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|Fixed Timestep||A framerate-independent interval that dictates when physics calculations and FixedUpdate() events are performed.|
|Maximum Allowed Timestep||A framerate-independent interval that caps the worst case scenario when frame-rate is low. Physics calculations and FixedUpdate() events will not be performed for longer time than specified.|
|Time Scale||The speed at which time progress. Change this value to simulate bullet-time effects. A value of 1 means real-time. A value of .5 means half speed; a value of 2 is double speed.|
Fixed time stepping is very important for stable physics simulation. Not all computers are made equal, and different hardware configurations will run Unity games with varying performance. Therefore, physics must be calculated independently of the game’s frame rate. Physics calculations like collision detection and Rigidbody movement are performed in discrete fixed time steps that are not dependent on frame rate. This makes the simulation more consistent across different computers or when changes in the frame rate occur. For example, the frame rate can drop due to an appearance of many game onscreen, or because the user launched another application in the background.
Here’s how the fixed time step is calculated. Before every frame is drawn onscreen, Unity advances the fixed time by fixed delta time and performs physics calculations until it reaches the current time. This directly correlates to the Fixed Timestep property. The smaller the value of Fixed Timestep, the more frequently physics will be calculated. The number of Fixed frames per second can be calculated by dividing 1 by Fixed Timestep. Therefore, 1 / 0.02 = 50 fixed frames per second and 1 / 0.05 = 20 fixed frames per second.
Simply put, a smaller fixed update value leads to more accurate physics simulation but is heavier on the CPU.
Fixed time stepping ensures stable physics simulation. However it can cause negative impact on performance if game is heavy on physics and is already running slow or occasionally dips to low frame rate. Longer the frame takes to process - more fixed update steps will have to be executed for the next frame. This results in performance degradation. To prevent such scenario Unity iOS introduced Maximum Allowed Timestep which ensures that physics calculations will not run longer than specified threshold.
If frame takes longer to process than time specified in Maximum Allowed Timestep, then physics will “pretend” that frame took only Maximum Allowed Timestep seconds. In other words if frame rate drops below some threshold, then rigid bodies will slow down a bit allowing CPU to catch up.
Maximum Allowed Timestep affects both physics calculation and FixedUpdate() events.
Maximum Allowed Timestep is specified in seconds as Fixed Timestep. Therefore setting 0.1 will make physics and FixedUpdate() events to slow down, if frame rate dips below 1 / 0.1 = 10 frames per second.