Draw and Erase lines on Desktop Window

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Hi,

I have a requirement in VC++6.0 where draw lines at desktop window between windows and erase them when is not needed. The lines are drawn from one window to another window passing through the desktop window. As part of the requirement the lines must be shown with different colors. I can use SetROP2 with R2_XORPEN which works perfectly for drawing and erasing lines using desktop widnow. But the line is on desktop is inverted with the combination of line color and desktop window color. I tried using Binary raster operations for suppressing this issue but none of the combination is resolved the issue. Could you help what is the solution for this?

regards
shanpalaniram

Posted 16-Mar-11 4:24am

Shanmugam R P

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Olivier Levrey · Answer 1 · 2011-03-16T04:47:00

Solution 1

If you want your line to be drawn in plain color, then you shouldn't use SetROP2 with R2_XORPEN. Just use the normal mode to draw your line.

To "erase" the line, just invalidate the corresponding area with one of the Invalidate functions. For example:

C++

HWND hDesktopWnd = GetDesktopWindow();
//redraw the rectangle area which contains the line
InvalidateRect(hDesktopWnd, &rect, TRUE);
//or with a region
InvalidateRgn(hDesktopWnd, rgn, TRUE);

Posted 16-Mar-11 4:47am

Olivier Levrey

v2

Comments

mbue 16-Mar-11 13:27pm

Thats right.
@OP: its not fine to paint over other windows. Create a single window for each line, that would be the better solution.
Regards

Sergey Alexandrovich Kryukov 16-Mar-11 21:49pm

This is very bad, too. All rendering should be in one windows, all elements are light-weighted (not windows).
--SA

mbue 17-Mar-11 6:27am

Your fail, you should to understand the intention of the op. hint: who's the owner of the desktop window?

Sergey Alexandrovich Kryukov 17-Mar-11 13:07pm

Absolutely does not matter who is the owner, even if nobody.
Can you explain the difference? I can draw in desktop, what's the problem?
--SA

Sergey Alexandrovich Kryukov 16-Mar-11 21:50pm

This is correct explanation, my 5, but the project leads nowhere. I saw many attempts to follow this line of "design". Nothing but epic fail each time.
--SA

Olivier Levrey 17-Mar-11 4:59am

Yes I agree. It will be hard to redraw the lines if another window (from another app) comes...

Sergey Alexandrovich Kryukov 17-Mar-11 13:00pm

Even in the same app. Disproportional effort required. Window objects are not controllable well enough for this purpose. It's like making surgery without cutting the body. At the same time, "lightweight" approach when all is rendered from scratch on the same canvas is easy enough.
--SA

Shanmugam R P 18-Mar-11 12:58pm

Hi Olivier Levrey,

Thanks for the response. I am currently using InvalidateRect for erasing the lines. But when I opened the application on top of any other windows/applications like IE, Word etc, the lines drawn on those windows can not be erased because I don't have control to repaint it, so I have to do Ctrl+Tab or min/max as to erase the lines. If use SetROP2 with R2_XORPEN, the lines are getting erased when my app is opened on top any windows. But it inverts the portion of line color which passing the desktop window. Typically both the solutions ( 1) SetROP2 with R2_XORPEN 2) drawing manually with default mode and using InvalidateRect) have issues. Hence looking for some solutions for either of the method. Help!

Shanpalaniram

Olivier Levrey 21-Mar-11 5:47am

I added a new answer. But I have no idea if it will work...

Olivier Levrey · Answer 2 · 2011-03-20T23:46:00

I have never done what I will describe and therefore I can't tell wether it will work ot not, but you can try to host all your windows in a main window and make this main window transparent.

You would then draw the lines inside your main window as non-transparent, and can invalidate the main window easily.

Check this link to know how to make transparent windows:
http://msdn.microsoft.com/en-us/library/ms997507.aspx[^]

I hope this will help.