[GSoC proposal] Airbrush and Calligraphy paintops

[Fela Winkelmolen]

I'm posting here a draft of my Google Summer of Code proposal, in case anybody 
has comments or suggestions

=========== begin ================
Implement real-media airbrush and calligraphy simulation paint operations in 
Krita.

== Expected Results ==
Real-media airbrush simulation support:
* usable with a mouse and with graphics tablets both using a graphics pen and 
a airbrush
* the engine will work with the following parameters: rate, tilt (only when 
the input device supports it), distance, size (of the area painted on the 
canvas)

Real-media western calligraphy simulation support:
* usable with both mouse and graphic tablets
* the engine will work with the following parameters: width, thinning 
(depending on the speed of the stroke, and optionally on the pressure of the 
input device), angle, fixation, mass & wiggle (determining how the 
tool "feels")


The user interface of both tools should be polished and usable. When using an 
input device that don't support a parameters directly (e.g. rate of the 
airbrush when using a mouse) there needs to be an easy way to change that 
parameter while drawing, for example using the keyboard and/or the mouse 
wheel.
============ end ===============


There are airbrushes that separate the function for air and paint flow (dual 
action airbrushes), but I'm not sure that makes sense in the digital version. 
Do wacom airbrushes support that?

I was also thinking about another parameter for the airbrush. My initial idea 
was to not simulate the single particles, mostly like the current airbrush 
looks like, but thinking more about it maybe we don't always want 
this "infinite granularity", but we want the particles to be actually seen. 
So the granularity might be configurable. Actually there may be the need of 
two parameters: size of the particles and opacity.
When the particle size is 0 it will use something like the fuzzy circle, the 
opacity in that case will have the same effect as the rate. If for example 
the opacity is 100% and the particle size is one pixel if would look a bit 
like the airbrush of MS paint (if I remember correctly).

Another parameter might be how "blurred" the stamp is around the border, or, 
in the case of particles, how concentrated the particles are at the center 
compared to the border.

Also the calligraphy tool may need to be a bit more sofisticated. The way it's 
described above it behaves mostly like the Inkscape calligraphy tool.