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Screen capture

Simple situations

If the code executed when the draw() function is called does not use the setContext() function, then you can make use of setContext() to capture the screen, as illustrated in the code below:

function captureScreen()
    local screenCap = image(WIDTH, HEIGHT) -- create an image the size of the screen 
    setContext(screenCap)                  -- set the context to the image
    draw()                                 -- draw a new frame (to the new context)   
                                           -- assumes draw will not alter the context 
    setContext()                           -- return the context to the screen     
    return screenCap                       -- return the image
end

(Credits: Simeon.)

More complex situations

If the code executed when the draw() function is called itself calls setContext(), then a more complex approach is required. See the example below.

-- Establish two global variables
_setContext    = setContext  -- one to preserve the built-in setContext() function
_screenContext = nil         -- if not nil, one to act as buffer to replace the screen

-- Overwrite the built-in setContext() function, before it is ever called
setContext = function(img)
    if img then return _setContext(img) end                       -- use img, if exists
    if _screenContext then return _setContext(_screenContext) end -- use buffer, if exists
    return _setContext()                              -- otherwise, call without arguments
end

function captureScreen()
    _screenContext = image(WIDTH, HEIGHT)  -- initialise buffer the size of the screen 
    draw()                                 -- draw a new frame (to the buffer)
    local screenCap = _screenContext       -- preserve the buffer
    _screenContext = nil                   -- set buffer to nil, restoring screen
    return screenCap                       -- return the image
end

The effect of the code is to substitute setContext(_screenContext) for setContext(), if _screenContext is not nil. The captureScreen() function takes care that _screenContext has a suitable image assigned to it, before draw() is called.