World
Each instance of p5play has its own world object, that can be used to control the
Box2D physics simulation. Its most important property is gravity, which has x and y
components.
Note that the physics simulation is deterministic. That means if you run the same code twice, unless you're using random values, you'll get the same result!
Sleeping
world.allowSleeping is true by default.
A sprite starts "sleeping" when it stops moving and doesn't collide with anything new. "Sleeping" sprites get ignored during physics simulation, which usually prevents the Box2D physics engine solver from having to make unnecessary calculations. While this is good for performance, sometimes it can cause problems.
You can wake up a sleeping sprite by setting sprite.sleeping to false. You can also
disable sleeping on a per sprite basis by setting sprite.allowSleeping to false.
Controlling Time
The world.timeScale ratio is set to 1 by default for real time physics simulation.
Set it lower for slow motion. Make it 0 to pause time!
Note that the Box2D physics engine solver is only stable up to a time scale of 2. If you want to
advance the simulation by a larger amount of time, run the world.physicsUpdate
function multiple times.
Click the canvas in the example to make the physics simulation progress in slow motion, 1/4th real time. The Matrix bullet time effect is achieved by making the background of each frame slightly transparent.
world.realTime stores how many real time seconds have elapsed since the start of the
world, including pauses.
world.physicsTime stores how many seconds have elapsed in the physics simulation.
Performance Testing
Set p5play.renderStats to true to display the number of sprites drawn, display rate, and
FPS calculations. For more comprehensive results, use your web browser's performance testing tools.
FPS in this context refers to how many frames per second your computer can generate, not including the delay between when frames are actually shown on the screen. The higher the FPS, the better your game is performing.
Having less sprites and using a smaller canvas will make your game perform better. For better
performance, don't use the q5.js clear function or sample the colors of pixels in your
canvas using canvas.get.
By default the Box2D physics engine performs 8 velocity iterations
world.velocityIterations and 3 position iterations
world.positionIterations. Decreasing these values will make the simulation faster but
also less accurate.
I've tested p5play in every web browser and found that Google Chrome performs the best.
Find Sprites
You can get an array of sprites found at a point with the world.getSpritesAt function.
Note that the sprites must have physics bodies to be found.
world.getSpriteAt returns the first sprite found at a point, the one with the highest
layer value.
Try moving the mouse to make sprites stop moving in the example!
Ray Casting
The world.rayCastAll function finds all the sprites that intersects a ray (line),
excluding any sprites that intersect with the starting point.
The world.rayCast function is similar, but only returns the first sprite.
Provide these functions with the ray's start and end points.
Alternatively, set the ray's starting point, direction, and optionally the maximum distance it should travel.
Try moving the mouse in the example, when the ray intersects with a sprite, it becomes orange.