Workshop › Scorched Earth-style Artillery › Step 9 of 23

Flight

What we are doing

Every frame gravity adds a little to the downward speed, and the shell moves by its speed. That is all it takes for a real curve. fly moves a shell one frame and says what it hit: 'away' off the side, 'ground', or null. When the shell hits something it disappears, and you can aim again.

```js
const GRAVITY = 0.15

// One frame of flight: gravity pulls down. Returns what it hit, or null.
function fly(s) {
  s.vy += GRAVITY
  s.x += s.vx
  s.y += s.vy
  if (s.x < 0 || s.x >= W) return 'away'
  if (s.y >= groundAt(s.x)) return 'ground'
  return null
}

function update() {
  if (state !== 'flying') return
  const hit = fly(shell)
  if (!hit) return
  shell = null
  state = 'aiming'
}

  update()
```

What it means

Gravity changes the speed, the speed changes the position: this makes a parabola, like a thrown ball.
The shell is below the surface when its y is at least the ground's y there.
fly takes the shell as a parameter, so later the computer can try imaginary shells with it too.

Your turn

Under H, write GRAVITY.
Above aimBy, write fly with its comment and update.
In loop, call update() before draw().

Skills: Velocity and gravity, The game loop

← Fire! Boom →

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