Simplified Painting App, made with Codea.

Color.lua

--# Color
-- modified version from https://github.com/EmmanuelOga/columns/blob/master/utils/color.lua

--[[
 * Converts an RGB color value to HSL. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes r, g, and b are contained in the set [0, 255] and
 * returns h, s, and l in the set [0, 1].
 *
 * @param   Number  r       The red color value
 * @param   Number  g       The green color value
 * @param   Number  b       The blue color value
 * @return  Array           The HSL representation
]]
function rgb2hsl(r, g, b)
    r,g,b = r/255, g/255, b/255
    local max, min = math.max(r,g,b), math.min(r,g,b)
    local h, s, l = 0, 0, (max + min)/2 -- achromatic
    if max ~= min then
        local d = max - min
        if l > .5 then s = d/(2 - max - min) else s = d/(max + min) end
        if max == r then
            h = (g - b)/d
            if g < b then h = h + 6 end
        elseif max == g then h = (b - r)/d + 2
        elseif max == b then h = (r - g)/d + 4
        end
        h = h/6
    end
    return h,s,l
end

--[[
 * Converts an HSL color value to RGB. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes h, s, and l are contained in the set [0, 1] and
 * returns r, g, and b in the set [0, 255].
 *
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  l       The lightness
 * @return  Array           The RGB representation
]]
function hsl2rgb(h, s, l)
    local r,g,b = l,l,l -- achromatic
    if s ~= 0 then
        local function hue2rgb(p, q, t)
            if t < 0   then t = t + 1 end
            if t > 1   then t = t - 1 end
            if t < 1/6 then return p + (q - p) * 6 * t end
            if t < 1/2 then return q end
            if t < 2/3 then return p + (q - p) * (2/3 - t) * 6 end
            return p
        end
        local q = l < .5 and l*(1 + s) or l + s - l*s
        local p = 2 * l - q
        r = hue2rgb(p, q, h + 1/3)
        g = hue2rgb(p, q, h)
        b = hue2rgb(p, q, h - 1/3)
    end
    return r*255, g*255, b*255
end

--[[
 * Converts an RGB color value to HSV. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSV_color_space.
 * Assumes r, g, and b are contained in the set [0, 255] and
 * returns h, s, and v in the set [0, 1].
 *
 * @param   Number  r       The red color value
 * @param   Number  g       The green color value
 * @param   Number  b       The blue color value
 * @return  Array           The HSV representation
]]
function rgb2hsv(r, g, b)
    r, g, b = r / 255, g / 255, b / 255
    local max, min = math.max(r, g, b), math.min(r, g, b)
    local h, s, v = nil, nil, max
    local d = max - min
    
    if max == 0 then s = 0 else s = d / max end
    if max == min then h = 0
    else
        if max == r then h = g < b and h + 6 or (g - b) / d
        elseif max == g then h = (b - r) / d + 2
        elseif max == b then h = (r - g) / d + 4 end
        h = h / 6
    end
    
    return h, s, v
end

--[[
 * Converts an HSV color value to RGB. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSV_color_space.
 * Assumes h, s, and v are contained in the set [0, 1] and
 * returns r, g, and b in the set [0, 255].
 *
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  v       The value
 * @return  Array           The RGB representation
]]
function hsv2rgb(h, s, v)
  local r, g, b
  local i = math.floor(h * 6);
  local f = h * 6 - i;
  local p = v * (1 - s);
  local q = v * (1 - f * s);
  local t = v * (1 - (1 - f) * s);

  i = i % 6

  if i == 0 then r, g, b = v, t, p
  elseif i == 1 then r, g, b = q, v, p
  elseif i == 2 then r, g, b = p, v, t
  elseif i == 3 then r, g, b = p, q, v
  elseif i == 4 then r, g, b = t, p, v
  elseif i == 5 then r, g, b = v, p, q
  end

  return r * 255, g * 255, b * 255
end

Main.lua

--# Main
Scale  = .25 -- image quality settings (may affect speed also)
Canvas = readImage("Documents:Canvas")

if not Canvas then
    supportedOrientations(CurrentOrientation, (CurrentOrientation~=3 and CurrentOrientation~=1) and CurrentOrientation+1 or CurrentOrientation-1)
    Canvas = image(WIDTH * Scale, HEIGHT * Scale)
else
    if Canvas.width < Canvas.height then
        supportedOrientations(PORTRAIT_ANY)
    else
        supportedOrientations(LANDSCAPE_ANY)
    end
end

function setup()
    noSmooth()
    displayMode(OVERLAY)
    
    parameter.boolean("Mirror", false)
    
    parameter.color("BG", loadstring(readLocalData("BG", "return color(221,219,199,255)"))(), function()
        if BG.a < 255 then BG.a = 255 end
        saveLocalData("BG", "return color"..tostring(BG))
    end)
    
