Paint select function...

[InNeedOfHelp]

I am writing an application using MFC and was wanting to include a function similar to the select function available in Paint.
I want to allow the user to grab the screen and drag it where they want to, can anyone give me some pointers as to how I would go about this?

Any help would be greatly appreciated.

[Banfa]

What you will need to do is this, when the set the first point (a rectangle is defined by 2 points) just record it, when they select the second point you have the rectangle to copy. At this point you will probably need to create 2 bitmaps, 1 cotaining the contents of the screen (with the selction removed and 1 containing the selection. These can be created with CreateCompatableBitmap, then you will also need a compatable device context (with the screen) for each bitmap and you will need to select the created bitmaps into the created device contexts. You can then fill in the bitmaps by using BitBlt to copy what is currently on the screen. Then as they are draging the selection around you can use BitBlt to fill in the Window client area from the 2 bitmaps until they have finished. Once finished you can Blt everything back onto the screen and delete the created device contexts and bitmaps.

[InNeedOfHelp]

What you will need to do is this, when the set the first point (a rectangle is defined by 2 points) just record it, when they select the second point you have the rectangle to copy. At this point you will probably need to create 2 bitmaps, 1 cotaining the contents of the screen (with the selction removed and 1 containing the selection. These can be created with CreateCompatableBitmap, then you will also need a compatable device context (with the screen) for each bitmap and you will need to select the created bitmaps into the created device contexts. You can then fill in the bitmaps by using BitBlt to copy what is currently on the screen. Then as they are draging the selection around you can use BitBlt to fill in the Window client area from the 2 bitmaps until they have finished. Once finished you can Blt everything back onto the screen and delete the created device contexts and bitmaps.

Hey Banfa,

This is what I have at the moment (which isn't much);

OnMouseMove(...){
...
CRect rect(0,0,wWidth,wHeight);
CreateCompatibleBitmap(m_hDC, wWidth, wHeight);
CDC compDevice; // device compatible to the screen
?
}
where wWidth & wHeight are the width and height of the screen in pixels.
HDC m_hDC =GetDC(m_hWnd); where m_Wnd is a handle to the window.

I am wanting the whole screen to be dragged which is why there is only a single rectangle - is this correct?

Also I am unsure as to how you select the bitmaps into the device context and use BitBlt?

Cheers

[Banfa]

Looks about right but don't mix WIN32 and MFC if possible, it will just get confusing, look up the methods

Get a Device Context for the Window
CDC* CWnd::GetDC( );

Create a bitmap compatable with the device context
BOOL CBitmap::CreateCompatibleBitmap( CDC* pDC, int nWidth, int nHeight );

Create a device contaxt compatable with another device context
virtual BOOL CDC::CreateCompatibleDC( CDC* pDC );

Select a bitmap object into a device context, having done this you can use the device context to draw to the bitmap
CBitmap* CDC::SelectObject( CBitmap* pBitmap );

Copy bits of a bitmap between different (or the same) device contexts
BOOL CDC::BitBlt( int x, int y, int nWidth, int nHeight, CDC* pSrcDC, int xSrc, int ySrc, DWORD dwRop );


If you are not using MFC then don't use CDC compDevice, use HDC compDevice and all the above functions have WIN32 API equivilents with the same name.

[InNeedOfHelp]

Banfa,
I have tried to add the code as you instructed however I am not getting it right, the code I have is below;

CRect rect(0,0,wWidth,wHeight);
CreateCompatibleBitmap(m_hDC, wWidth, wHeight);
CDC *compDevice = GetDC();
CreateCompatibleBitmap(*compDevice, wWidth, wHeight);
CreateCompatibleDC(*compDevice);
CBitmap *bitmap;
compDevice->SelectObject(*bitmap);
compDevice->BitBlt(0,0,wWidth,wHeight,compDevice,0,0,SRCCOPY) ;

While the code compiles the SelectObject line throws an exception when it is executed (because it has no value, I assume) however I am unsure as to what should be there. Also I don't think that the compDevice (5th arg in BitBlt) that I have put in is correct?

Again any help would be greatly appreciated.

[InNeedOfHelp]

Sorry should also mention that it is MFC that I am using.

[Banfa]

OK, you are using MFC so I assume you are calling this from a class sub-classed from CWnd

Creation Code

CClientDC wndDC; //CClientDC autoatically calls GetDC and ReleaseDC
 
CBitmap bitmap;
bitmap.CreateCompatibleBitmap(&wndDC, wWidth, wHeight);
 
CDC bitmapDC;
bitmapDC.CreateCompatibleDC(&wndDC);
 
CBitmap *pOldBitmap = bitmapDC.SelectObject(&bitmap);
 
bitmapDC.BitBlt(0,0,wWidth,wHeight,wndDC,0,0,SRCCOPY); 
 

however these variables

CBitmap bitmap;
CDC bitmapDC;
CBitmap *pOldBitmap;

need to be persistent so that you can call the deletion code

bitmapDC.SelectObject(pOldBitmap);
 
bitmap.DeleteObject();
 
bitmapDC.DeleteDC();