Coding Math: Episode 13 - Friction

Показать описание

Continuing the basic physics for programmers series, this time we look at friction, two ways to accomplish it, one right, one wrong, and the pros and cons of each.

Рекомендации по теме

Комментарии

To get friction you could also make a vector that is the velocity normalized then multiply it with some value and subtract it from the velocity vector. vel -= normalized_vel*1.5f;

vertexbyte

You can optimize the first friction method slightly by comparing lengths without using Math.sqrt(). Effectively you are comparing length^2 rather than length, but the relationships >, =, and < remain equivalent.

Technomancr

Presumably method 2 is trickier to deal with if you are using delta times to make your game fram rate independent?

Arganoid

Interesting, thx. You've made an error though : if k is the friction factor and dt the step time then
Vx = Vx - k * Vx* dt and
Vy = Vy - k * Vy * dt
is the right velocity formula : no need for cos/sin/atan or to approximate the friction. Cheers !

TheVinchebleu

I think you can get correct friction with only one `sqrt` by doing this:
v' = max{ (|v|-f)/|v|, 0 } · v
where f is the magnitude of the friction vector.
Still not as efficient as the incorrect but good enough version, of course. Also you need to check that the current velocity is non-zero.

I'm not used to JavaScript, and mainly use Common Lisp, so sorry for the code that's probably less readable for most people, but here is a CL implementation either way (assuming usage of rtg-math):
```lisp
(defun friction (velocity friction)
(let ((v (v2:length velocity)))
(if (v2:0p v)
velocity
(v2:*s velocity (max (- v friction)
0.0))))
```

thomasbartscher