Pymunk body-specific damping

Question

How to use pymunk.Body.update_velocity(body, gravity, damping, dt) to dampen the mass. I know I can use global damping here but I want to add more masses later on and have custom damping for each one, and so I want to learn how to deploy body-specific damping. I have the following questions

1. How to use pymunk.Body.update_velocity
2. Can I leave gravity blank if I don't want custom gravity. Or should I just write space.gravity
3. What is dt here and how do I determine that

Answer 1

Did you find the example in the documentation here: http://www.pymunk.org/en/latest/pymunk.html#pymunk.Body.velocity_func ?

1. You need to define a custom velocity function somewhere, and then set it on the body you want it. So preferably you define it before you create the bodies. In this function you can call the existing default velocity function (pymunk.Body.update_velocity) if you want, but that is not a requirement. It depends on if you want to write all code to update velocity of the body yourself, or if you want to just modify it a bit with the existing code as a base. Since you only want to modify the damping I think its easiest to call the default function.

2. When you call the default v function you can send in the same gravity unmodified as you get it in your custom function.

3. dt is the delta time. I see now the docs could be improved here, I will make a note. So the dt is the same value as the dt sent to space.step(dt) function in your simulation loop.

All in all, I think something like this would do (you just need to adjust the calculation of modified_damping to the logic you want):

def custom_damping(body, gravity, damping, dt):
    modified_damping = body.custom_damping * damping
    pymunk.Body.update_velocity(body, gravity, modified_damping, dt)

body.custom_damping = 0.31415
body.velocity_func = custom_damping

(if you dont want to set a variable on the body you can of course do it in some other way. Lets say you want all bodies to have 2x the damping compared to normal you could do modified_damping = 2 * damping instead, and remove the body.custom_damping property.