declarative image composition using sdl2


* Introduction
  This package provides tools for simple image composition through the
  means of the SDL library, version 2.  You can combine, translate,
  rotate, blend, modulate colors and draw in a declarative way.

* Example

  The following program draws rectangles to the screen, pauses for 5
  seconds and terminates afterwards.  See the comments for further

  #+begin_src haskell :tangle example.hs
    import Control.Concurrent (threadDelay)
    import Data.Text (pack)
    import Linear.V2 (V2(..))
    import SDL (initialize, InitFlag(InitVideo), createWindow, defaultWindow,
                createRenderer, RendererConfig(rendererTargetTexture),
                defaultRenderer, rendererLogicalSize, ($=), present, clear, quit,
                BlendMode(BlendAlphaBlend), Texture)
    import SDL.Compositor (filledRectangleC, translateA, runRenderer, overC,
                           modulateAlphaM, blendMode, rgba, CompositingNode)

    main :: IO ()
    main = do
      -- intialize SDL
      initialize [InitVideo]
      -- create a window
      window <- createWindow (pack "test window") defaultWindow
      -- create a renderer for the window
      rend <- createRenderer window (-1) (defaultRenderer {rendererTargetTexture = True})
      -- set the logical size of the window to 800x600
      rendererLogicalSize rend $= Just (V2 800 600)
      -- clear the renderer
      clear rend
        -- 3 rectangles, one in red, one in green and one in blue, width:
        -- 100, height 150
        rectRed, rectGreen, rectBlue :: CompositingNode Texture
        rectRed = filledRectangleC (V2 100 150) (rgba 255 100 100 255)
        rectGreen = filledRectangleC (V2 100 150) (rgba 100 255 100 255)
        rectBlue = filledRectangleC (V2 100 150) (rgba 100 100 255 255)
        -- translate an image by (250,300) pixels
        translateToCenter = translateA (V2 250 300)
      -- draw everything
      runRenderer rend
        ( ( -- enable alpha blending for the whole subtree
            blendMode BlendAlphaBlend .
            -- make all the images slightly transparent
            modulateAlphaM 150 .
            -- align the images in the center
          ( -- red, green and blue
            rectRed `overC`
            translateA (V2 150 0) (rectGreen `overC`
                                   translateA (V2 150 0) rectBlue)
      -- show the drawn image on the screen
      present rend
      -- pause for 5 seconds
      threadDelay (5 * (10::Int)^(6::Int))
      -- quit SDL

* Usage
  You compose images from two kinds of primitives: textures and
  shapes.  Generate a texture through the usual means of SDL.  Use
  =sizedC= generate a compositional element of the
  texture.  You can also create shapes by using =rectangleC=,
  =filledRectangleC= and =lineC=.

  You can combine images with the =overC= function.  It takes two
  arguments and puts the first one above the second one when
  rendering.  When you have a list of images that you want to combine
  you can use for example =foldl1 overC= to combine all of them.  You
  should also check out the =withZIndex= function that allows you to
  specify the z-ordering explicitly and independent of the order of
  function arguments.

  Color modulation can be archived via the following commands:

  | =modulateAlphaM= | for alpha channel |
  | =modulateRedM=   | for red channel   |
  | =modulateGreenM= | for green channel |
  | =modulateBlueM=  | for blue channel  |

  For further documentation and the API see hackage.

* Resolution Independent Drawing
  This package provides an implementation for resoltution independent
  image composition.  When using the resolution independent
  implementation another coordinate system is used.  When you have a
  quadratic window then the point (0,0) is the top left corner and
  (1,1) is the bottom right corner.  In case the screen is not
  quadratic, then the algorithm will find the biggest square that fits
  on the screen, put it in the center and assignes the coordinates
  according to above rule.  The coordinate system is extended linearly
  beyond the boundaries of the square.  Execute the example program
  given below familiarize with this concept.  You should be able to
  execute it with =runhaskell=, it is really small.

  Here are some example calls:
    runhaskell resultion-independent.hs 800 600
    runhaskell resultion-independent.hs 40 30
    runhaskell resultion-independent.hs 20 600

  This is the source code for the above mentioned example:
  #+begin_src haskell :tangle resolution-independent.hs
    import Data.Text (pack)
    import Linear.V2 (V2(..))
    import SDL (initialize, InitFlag(InitVideo), createWindow,
                WindowConfig(windowInitialSize), defaultWindow,
                createRenderer, defaultRenderer, clear, present,
                quit,($=), rendererLogicalSize, Texture,
    import System.Environment (getArgs)
    import SDL.Compositor.ResIndependent (translateR, filledRectangleR,
                                          fromRelativeCompositor, ResIndependent)
    import SDL.Compositor (runRenderer, rgba, CompositingNode)
    import Control.Concurrent (threadDelay)

    main :: IO ()
    main = do
      -- Initialize sdl
      initialize [InitVideo]
      -- read window dimensions from command line args
      dims <- (\(w:h:_) -> V2 (read w) (read h)) <$> getArgs
      -- create a window with the given dimensions
      window <- createWindow (pack "test window") (defaultWindow {windowInitialSize = dims})
      -- create a renderer for the windows surface, software for
      -- compatibility reasons
      renderer <- createRenderer window (-1)
                  (defaultRenderer {rendererTargetTexture = True
                                   ,rendererType = SoftwareRenderer})
      -- setting the logical size is optional
      rendererLogicalSize renderer $= Just dims
      -- clear the window
      clear renderer
      -- draw a white square with length 0.99 for the edges
      let square :: ResIndependent CompositingNode Texture
          square = filledRectangleR (V2 0.99 0.99) (rgba 255 255 255 255)
      -- draw everything
        ( fromRelativeCompositor (fromIntegral <$> dims) $
          translateR (V2 0.5 0.5) -- move the square to the center of
      -- present what we have drawn to the user
      present renderer
      -- wait for 5 seconds
      threadDelay (5 * 10^(6::Int))
      -- quit sdl

* License
  Copyright (C) 2015  Sebastian Jordan

  This program is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see