    parameter.color("Color", loadstring(readLocalData("Color", "return color(41,151,176,255)"))(), function()
        if Colorpicker then Colorpicker = false end
        if Color.a > 0 then Color.a = 255 end
        Schade = select(3, rgb2hsl(Color.r, Color.g, Color.b))
        saveLocalData("Color", "return color"..tostring(Color))
    end)
    
    parameter.boolean("Colorpicker", false)
    
    parameter.number("Schade", .001, .999, Schade, function()
        if Colorpicker then Colorpicker = false end
        local h, s, l = rgb2hsl(Color.r, Color.g, Color.b)
        Color = color(hsl2rgb(h, s, Schade))
        saveLocalData("Color", "return color"..tostring(Color)) -- fix because Color = color() doesn't trigger its prameter callback
    end)
    
    parameter.integer("Size", 1, 50, loadstring(readLocalData("Size", "return 2"))(), function()
        saveLocalData("Size", "return "..tostring(Size))
    end)
end

function orientationChanged()
    if _allowFlipping then
        local flipped = image(Canvas.width, Canvas.height)
        setContext(flipped)
        sprite(Canvas, WIDTH/2 * Scale, HEIGHT/2 * Scale, -Canvas.width, Canvas.height)
        setContext()
        Canvas = flipped
    else
        _allowFlipping = true
    end
end

local function deviceShaking()
    local acceleration = UserAcceleration
    local intensity    = 2 -- adjust min. shaking intensity to trigger this event!
    if UserAcceleration.x > intensity or acceleration.y > intensity or acceleration.z > intensity then
        _deviceShakingUpdatedAt = ElapsedTime
        _deviceShakingBegunAt   = _deviceShakingBegunAt or ElapsedTime
        
        -- first shake triggers listening process
        -- listen if shaking continues for [x] seconds then device is considered shaking on purpose
        if (ElapsedTime - _deviceShakingBegunAt) >= .5 then return true end
    end
    if  _deviceShakingUpdatedAt and (ElapsedTime > (_deviceShakingUpdatedAt + .5)) then
        _deviceShakingUpdatedAt = nil
        _deviceShakingBegunAt   = nil
    end
    return false
end

function draw()
    background(BG)
    sprite(Canvas, WIDTH/2, HEIGHT/2, WIDTH, HEIGHT)
    
    if Mirror then
        stroke(127)
        strokeWidth(1)
        line(WIDTH/2, 0, WIDTH/2, HEIGHT)
    end
    
    if Touch then
        local size = not Colorpicker and Size*2 or 50
        local pos  = not Colorpicker and vec2(Touch.x, Touch.y) or vec2(Touch.x, Touch.y + size)
        fill(Color)
        strokeWidth(1)
        stroke(0)
        ellipse(pos.x, pos.y, size + strokeWidth())
        stroke(255)
        ellipse(pos.x, pos.y, size)
    end
    
    if deviceShaking() then
        saveLocalData("BG", "return color(221,219,199,255)")
        saveLocalData("Color", "return color(41,151,176,255)")
        saveLocalData("Size", "return 2")
        saveImage("Documents:Canvas", nil)
        restart()
    end
end

function touched(touch)
    if not Touch or Touch.id == touch.id then Touch = touch end
    if Touch.state  == BEGAN then tween.resetAll() end
    if Touch.state  == ENDED then
        if Touch.id == touch.id then
            Touch   =  nil
            if Colorpicker then Colorpicker = false end
            if _SaveCanvas then
                tween.delay(.375, function() saveImage("Documents:Canvas", Canvas) end)
                _SaveCanvas = nil
            end
        end
    else
        if not Colorpicker then
            setContext(Canvas)
            noFill()
            strokeWidth(Size/2)
            stroke(Color)
            if Color.a == 0 then
                stroke(0,0,0,255)
                blendMode(ZERO, ONE_MINUS_SRC_ALPHA)
            end
            line(Touch.prevX * Scale, Touch.prevY * Scale, Touch.x * Scale, Touch.y * Scale, ROUND)
            if Mirror then line((WIDTH - Touch.prevX) * Scale, Touch.prevY * Scale, (WIDTH - Touch.x) * Scale, Touch.y * Scale, ROUND) end
            setContext()
            blendMode(NORMAL)
            _SaveCanvas = true
        else
            local col = color(Canvas:get(Touch.x * Scale, Touch.y * Scale))
            if col == color(0,0,0,0) then col = BG end
            if col.a < 255 then col = Color end
            Color = col
            
            Schade = select(3, rgb2hsl(Color.r, Color.g, Color.b))
            saveLocalData("Color", "return color"..tostring(Color)) -- fix because Color = color() doesn't trigger its prameter callback
        end
    end
end