I use the Jbullet physics engine, but I have some problems with it. As the parameter for stepSimulation I use 1/60. Gravity I have set on -9.81. The mass of my ball is 10. Now when I drop the ball with radius 10 from height: 40. The camera, which is 150 away, shows the dropping ball, but it takes 6 seconds before the ball hits the ground. I seems that it is shown in slow motion. Setting gravity to -100 resolves this, but of course I want to solve it correctly. Can anyone help me with this?
Because it’s a fixed rate step function, if you can’t call stepSimulation as fast as 1/60 then the simulation will run slower than real life and it will appear in slow motion. How frequently do you call stepSimulation?
Distance moved = .5 a t^2, so if you’ll permit me to approximate gravity as -10…
distance = .5 a t^2
40 = .5 (10) t^2
8 = t^2
=> t ~= 2.8
So the ball should take about 3 seconds to hit, or about 180 frames - can you check the frame count and see what the actual count is? The rest of the difference may be caused by frame rate issues, as lhkbob mentioned.
If I had to guess, you probably don’t want to use a ball that’s 10 meters in radius. That’s the size of a barn, and barn sized balls don’t tend to be what you want in games.
How exactly are you calling it? Do you pass delta time from last frame to first param? (J)Bullet automatically handles maintaining of fixed rate of 60Hz (by default, can be changed by third param).
The game engine we made for this makes sure the game runs at 60 frames per second. If we print out delta time it is around 1/60. We print our fps which differs between 58 and 62. Still the (gigantic) ball takes so long before touching the ground. We have other demos as well including chains(build of cylinders) and still we have this slowmo effect.