Opacity vs Flow

[malle_yeno]

This is probably a stupid question, but what is the difference between opacity and flow? What’s the use case for making the distinction between the two?

For background:
The Krita manual says:

Opacity
The transparency of a stroke.
Flow
The transparency of separate dabs. Finally separated from Opacity in 2.9.

I’m a little confused as to how these are separate things – don’t you need to change the transparency of dabs to make a full stroke transparent? The manual has the following image to illustrate the difference at separate values, but I’m not sure that I fully understand it either.

[wojtryb]

10% flow means that you get 10% opacity if you paint a line (sort of, as those dabs - ‘brush stamps’ can overlap). But if you draw in the same place over and over again, without lifting up your pen, you can still get 100% opacity in one stroke.
Having 10% opacity means that your highest opacity you can get in one stroke is 10% - even if you draw over one place many times. You can lift your pen and make it more opaque in other stroke.
I’m no expert, but I heard that basically opacity is more advanced in terms of computing, as your computer needs to remember more info about the stroke. But it seems more intuitive, easier to use, and works closer to paint, while flow is more like a spray or airbrush I guess.
But if you need it in your brush, you can still use the flow changes, though for me opacity seems to be safe and default choice.

[tiar]

There is also a difference between “Wash” and “BuilUp” painting modes… and the fact that Flow was changes recently (you can switch to the old one in Configure Krita, but the new one is more useful)…

For the old one, I made this chart:

https://www.deviantart.com/tiarevlyn/art/T-Flow-vs-Opacity-vs-Painting-Mode-Tutorial-692265526

And the description below helps a bit, too.

On the other hand… now Flow works a bit differently (smoother), and I really need to update that graph to reflect the changes.

Still, I understand this way:

I’m not exactly 100% sure… especially with those painting modes coming into play. But from my experience it is something like that.

When I learn for sure - I’m Krita’s programmer for art’s sake, I should know that - and based on code, because wordsy explanation just aren’t enough in this case I believe, I will make a proper chart explaining all the math, but for now, I can only share my guesses…

(With those painting modes, I believe it is kinda similar but with with parts of the stroke instead of separate dabs: in Wash mode with lowered Opacity you won’t be able to paint over a stroke and get it more and more opaque, but with Build Up mode it is possible).

[raghukamath]

I know I am asking too much here, can you make that chart and add it to the manual.

[tiar]

Sure, but which one? And I’d need a confirmation from @halla, @dkazakov or @wolthera that it’s correct or alternatively actually try to understand it from the math/code point of view, because what I knew before was partially from the manual, partially from guessing from the results.

[dkazakov]

Hi, @tiar!

The chart looks nice and informative. The only addition I can make is that in BuildUp mode “Flow” is not used at all (unless you select AlphaDarken blending mode, which noone ever did, I guess).