025 : Jump

First define how much gravity there should be in this world:

script.js: Define a gravity variable.

function init_game() { ... // Global world parameters. _game.friction = 0.15; _game.gravity = 0.5; }

Run your code in a browser and verify that there are no errors.

We need to set the initial jump velocity and the max allowed velocity for the player

script.js: Define the initial jump velocity.

function init_player() { ... _player.velocity_x = 0; _player.velocity_x_delta = 0.8; _player.velocity_x_max = 3.5; _player.velocity_y = 0; _player.velocity_y_jump = -10; _player.velocity_y_max = 10; }

Run your code in a browser and verify that there are no errors.

We need to apply gravity to the player (just like we did with friction).

script.js: Update the player y-velocity to account for gravity.

function update_player() { ... // Apply the global world effects on the player. _player.velocity_x *= (1.0 - _game.friction); _player.velocity_y += _game.gravity; if (_player.velocity_y > _player.velocity_y_max) _player.velocity_y = _player.velocity_y_max; ... }

Run your code in a browser and verify that there are no errors.

and finally check for the jump key to initiate the jump. In this case, Up arrow, 'w' and spacebar:

script.js: Add support for the jump keys.

function check_input() { ... // Right arrow or 'd' to move right. if (_game.keymap[39] || _game.keymap[68]) { _player.velocity_x += _player.velocity_x_delta; if (_player.velocity_x > _player.velocity_x_max) { _player.velocity_x = _player.velocity_x_max; } } // Up arrow, 'w' and spacebar to jump. if (_game.keymap[38] || _game.keymap[87] || _game.keymap[32]) { _player.velocity_y = _player.velocity_y_jump; } }

Run your code in a browser and verify that there are no errors.

Go ahead and run your code now.

The player falls off the bottom of the screen!

That's because gravity is pulling the player down - that't what gravity does.

But it was supposed to stop once it hit the platform. How are we going to fix it?

GOTO 029