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Setting desktop pixel colors using Win API

Posted on 2009-05-06
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Last Modified: 2013-12-26
Hi everyone. I am working on a project written using Windows API. The program currently captures the full screen (like printscreen) and draws new pixels to it without a problem.  I need to find a way to visibly set pixels on the screen but NOT change printscreen calls. Like two layers stacked on top of each other. The top layer being separate from the bottom.  My changes must only occur in the top layer. Is this possible? If so, I'm looking for the windows api functions that will work. Thanks a bunch.
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Question by:secondeff
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by:debuggerau
ID: 24322829
are you using bit blit?
http://en.wikipedia.org/wiki/Bit_blit
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by:secondeff
ID: 24322870
Thanks for the response. I'm using bit blit to get the screen information. I'm using setpixel to write to it.
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by:debuggerau
ID: 24322924
ok, so you want to write on the screen, but have printscreens ignore the added layer?
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by:debuggerau
ID: 24322982
goto go, but it cant be done without control of the computer....
back tomorrow...
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by:secondeff
ID: 24323070
Yea, you summed it it. What do you mean "without control of the computer"? I'm writing the software for my own computer.
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by:debuggerau
ID: 24331929
you say that you don't want to change printscreen calls, however that feature will capture all layers onscreen, unless you disable that function while displaying your particulars.
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Author Comment

by:secondeff
ID: 24332275
I'll concede that I can't draw on the desktop without it being captured. I could work around this if I knew when any program captured this data (maybe a windows hook)? I can monitor the printscreen key by re-registering its hotkey in my application. However, what happens in the case of a CreateDC("Display") call? How would I monitor this?- Thanks
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by:debuggerau
ID: 24332394
a createDC call for the display would need a windows helper hook to override the call, but I think you'd be better off using this approach:

http://www.faqs.org/faqs/msdos-programmer-faq/part2/section-17.html

The standard handler, in BIOS versions dated
 December 1982 or later, uses a byte at 0040:0100 (= 0000:0500) to
 determine whether a print screen is currently in progress. If it is,
 pressing PrintScreen again is ignored. So to disable the screen
 snapshot, all you have to do is write a 1 to that byte. When the user
 presses PrintScreen, the BIOS will think that a print screen is already
 in progress and will ignore the user's keypress. You can re-enable
 PrintScreen by zeroing the same byte.

 Here's some simple code:

   void prtsc_allow(int allow) /* 0=disable, nonzero=enable */
   {
     unsigned char far* flag = (unsigned char far*)0x00400100UL;
     *flag = (unsigned char)!allow;
   }

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by:secondeff
ID: 24332536
I tried the code snippet you posted and when I try to write to 0040:0100 I get an access violation.
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debuggerau earned 500 total points
ID: 24332970
yea, ok, its a DOS function, protected from windows.

So looking at other solutions, this came up:
http://labnol.blogspot.com/2004/08/disable-print-screen-key-in-windows.html
It talks about reallocating predefined keys for printscrn..

But it also talks about limitation with it too, like how it will only capture the current screen..
Obviously it would be difficult to configure every system with a second screen, but I have written data to outside the current window, but it is not displayed, yet still accessible. Would this help do you think?
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