Currently, the brush engine provides a variety of sensors which can affect the brush engine in realtime. eg: pen tilt, rotation, tilt direction, pressure, etc. As well as a variety of calculated "sensors" such as the speed, duration or distance of the brush stroke, stroke direction, randomness, etc.

These sensors can be tied to various properties of the brush engine (size, opacity, flow, brush angle, etc.), and will affect the brush output in real time.

Since there the number of "real" sensors is currently more or less limited to the number of sensors provided by your graphics tablet, the ways in which you can manipulate the brush strokes in real time is pretty limited, and you often wind up having to control more than one brush parameter with one sensor input (eg: pressure being used to control both size and opacity, etc.). As a natural result of the limited number of "real" sensors, the number of brush engine properties that can be controlled by sensors is also limited (because there are only so many sensors to assign, it would be of limited value to add many more properties that could be controlled - the core properties like brush size and opacity tie up pressure inputs, etc... So from a UX perspective, adding more controllable parameters would not be all that useful without more sensor inputs to assign them to).

It would be useful to add some extra sensors to Krita's brush engine. This would allow the brushes to be more interactive, more sensors would mean that you could assign one sensor to one property rather than having to have one sensor control several properties, and extra sensor inputs would also allow Krita to develop in a direction where more brush properties could eventually be made to be controllable in realtime.

Another advantage of these extra sensors would be that users wouldn't necessarily have to buy expensive specialty styluses (like the wacom art pen or airbrush) in order to get a bit of extra control over their brushes - extensible realtime control would be available to anyone, even if their tablet doesn't provide extra sensors like pen tilt, rotation, tangential pressure, etc. Even mouse users would have access to extra sensor input.

TOUCHSTRIP SENSORS
Initially, I think it would be useful to add sensors that could be tied to the touchstrips on intuos tablets. For example, you could control brush rotation with the circular touchstrip on an intuos4, or you could control saturation in realtime with an intuos3 touchstrip.

VIRTUAL SENSORS
Adding a few "virtual sensors" to krita would also be useful. These would be variables in Krita that would store a value that could be incremented or decremented using keyboard shortcuts (configurable (much like opacity, etc.) in the "configure shortcuts" menu). This would be controller-independent, so any user could access these sensors through the existing Krita keyboard shortcut system. The important difference being that, since these would be brush engine sensors, the user could bind the virtual sensor to whatever brush property they choose, and it would affect the brush stroke in real time (unlike the current opacity, lighten, etc. shortcuts, which affect the next brush stroke, but don't affect the current stroke in real time.)

MIDI SENSORS
Eventually, adding the ability to connect a MIDI device to Krita would be the most useful aspect of the new sensors - MIDI control has been suggested before, but I haven't seen it suggested in the sensor context. Imagine being able to connect a Korg Nanokontrol (inexpensive midi controller with 8 faders and 8 potentiometers - http://www.korg.com/us/products/control ... okontrol2/ ) to 16 realtime brush sensors - one fader could control opacity, another could control saturation, etc... all in realtime. Or you could use a rotary encoder on a MIDI controller to control brush rotation... etc. (With some tweaks to the brush system, you could even have faders controlling RGB values independently...) With one hand on the MIDI controller, and one hand for the drawing tablet stylus, users could have a huge amount of control over brush parameters as they paint.


(Not related to sensors, but if MIDI controllers become usable in Krita, it would also be great to be able to configure MIDI buttons to trigger shortcuts in the "configure shortcuts" menu.)


OTHER IDEAS
Maybe a mousewheel sensor would be useful too, for mouse users...

Or setting up an extra touchpad as an x-y sensor controller for krita...

Or tying touchscreen input to Krita sensors in various ways...

was thinking about this for Midi/Osc controller http://hexler.net/software/touchosc then use a smartphone/tablet to control brush properties or shortcuts.
But a midi/osc input device plugin would be better as a calligra plugin instead of krita so all applications could benefit from it.

Registered just to throw my vote in for this. I got so excited when I saw that I could bind value and saturation to keys, thinking I could map those to my tablet touch strips and control those parameters mid-stroke, but the brush won't update until it's lifted. Surprising, every program I've used at least allows you to do this with brush size.

Here's a guy demonstrating the effectiveness of MIDI interfaces for sensory input. People want this!