StretchBlt MFC C++ problem

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Hello! I use StretchBlt to load bitmap image and show it on screen.
I also use double buffer in my application, which is dialog based.

I wrote a function that displays the image:

void CPlay::OnBackground(CBitmap* p, CDC* pDC)
{
    CDC dcMemory;
    BITMAP bm;
    dcMemory.CreateCompatibleDC(pDC);
    dcMemory.SelectObject(p);
    p->GetBitmap(&bm);

	pDC->StretchBlt(0, 0, screenX, screenY, &dcMemory, 0, 0, bm.bmWidth, bm.bmHeight, SRCCOPY);
}

Then I make CBitmap variable and load the image:

variable.LoadBitmap(RESOURCE);

I call the function OnBackground(&variable, &pDC);
pDC is CDC variable;
screenX, screenY are my screen resolution:

	screenX = GetSystemMetrics(SM_CXSCREEN);
	screenY = GetSystemMetrics(SM_CYSCREEN);

If I keep my resol(1280x1024), it works well. But if I change it, my CPU starts using 50-60% and it is slowed down.
If I'm wrong using StretchBlt, please let me know.
I hope you can get my point, that's why I used to write it in a detailed manner.
Thanks!

Posted 16-Apr-10 9:31am

George Findulov

Updated 16-Apr-10 21:14pm

Moak

v4

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CPallini · Answer 1 · 2010-04-20T20:40:00

Solution 3

You may create at once a resized bitmap instead of stretching again and again the original one. See if my "Plain C Resampling DLL"[^] article helps.
:)

Posted 20-Apr-10 20:40pm

CPallini

Christian Graus · Answer 2 · 2010-04-16T18:56:00

Solution 1

Do you mean AS you change it, or AFTER you change it ? If you're changing it, it's drawing the image over and over, so of course that uses the processor.

Posted 16-Apr-10 18:56pm

Christian Graus

@Intersect☺™ · Answer 3 · 2010-04-20T18:53:00

Solution 2

Try to clean your DC before you change your resolution. It will produce a great effect I believe.

Posted 20-Apr-10 18:53pm

@Intersect☺™

Sauro Viti · Answer 4 · 2010-10-15T03:06:00

It's not related to your question, but there is a conceptual error in your function: when you select a GDI object, like a bitmap onto a device context you must keep trace of the previously selected one and restore it, before destroying the device context.

C++

void CPlay::OnBackground(CBitmap* p, CDC* pDC)
{
    CDC dcMemory;
    BITMAP bm;
    dcMemory.CreateCompatibleDC(pDC);
    CBitmap *pOriginalBmp = dcMemory.SelectObject(p);
    p->GetBitmap(&bm);
	pDC->StretchBlt(0, 0, screenX, screenY, &dcMemory, 0, 0, bm.bmWidth, bm.bmHeight, SRCCOPY);
    dcMemory.SelectObject(pOriginalBmp);
}