I’ll be explaining what really differentiates these 3 programs from each other and what their strengths are. Firstly to cover some grounds before diving into the programs.
The First thing people people will try to learn with drawing/painting programs are 2 main brush engines that your workflow needs to consist of. One system is a very direct approach to drawing/painting. It allows you to control 2 main properties and they go by a few names, namely Opacity/Transparency and Flow/Density/Strength. The other system Controls other variables other than just Opacity and Flow.
*note that Sai’s default brush does not support Opacity, and for awhile CSP did’t support it either until a few patches ago.
The first system is quite easy to understand since it only affects the Brush.
Opacity Controls only the transparency of the brush. It is important to note that it does not affect each individual dot of the brush only the brush itself as if it was a filter.
Some key things about opacity
Sharp edges regardless of brush shape and amount
Allows for Glazing which only affects color and value, not the edges
Flow controls each individual dot of the brush unlike opacity. Since it controls each dot There is no transparent quality with this property, it is simply meant to control how strong or weak the brush actually is, while opacity does not. A lot of people make the mistake of drawing and painting with this property alone, especially with Sai and CSP. Usually this property works a whole lot better with Opacity since Flow allows for a better transition from the lower and higher ends of the pen pressure curve.
Some key things
Not transparent, completely opaque property
Smoother transitions with other properties
Now these 2 properties also have a general control that sets a threshold(first 3 pics). With opacity at 80% you cannot exceed the overall opacity of the brush in one stoke. With Flow you cannot paint stronger than the set threshold. Generally you do not touch opacity unless you want to deliberately overlap your strokes which in some cases you will want to since a set threshold will give you a more solid color at a lower percentage, In contrast to just painting it with pen pressure. Flow will be the only one you will lower just in case you want a smoother transition with opacity, whether you paint with opacity and flow or just opacity alone. Note if you do lower flow below 50% you will start to see it affect each dot.
This is just the general premise of the first system. The second system is a bit more complex but it allows us to paint in a way that we simply just cant with the first system.
The Second System involves 2 main properties that affect our brush greatly. The wetness and Mix values. It is a bit harder to understand because it does not only affect the brush but also surrounding colors. Other programs make it increasingly difficult since they require layers to dilute the paint, however dilution in digital and with these progarms(namely Sai, and CSP) is entirely different. Dilution with these programs act as opacity, while dilution in analog affects the consistency and how well it will mix painting wet on wet. With analog you would have to buy specific colors with already transparent qualities or heavily dilute the paint to act as a transparent color to change the underlying colors.
Wetness is a pretty straight forward property, it just controls how much paint we push.
Mix however is something that will confuse everyone no matter what. Especially with Sai and CSP users. In general Mix takes two variables, the Canvas color and your Brush color and allows you to blend these colors accordingly. Usually a lower mix value will paint mostly from your brush color, A higher mix value will paint mostly from the Canvas color. Somewhere in between you will get a color halfway, which simulates real paint. With real paint you dilute pigments and this dilution will take the underlying color and the diluted pigment and create a new color.
This is just a basic breakdown, all three programs have their nuances of how they tackle these basic properties and ill try my best to cover them.