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docs-mini/README.Joystick
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@ -1,5 +1,5 @@
Users Guide to Joystick Usage Under FlightGear Flight Simulator Users Guide to Joystick Usage Under FlightGear Flight Simulator
First Draft version 0.2 9/23/2000
Author John Check <j4strngs@rockfish.net> Author John Check <j4strngs@rockfish.net>
This document is written with versions of FlightGear 0.7.5 and greater This document is written with versions of FlightGear 0.7.5 and greater
@ -9,6 +9,7 @@ nature of FGFS should ensure the information presented is useful on other
platforms. I'd like to say thanks to all the developers who make FGFS happen platforms. I'd like to say thanks to all the developers who make FGFS happen
and forgive me for not giving credit with regard to the property and forgive me for not giving credit with regard to the property
manager and js_demo. Corrections and additions are encouraged. manager and js_demo. Corrections and additions are encouraged.
The most current version can be found at http://rockfish.net/shell/aboutjoy.txt
Some History: Some History:
Earlier versions of FGFS had assignments of joystick axis/buttons Earlier versions of FGFS had assignments of joystick axis/buttons
@ -86,50 +87,50 @@ that can be passed to FGFS.
Axis properties Axis properties
dead-band dead-band
This is an area where signals are ignored. It is used to compensate This is an area where signals are ignored. It is used to compensate
for noise or potentiometers of dubious quality by creating a threshold for noise or potentiometers of dubious quality by creating a threshold
below which any signal is ignored. It it written as a decimal number or "float" below which any signal is ignored. It it written as a decimal number or "float"
with a typical value of 0.1 for elevators and ailerons, 0.0 for throttle with a typical value of 0.1 for elevators and ailerons, 0.0 for throttle
factor factor
This number, also written as a float, will control sensitivity of an axis. This number, also written as a float, will control sensitivity of an axis.
Negating the number will result in the control moving counter to the default. Negating the number will result in the control moving counter to the default.
A typical value is 1.0. In my case, throttle behaviour was inverted from what A typical value is 1.0. In my case, throttle behaviour was inverted from what
I preferred. I set this value to -1.0 and everything was groovy. I preferred. I set this value to -1.0 and everything was groovy.
offset offset
Also a float. Used to maximize a controls use of it's axis, as in the case of a Also a float. Used to maximize a controls use of it's axis, as in the case of a
throttle where zero would be a minimum and not a center point like in the case throttle where zero would be a minimum and not a center point like in the case
of a rudder. Typical value -1.0 (Am I close? Anybody?) of a rudder. Typical value -1.0.
Button properties Button properties
switch switch
A button designated a switch is either on or off. While the button is A button designated a switch is either on or off. While the button is
held in the switch is engaged. Brakes are described as a switch and take held in the switch is engaged. Brakes are described as a switch and take
additional parameters [ step, repeatable ] additional parameters [ step, repeatable ]
adjust adjust
A button designated adjust is for controls that have a range A button designated adjust is for controls that have a range
of settings, for example elevator trim and flaps. These are found of settings, for example elevator trim and flaps. These are found
in pairs having opposing values for the parameter 'step'. in pairs having opposing values for the parameter 'step'.
The 'repeatable' parameter should be appropriate to the type of control. The 'repeatable' parameter should be appropriate to the type of control.
step step
This defines how much adjustment is applied when the button is activated This defines how much adjustment is applied when the button is activated
Default values are 1.0 for brakes (full on), 0.001 / -0.001 for Default values are 1.0 for brakes (full on), 0.001 / -0.001 for
elevator trim and -0.34 / 0.34 for flaps elevator trim and -0.34 / 0.34 for flaps
repeatable repeatable
In this case repeatable means when the button is held down the value continues In this case repeatable means when the button is held down the value continues
to increment. repeatable is a true / false value. The default for brakes is to increment. repeatable is a true / false value. The default for brakes is
false. This is appropriate since by default brakes are a switch that are full false. This is appropriate since by default brakes are a switch that are full
on (right?) Elevator trim on the other hand defaults to true. Holding down the on. Elevator trim on the other hand defaults to true. Holding down the
button for elevator trim will cause a continuous adjustment until the button button for elevator trim will cause a continuous adjustment until the button
is released. Being a fine adjustment this is appropriate behaviour. Flaps is released. Being a fine adjustment this is appropriate behaviour. Flaps
on the otherhand default to false. Clicking the flaps button will cause the on the otherhand default to false. Clicking the flaps button will cause the
@ -202,5 +203,42 @@ In my case I had to make entries to put the throttle on axis2.
--prop:/input/js0/button6/step=0.34 --prop:/input/js0/button6/step=0.34
--prop:/input/js0/button6/repeatable=false --prop:/input/js0/button6/repeatable=false
Determining approriate values for axes:
FlightGear uses the PLIB library to handle the joystick input.
PLIB applies the values for deadband then hands the result over to FGFS,
where offset and factor are applied, the result is passed to the FG control
property.
It's important to understand how the dead-band, offset and factor properties
work together for axes in order to determine apropriate values.
The full order of precedence for axis properties is
1. The raw PLIB axis value ...
2. is adjusted to dead-band, then passed to FGFS, which ...
3. applies the offset, then the result...
4. is multiplied by factor, which ...
5. is assigned to the FlightGear control property.
Put another way....
PLIB lightly_toasted_value = ( raw_value > dead-band )
FGFS cooked_value = ( lightly_toasted_value + offset) * factor
Well, kind of. The dead-band value is applied +/- 0. Which is to say
when we assign it a value of 0.1 the effective value is -0.1 to 0.1
If the raw value is less than the dead-band then the raw_value is 0.
Here's a visual aid.
-1 0 1
.......................
-1 | | 1
^
deadband
This diagram represents a dead-band value of 0.1. Let's say we
have an el cheapo joystick with noisy pots. The noise level is +/-0.09.
With the raw signal our control would constantly be moving slightly.
A deadband value of 0.1 acts as a filter suppressing the noise.