fix air friction calculation

[xi]

The code is slghtliy confusing because velocityPrevX and velocityPrevY actually define a distance. The real velocity can be calculated as velocityPrevX / deltaTimePrev and velocityPrev / deltaTimePrev where deltaTimePrev = deltaTimeScaled / correction.

This is the formular without air friction:

velocity.x = velocityPrevX * correction + body.force.x / body.mass * deltaTimeSquared

Air friction is a force that acts in the opposite direction of the velocity. It is proportional to velocity (for small velocity):

var force = body.force.x - (body.frictionAir * velocityPrevX / deltaTimeScaled * correction);
velocity.x = velocityPrevX * correction + force / body.mass * deltaTimeSquared;

which can be converted to:

var frictionAir = 1 - body.frictionAir * deltaTimeScaled / body.mass;
velocity.x = velocityPrevX * frictionAir * correction + body.force.x / body.mass * deltaTimeSquared;

[getkey]

Interesting, could this be the fix for #148?

[liabru]

Thanks for raising this and sorry it's taken a while. I've been working on a wider scale update that touches on this and other related issues to do with timing and frame-rate, please see #777.

If you have any time, it would be great if you could try it and see if it fixes the problem raised here at least, thanks again.

[xi]

As far as I can see, #777 only refactors the code but does not address the faulty equations as described in the pull request.

I will still try the refactoring sometime this week and give feedback.