HTML5 Games Workshop: Make a Platformer Game with JavaScript

Re:HTML5 Games Workshop: Make a Platformer Game with JavaScript by buzzybee on Wednesday April 05, 2017 @06:55PM (#14046448)

The reason why you don't see an issue while using Box2D is because its integration methods are low-error. A simple platform game using Euler style integration of the type

   each step, add forces from input and gravity. add the resulting acceleration multiplied by time to my position.

will trivially produce jump heights of 50% variation when subjected to a variable timestep. On a quick search, Box2D uses semi-implicit Euler [0], which much more accurately fits the curve. It does not guarantee zero error: to do that you have to have an analytic method, which isn't applicable to a general-purpose physics simulation.

[0] https://en.wikipedia.org/wiki/Semi-implicit_Euler_method

edit: i did forward euler wrong

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Re:HTML5 Games Workshop: Make a Platformer Game with JavaScript by wareya on Wednesday April 05, 2017 @07:00PM (#14046482)

I covered integration error in my first post here, point 3. The thing I was calling Box2D out for is the fact that it doesn't use a hacky way of rectifying collisions. It seeks out the point in time that they occur.

>It does not guarantee zero error: to do that you have to have an analytic method, which isn't applicable to a general-purpose physics simulation.

If you have constant acceleration, the analytic way to get the point you want to be at at the end of the frame is simple: just pretend your current frame is using half the added speed from the acceleration that you're going to undergo this frame. Or you could use a hermite curve or something.

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Re:HTML5 Games Workshop: Make a Platformer Game with JavaScript by buzzybee on Wednesday April 05, 2017 @07:23PM (#14046608)

Those are the integration methods I was talking about. An analytic method without error is one which can describe the curve at any moment in time, not "constant acceleration per frame".

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