is simple math: gamma is used as exponent applied to the current lightness value. Higher gamma values will lead to lower output, because lightness is normalized to a range from zero to 1. So your gut feeling is wrong. It is kind of same wrong as wanting a larger share of a cake, and choosing 1/3 instead of 1/2. larger denominator results in lower share.

That makes sense. But why is white on the right of the Gamma slider, and black on the left? Wouldn't it make more sense if that layout was reversed?

[smadell]

Interesting question, Walt. But the same question could be asked about the Black and White sliders. After all, sliding the Black slider to the right doesn’t make the image whiter - it makes it blacker. And vice versa for the White slider. Obviously the gradients under the sliders correspond to the underlying image, and not the result. And..  at this point, changing it would throw the Affinity world into an absolute frenzy!

[walt.farrell]

Good point, @smadell.

[NotMyFault]

1 hour ago, walt.farrell said:

That makes sense. But why is white on the right of the Gamma slider, and black on the left? Wouldn't it make more sense if that layout was reversed?

If you want to use something like gamma to the purpose of brightening, and want higher numbers for more brightness then you need to use inverse gamma. Possible, but don’t count me in.
 

[walt.farrell]

15 minutes ago, NotMyFault said:

If you want to use something like gamma to the purpose of brightening, and want higher numbers for more brightness then you need to use inverse gamma. Possible, but don’t count me in.
 

It just seems to me that having a slider where sliding in the "bright" direction darkens the image, and slicing to the "dark" direction lightens the image, is backwards and would contribute to one's gut feeling that it's working inversely.

[smadell]

I think the problem is that we are conflating the "Gamma" slider with a "Brightness" slider, and it really isn't that (although admittedly that's what it "feels like.") Methinks it's just a matter of getting used to what the Gamma slider actually does, and coming to understand it on its own terms.

Can you imagine how many posted complaints there would be if the Gamma/InverseGamma/Brightness slider (whatever we would call it – you know, the third slider down) was reversed to work the way the OP describes? I can hear the complaints now - "how can I use this ridiculous program that doesn't work exactly the way Photoshop does?"

Also, why not just use the "Brightness and Contrast" adjustment if the Gamma slider in Levels is too much to bear?

[chip2]

I understand the math. This thread isn't about math.

Obviously, gamma isn't the same as "brightness" or "contrast", but it does make the image brighter or darker. And this is where I think it makes sense to have it behave in a way that resembles a brightness control: That increasing means "brighter" and decreasing means "darker".

I'm not talking what is correct or wrong, but what feels correct or wrong to normal non-technical people.

Edited December 2, 2023 by chip2

[smadell]

You're not wrong, @chip2. You're just swimming against a rather strong tide. I would dare say that every photo editing software out there that includes a Levels adjustment with a "Gamma" or "Grey Point" slider works in the anti-intuitive manner you're describing. There are literally decades of precedent for a Gamma/Grey Point/Midtones slider that works the way Affinity Photo presents it. Doing it a different way, no matter how intuitive it may feel to "non-technical people," will alienate a large crowd of editors with experience in Affinity Photo and a plethora of other applications.

It is said that "haters gotta hate," and switching up the Levels slider(s) would just be another aspect of AP for naysayers to point at and claim that Serif doesn't know what it's doing. Like I said, it's not that you're wrong – it's just that there are some fights that are not worth fighting.

[fde101]

All being said, the way that it responds vs. the way it is visualized are definitely in contrast.

If it is to work the way it does, it would make sense to reverse the shading, or failing that, to remove it and leave it as a plain ordinary slider.

Note also that there are Lift, Gamma and Gain controls in traditional color grading toolsets such as Resolve, and the Gamma control in Resolve does work the opposite of what is described here - moving it to the right makes things brighter, to the left darker - so there is precedent in other software for making it work more logically and less mathematically.  Most of the people using the software are more interested in the creative effect of the tool than in the math behind it.

[chip2]

Blender's gamma works the same way as Resolve btw. (left = darker, right = brighter):

So 1.0 is normal, 1.3 is "slightly pale" and 0.7 is "darker than usual".

So yes, I might be swimming against a strong tide (great way of putting it!), but sometimes I think it's healthy to revise one's own habits.  Especially if they're causing confusion for people who don't yet have any habits at all.

I'm also not here to dictate how things should be... I just wanted to bring up the topic in case it was not an intentional design.