friday-0.1: example/Delayed.hs

import System.Environment (getArgs)

import Vision.Image
import Vision.Primitive (Z (..), (:.) (..), Rect (..), ix2)

-- Reads an image from a file, applies a composition of transformations to
-- create a centred and squared miniature and then writes the result to a file:
--
-- usage: ./delayed input.png output.png
main :: IO ()
main = do
    [input, output] <- getArgs

    -- Loads the image. Automatically infers the format.
    io <- load Nothing input

    case io of
        Left _err -> putStrLn "Error while reading the image."
        Right img -> do
            let -- Convert the StorageImage (which can be Grey, RGB or RGBA) to
                -- an RGB image.
                rgb = convert img :: RGB

                -- Gets the size of the image.
                Z :. h :. w = shape rgb

                -- Creates a Rect object which will be used to define how we
                -- will crop our image. The rectangle is centered on the largest
                -- side of the image.
                rect | w > h     = Rect ((w - h) `quot` 2) 0 h h
                     | otherwise = Rect 0 ((h - w) `quot` 2) w w

                -- Crops the image. Doesn't compute the image into a "real"
                -- image: by using a delayed representation, this intermediate
                -- image will not exist in the computer memory as a large array.
                cropped = delayed $ crop rect rgb

                -- Resizes the image. By using the delayed representation of the
                -- cropped image, our compiler should be able to fuse these two
                -- transformations into a single loop.
                resized = manifest $ resize Bilinear (ix2 250 250) cropped

            _ <- save output resized
            return ()