When to apply vignetting? How to achive a correct perceptual vignetting?

@anon41087856 has stated in his great article about migration from color lab to linear RGB that vignettes should be applied while still in linear space.

If you apply it after the base curve/filmic curve as the vignette module does you can create nos smooth transitions an bandings.

OK, I can understand that and agree.

I usually apply vignetting manually drawing an ellipse as mask applied over a darkening layer (in darktable or other programs) and if I nee it, use a parametric mask to protect deep shadows and brighter lights from darkening.

In darktable I can do it using the exposure module (that works in the linear space) or the contrast/brightness/saturation (moving it in the pipeline to a position before the base curve/filmic in the pipeline).
But the problem is that results are different with no smooth and uniform darkening (perceptually).

The problem is not the darkening itself, the problem is the distribution of the gray tones in the mask used for vignetting.
The gradient generated by the mask is linear, but a linear mask does not produce a perceptually uniform darkening.

If we are going to use vignetting in the linear pipeline we need a logarithmic distribution of grays in mask in order to get a perceptually constant darkening.

But as long as I know you cannot change the way the mask is smooth, it is always linear.

So how are we supposed to get proper vignetting? Is it a work in progress?
In that case, until there is an option to get non linear distribution of shadows in masks, we have no option but applying it after the base curve when the image is more similar to a perceptual image.

Image with a vignette applied using brightness after base curve.
You can see a perceptually constant darkening, as the luminosity has been converted to by the base curve to a value similar to what we perceive as lightness.

Same image with vignette applied in linear workspace, before the base curve.
The gradient is to harsh, it is not a smooth and uniform transition.

I use the exposure module for vignetting

I would argue that there is no “correct” vignetting, as real lenses have all sorts of different brightness falloff curves.

The restults are similar with the exposure module.

The problem is not how you darken the image, but how the darkening effect is distributed over the mask.
If you use a linear gradient in a linear space you do not get a perceptual uniform distribution of tones.
To get that in a linear space you need a logarithmic gradient.

Exposures darkens all tones equally, bright uses a differente curve that I usually prefer (I have not tested yet how it exactly work in darktable).
But the problem is not that.

I don’t care about how the lens fallsoff, that is not the point.
I am not trying to replicate the lens behaviour.

I am just trying to get an effect that results pleasant for me, to get a darkening with a gradient that is perceptually uniform.

Others may prefer the linear gradient.
It is good to have choice, but you cannot choice now.

The vignetting module produces that perceptually uniform gradient (just because is applied after base curve/filmic).
You can get it appliying the darkening (using exposure or brightness) after the base curve.

But that can lead to the posterizing problems that @anon41087856 comments in his article.

So having differente options to distribute the gradients in the mask would be great.

I use the vignette module with a low opacity, ~12%, with the blend mode on lightness.

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You can use the color balance module to have good control over the perceptive appearance of the vignette:

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Thank you, @s7habo

I like the results I can get using the color balance and applying a midtones lighness reduction and some contrast.

What seems strange to me for a vignette is making contrast so high. Vignetting effect of a lens should be reducing contrast not giving more contrast, am I wrong?

But the results look pleasant to me.

I don’t like color balance for general use because it has too many things to control (adjust relative light in midtones, shadows and lights). It is great when what you want is that, adjust relative relations between them or give some dominance (or get rid or it) in shadows or lights.

You should get a similar result with a simple contras/brightness/saturation or a curve applied using lightness and the same mask.

But it does not seem to be the case. Is it not due to being the transitions in the mask linear instead of logaritmic?

No. Vignetting is a complex problem and depending on how it occurs, the result will change:

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Thank you.

I am not indeed interested in reproduce the effects of the lens.

And I don’t like usually notorious vignetting .

I use it as a subtle way to center the attention in the subject and create a bit mor volume and 3D effects.

I try to get a natural and subtle effect in a perceptual way.

Thas is why I had thought that making it int a perceptual space is the better way (you may have problems with posterization), and not in the linear space or better: if you make it in the linear space not with a linear gradation of the mask.

I suppose what you find pleasant is the natural vignetting that occurs when the main subject is slightly illuminated with diffuse light and only stands out subtly from the surroundings. In my opinion, the vignette looks most pleasant when this effect can be imitated.

In this case, I would like to emphasize the riders.

Before:

But since we know that we perceive the contrasts much more strongly in darker than in light areas, correspondingly darker areas must be richer in contrast.

After:

It is even better if the vignette is not round, but adapted to the subject. In this case, the meadow seems to be more luminous than the rest, as if there were a hole in difuse clouds ( I’ve taken it to extremes here to make the effect clear):

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Color Balance Module does indeed look very overloaded, but it offers a lot of processing possibilities immediately and is therefore - at least for my needs - completely sufficient to get the desired results quickly. I will even highly recommend getting to know the module very well, as it saves a lot of subsequent work.

For me it is - in a single instance - a replacement for base curve, contrast and brightness, color corrections etc.

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Thank you.
You are right, that is the kind of vignette I use to do.

Even in the strong case it is not disruptive or unnatural.

I like the results of your way of vignetting using color balance, i will keep using it.

Color balance is a great tool to equilibrate shadows/lights/midtones or eliminate (or produce) dominants in light or shadows in an easy and quick way (whithout using masks and curves)

But it is a bit bloated for simple things like altering general light or this kind of vignetting.

But on the other hand I usually introduce a soft masking in my vignets to protect deep shadows and strong lights (strong lights are not darken as strongly in a lens).
So may be darkening just midtones and not light or shadows (or doing less) will save me from using the parametric mask.

May be color balance would be easier to use, less bloated separating the adjustments in tree tabs (much like filmic in its last incarantion) and it would be great to have a general tab to adjust the whole image, not just midtones/shadows/lights.

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Maybe an interesting fact: Color Balance Module, which has been changed recently, is based on the Color Decision List (ASC CDL) a process used in the film industry.

These three controls are not identical in their function, but (according to their respective function) have the greatest influence on different areas of the image. Here are the descriptions:

So, they are not three identical functions applied to shadows, mid tones and highlights. Accordingly, if you know the functions, you can combine them to get a wide range of results. But, I admit that

  • it’s not obvious what they do and
  • documentation of the module in darktable user manual is very sparse

Here is an example of what this module typically looks like in video editing software:

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