UFO Game

We'll be creating a little demo that sees a UFO move around the screen using the keyboard. Create a new folder, then add the following code to a new main.lua file:

function spry.conf(t)
  t.window_title = 'UFO Demo'
end

kenney.nl provides free (public domain) game assets. One of the asset packs includes a green UFO. Add it to the game folder, and load it using spry.image_load.

function spry.start()
  ufo = spry.image_load 'green-ufo.png'
end

Player Class

We'll create a new file called player.lua with the following contents:

class 'Player'

function Player:new()
  self.x = spry.window_width() / 2
  self.y = spry.window_height() / 2
end

function Player:draw()
  ufo:draw(self.x, self.y)
end

Inside spry.start, We'll create a new player back in the main.lua file:

function spry.start()
  ufo = spry.image_load 'green-ufo.png'
  player = Player()
end

Player() calls the Player:new method. The player's x and y position is one half of the window's width and height. In other words, we're setting the player's position to the center of the screen.

Player:draw uses the ufo image that we load during program startup in spry.start. The image is drawn with the player's x and y position.

Let's create spry.frame in main.lua. In this function, we'll draw the player:

function spry.frame(dt)
  player:draw()
end

You do not need to include the newly created file inside main.lua. All Lua scripts in your project folder are loaded on startup. Once you run your game, you should see a ufo on the screen:

spry.exe my_folder

Keyboard Input

We'll use keyboard input to move the player. Create a new Player:update method:

function Player:update(dt)
  local vx, vy = 0, 0

  if spry.key_down 'w' then vy = vy - 1 end
  if spry.key_down 's' then vy = vy + 1 end
  if spry.key_down 'a' then vx = vx - 1 end
  if spry.key_down 'd' then vx = vx + 1 end
end

vx and vy stands for velocity x, and velocity y. These variables represent the player's horizonal and vertical speed. When the user presses any of the WASD keys, the velocity changes appropriately. In the case of the D key, vx is set to itself, plus 1, representing movement towards the right side of the screen.

Now we'll move the player by setting the x and y position. Put the following code inside Player:update, just before the function ends:

local move_speed = 200
self.x = self.x + vx * move_speed * dt
self.y = self.y + vy * move_speed * dt

Whenever we move something, we need to make sure the movement is framerate independent. This is done by multiplying movement speed by delta time dt (the time spend between this frame and last frame). We'll pass dt to the player's update method through spry.update:

function spry.frame(dt)
  player:update(dt)
  player:draw()
end

Run the game again and try moving the UFO using the WASD keys.

Image Origin

When drawing the UFO, the image origin starts at the top-left. If you want the player to be exactly centered, change Player:draw:

function Player:draw()
  local rotation = 0

  -- scale
  local sx = 1
  local sy = 1

  -- image origin
  local ox = ufo:width() / 2
  local oy = ufo:height() / 2

  ufo:draw(self.x, self.y, rotation, sx, sy, ox, oy)
end

Here, we're adding additional info to ufo:draw. To set the image origin to the center of the image, get the image dimensions with ufo:width and ufo:height, and divide by 2. In order to set the image origin, we also need to explicitly set the image's rotation, horizontal scale, and vertical scale first.

Movement Fix

You might notice that moving diagonally is a little faster compared to vertical or horizonal movement. This is because vertical and horizontal movement has a speed of 200, but using both vertical and horizontal speeds together results in a speed of around 282 (square root of 200² + 200²). We need to normalize vx and vy so that the combined speed of both vx and vy is always 1.

After setting keyboard input, but before moving the player, add the following line:

vx, vy = normalize(vx, vy)

Run the game again, and you'll see that the player moves at the same speed regardless of direction.