What is Screen Scraping?
Screen Scraping is a term used to describe the process of a computer program or macro extracting data from the display output of another application. Rather than parsing data from the database or data files belonging to an application, Screen Scraping pulls the data from the screen itself, extracting data that was intended to be displayed to the end-user as opposed to data designed for output to another application or database.
Screen Scraping is necessary when there is a need to access the information displayed by the application but there is no method provided to access it behind the scenes. The database or data files may not be accessible, or may be undocumented or proprietary and therefore cannot be parsed easily; the costs associated with interacting with the database may be too high; or the license agreement or warrenty prohibits it. In the case of legacy systems that are no longer supported there may be no knowledge of the data structures, or the technology used is no longer compatible with current technology. In these cases we are resorted to extracting the data from the screen – from the windows of the application.
The term Screen Scraping probably originates from the era of computer terminals when you could connect the terminal output of a computer to an input port on another and therefore record the screen data.
Screen Scraping Methods
There are a number of ways we can retrieve information from the screen using Macro Scheduler, depending on the type of application the data is in.
Screen Scraping Web Applications
Applications like Macro Scheduler’s WebRecorder can access the data and objects inside an Interner Explorer window and can therefore be used to extract the data. Technically speaking I would not call this screen scraping since WebRecorder is using an API interface provided by Internet Explorer, but the process of extracting information from web sites is commonly refered to as Screen Scraping. With WebRecorder we can use the ExtractTag wizard to create code that extracts the text from a particular element in the page. While WebRecorder is the easiest way to do it, it is also possible to automate IE and extract data from web pages by using VBScript. The following forum posts may help:
Screen Scraping Microsoft Office Applications
Microsoft Office Applications, like Internet Explorer, have a COM interface that allows scripts to manipulate them and access the data held within them. Again, not really scraping data from the screen itself, as you are getting it directly from a programming interface. There are a number of examples in the forums and blog archives and also some sample scripts that come with Macro Scheduler which demonstrate how to automate Office applications and retrieve data from them.
Screen Scraping Regular Windows Applications
Most other applications don’t offer a scripting interface like MS Office or Internet Explorer. This is where we really need to work directly with the screen. There are a number of ways we can do this kind of Screen Scraping with Macro Scheduler.
The Text Capture Functions
Macro Scheduler includes some Text Capture functions which can be used to extract text from a given window, rectangular screen area or screen point. These functions use low level system hooks which monitor applications calling the various “TextOut” functions that Windows uses to output text to the screen. By doing so they are able to capture this text. The Text Capture functions return the text to a variable which you can then use as needed.
However, a few applications don’t use the Windows built-in functions to create and output text. Don’t worry –Most do, but a few use their own techniques. When you realise that text on the screen is just a sequence of small dots, if the application programmer decided to build his own routine to assemble text from dots rather than calling the Windows functions which already do that for you, you’re not going to be able to capture it.
The text capture functions and their limitations are explained here. There is an example application, here, created with Macro Scheduler, which you can use to determine whether or not the text you want to capture can be captured using the text capture functions.
Using the Clipboard for Screen Scraping
If the text you want to capture is selectable then you can use the clipboard to retrieve it. A Macro Scheduler macro can send the keystrokes necessary to highlight and copy the text to the clipboard and then use the GetClipboard function to retrieve that text to a variable. This is far less elegant than using the Text Capture functions but might be necessary if the application concerned is not utlising any of the Windows text out functions to create the text.
SetFocus>Notepad* //Select ALL Press CTRL Send>a Release CTRL //Copy to clipboard Press CTRL Send>c Release CTRL //Get and display the data WaitClipboard GetClipboard>theData MessageModal>theData
Screen Scraping via Optical Character Recognition
A final resort might be Optical Character Recognition (OCR). If the text is part of an image file neither the Text Capture or Clipboard methods are going to be able to extract it. But, depending on the quality of the image and the fonts used, OCR might. There are a number of OCR engines that can be used by Macro Scheduler, one of which is part of Microsoft Office. The following posts demonstrate how this can be done:
More on Screen Scraping.