Screen capture in Haskell?

Question

Is it possible to capture the screen (or a window) using Haskell in a Windows environment? (ie, taking a screenshot every few minutes or so). If so, how would one go about doing this (again, in Haskell, for a Windows environment)?

More info: I'm a beginner to Haskell. A friend wants to cut development costs by having me whip together some programs for his accounting firm, but he insists that I use Haskell. He wants a tool that will allow him to monitor the desktops of different Windows XP workstations. It would likely have to be a client/server type application. He only needs to monitor desktop activity, hence why he doesn't want any of the expensive management software that is already on the market. I have sifted through lots of documentation, and only got as far as finding wxHaskell, but I couldn't find much on capturing the screen, especially for Windows environments.

Answer 1

You should be able to do this with the Win32 API. Based on What is the best way to take screenshots of a Window with C++ in Windows?, you need to get the context of the window and then copy the image from it using GetWindowDC and BitBlt respectively.

Looking around the Haskell Win32 API documentation, there is a getWindowDC function in Graphics.Win32.Window. This returns an IO HDC. There is a bitblt function in Graphics.Win32.GDI.Graphics2D. This function takes an HDC along with a bunch of INTs which presumably correspond to the arguments it takes in C++.

Unfortunately, I don't have a Windows machine handy, so I can't write the actual code. You'll have to figure out how to use the Win32 API functions yourself, which might be a bit of a bother.

When you do, it would be great if you factored it into a library and put it up on Hackage--Windows does not usually get much love in Haskell land (as I myself show :P), so I'm sure other Windows programmers would be grateful for an easy way to take screenshots.

Answer 2

You can also do it in a cross-platform way with GTK.

That would not be much different from doing it with C: Taking a screenshot with C/GTK.

{-# LANGUAGE OverloadedStrings #-}

import Graphics.UI.Gtk
import System.Environment
import Data.Text as T

main :: IO ()
main = do
    [fileName] <- getArgs
    _ <- initGUI
    Just screen <- screenGetDefault
    window <- screenGetRootWindow screen
    size <- drawableGetSize window
    origin <- drawWindowGetOrigin window
    Just pxbuf <-
        pixbufGetFromDrawable
            window
            ((uncurry . uncurry Rectangle) origin size)
    pixbufSave pxbuf fileName "png" ([] :: [(T.Text, T.Text)])

Answer 3

The Approach Tikhon mentioned is correct. Just to add some code to the answer he gave above

import Graphics.Win32.Window
import Graphics.Win32.GDI.Bitmap
import Graphics.Win32.GDI.HDC
import Graphics.Win32.GDI.Graphics2D

main = do desktop   <- getDesktopWindow -- Grab the Hwnd of the desktop, GetDC 0, GetDC NULL etc all work too
          hdc       <- getWindowDC (Just desktop) -- Get the dc handle of the desktop
          (x,y,r,b) <- getWindowRect desktop -- Find the size of the desktop so we can know which size the destination bitmap should be
                                             -- (left, top, right, bottom)
          newDC     <- createCompatibleDC (Just hdc) -- Create a new DC to hold the copied image. It should be compatible with the source DC
          let width  = r - x -- Calculate the width
          let height = b - y -- Calculate the Height
          newBmp    <- createCompatibleBitmap hdc width height -- Create a new Bitmap which is compatible with the newly created DC
          selBmp    <- selectBitmap newDC newBmp -- Select the Bitmap into the DC, drawing on the DC now draws on the bitmap as well
          bitBlt newDC 0 0 width height hdc 0 0 sRCCOPY -- use SRCCOPY to copy the desktop DC into the newDC
          createBMPFile "Foo.bmp" newBmp newDC  -- Write out the new Bitmap file to Foo.bmp
          putStrLn "Bitmap image copied" -- Some debug message
          deleteBitmap selBmp -- Cleanup the selected bitmap
          deleteBitmap newBmp -- Cleanup the new bitmap
          deleteDC newDC      -- Cleanup the DC we created.

This was just quickly put together, but it saves a screenshot of to a file named Foo.bmp. Ps. To whomever wrote the Win32 Library, nicely done :)