How to create a large Compatible Memory DC in GDI programming?

Question

I want to create a large CompatibleDC, draw a large image on it, then bitblt part of the image to other DC, in order to achieve high performance.

I am using the following code to create compatible Memory DC. But when the rect becomes very large, etc: 5000*5000, the CompatibleDC created become unstable. sometimes it is OK, sometimes it failed. is there any thing wrong with my code?

input :pInputDC
output:pOutputMemDC

{
    pOutputMemDC=new CDC();
    VERIFY(pOutputMemDC->CreateCompatibleDC(pInputDC));

    CRect rect(0,0,nDCWidth,nDCHeight);
    CBitmap bitmap; 
    if (bitmap.CreateCompatibleBitmap(pInputDC, rect.Width(), rect.Height()))
    {
        pOutputMemDC->SetViewportOrg(-rect.left, -rect.top);
        m_pOldBitmap = pOutputMemDC->SelectObject(&bitmap);
    }
    CBrush brush;
    VERIFY(brush.CreateSolidBrush(RGB(255,0, 0)));
    brush.UnrealizeObject();
    pOutputMemDC->FillRect(rect, &brush);
}

Answer 1

Instead of creating a large DC and then blitting a portion of it another, smaller DC, create a DC the same size as the destination DC, or at least the same size as the blit destination. Then, offset all your drawing commands by the (-x,-y) of the sub section you want to copy. If your destination is (100,200)-(400,400) on the source then create a DC (300x200) and offset everything by (-100,-200).

This has two big advantages: firstly, the memory required is much smaller. Secondly, GDI will clip your drawing operations to the size of the DC (it always clips anyway). Although the act of clipping takes CPU time, the time saved by not drawing pixels that aren't seen more than makes up for it.

Now, if this large DC is something like an image (JPEG for example) then you need to look into other methods. One technique used by many image editing programs is to split the image into tiles and page the tiles to/from memory/hard disk. Each tile is its own DC and you only have enough source DCs to fill the target DC. As the view window moves across the large image, unload tiles that have moved out of the target rectangle and load tiles that have become visible.