I try to improve in lua. Therefore i am looking at some sourcecode and try to understand, how it works.

Right now I don't understand the sourcecode of:

http://hump.readthedocs.io/en/latest/camera.html

at the beginning of the code it has:

local camera = {}

camera.__index = camera

i do understand, that __index is a methamethod....but it would usually be used to add properties or function from a metatable to annother table....

but in this example camara is a table and camara.__index points to camera. I don't understand, what it exactly does?

Can somebody explain?

Here is the complete source-code

Code: Select all

```
local cos, sin = math.cos, math.sin
local camera = {}
camera.__index = camera
-- Movement interpolators (for camera locking/windowing)
camera.smooth = {}
function camera.smooth.none()
return function(dx,dy) return dx,dy end
end
local function new(x,y, zoom, rot, smoother)
x,y = x or love.graphics.getWidth()/2, y or lo-ve.graphics.getHeight()/2
zoom = zoom or 1
rot = rot or 0
smoother = smoother or camera.smooth.none() -- for locking, see below
return setmetatable({x = x, y = y, scale = zoom, rot = rot, smoother = smoother}, camera)
end
function camera:lookAt(x,y)
self.x, self.y = x, y
return self
end
function camera:move(dx,dy)
self.x, self.y = self.x + dx, self.y + dy
return self
end
function camera:position()
return self.x, self.y
end
function camera:zoom(mul)
self.scale = self.scale * mul
return self
end
function camera:zoomTo(zoom)
self.scale = zoom
return self
end
function camera:attach(x,y,w,h, noclip)
x,y = x or 0, y or 0
w,h = w or love.graphics.getWidth(), h or love.graphics.getHeight()
self._sx,self._sy,self._sw,self._sh = love.graphics.getScissor()
if not noclip then
love.graphics.setScissor(x,y,w,h)
end
local cx,cy = x+w/2, y+h/2
love.graphics.push()
love.graphics.translate(cx, cy)
love.graphics.scale(self.scale)
love.graphics.rotate(self.rot)
love.graphics.translate(-self.x, -self.y)
end
function camera:detach()
love.graphics.pop()
love.graphics.setScissor(self._sx,self._sy,self._sw,self._sh)
end
function camera:draw(...)
local x,y,w,h,noclip,func
local nargs = select("#", ...)
if nargs == 1 then
func = ...
elseif nargs == 5 then
x,y,w,h,func = ...
elseif nargs == 6 then
x,y,w,h,noclip,func = ...
else
error("Invalid arguments to camera:draw()")
end
self:attach(x,y,w,h,noclip)
func()
self:detach()
end
-- world coordinates to camera coordinates
function camera:cameraCoords(x,y, ox,oy,w,h)
ox, oy = ox or 0, oy or 0
w,h = w or love.graphics.getWidth(), h or love.graphics.getHeight()
-- x,y = ((x,y) - (self.x, self.y)):rotated(self.rot) * self.scale + center
local c,s = cos(self.rot), sin(self.rot)
x,y = x - self.x, y - self.y
x,y = c*x - s*y, s*x + c*y
return x*self.scale + w/2 + ox, y*self.scale + h/2 + oy
end
-- camera coordinates to world coordinates
function camera:worldCoords(x,y, ox,oy,w,h)
ox, oy = ox or 0, oy or 0
w,h = w or love.graphics.getWidth(), h or love.graphics.getHeight()
-- x,y = (((x,y) - center) / self.scale):rotated(-self.rot) + (self.x,self.y)
local c,s = cos(-self.rot), sin(-self.rot)
x,y = (x - w/2 - ox) / self.scale, (y - h/2 - oy) / self.scale
x,y = c*x - s*y, s*x + c*y
return x+self.x, y+self.y
end
-- camera scrolling utilities
function camera:lockX(x, smoother, ...)
local dx, dy = (smoother or self.smoother)(x - self.x, self.y, ...)
self.x = self.x + dx
return self
end
function camera:lockY(y, smoother, ...)
local dx, dy = (smoother or self.smoother)(self.x, y - self.y, ...)
self.y = self.y + dy
return self
end
function camera:lockPosition(x,y, smoother, ...)
return self:move((smoother or self.smoother)(x - self.x, y - self.y, ...))
end
function camera:lockWindow(x, y, x_min, x_max, y_min, y_max, smoother, ...)
-- figure out displacement in camera coordinates
x,y = self:cameraCoords(x,y)
local dx, dy = 0,0
if x < x_min then
dx = x - x_min
elseif x > x_max then
dx = x - x_max
end
if y < y_min then
dy = y - y_min
elseif y > y_max then
dy = y - y_max
end
-- transform displacement to movement in world coordinates
local c,s = cos(-self.rot), sin(-self.rot)
dx,dy = (c*dx - s*dy) / self.scale, (s*dx + c*dy) / self.scale
-- move
self:move((smoother or self.smoother)(dx,dy,...))
end
-- the module
return setmetatable({new = new, smooth = camera.smooth},
{__call = function(_, ...) return new(...) end})
```