Cannot paint into offscreen DC

Hi,

I have this code for drawing my custom control. I am going to do my drawing into an off-screen DC and then blit it to the screen DC. However, it won't work. If i pass the screen DC to the Rectangle() function it will draw a rectangle, but when passing the memory DC it will not. I have done this many times before, but I can't get it to work in this particular case. Does it have anything to do with my OO approach?

Edit: What I mean by "won't work" is that seemingly nothing gets drawn to the memory DC. No function calls fail either. I have checked return values and GetLastError(). I can for instance use the BLACKNESS raster operation on the BitBlt function and the control gets painted black.

Code:

LRESULT Button::OnPaint(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    PAINTSTRUCT ps;
    m_hdc = BeginPaint(hwnd,&ps);

    OffscreenBitmap(&ps);

    EndPaint(hwnd,&ps);

    return FALSE;
}

void Button::OffscreenBitmap(PAINTSTRUCT *ps)
{
    HDC hdcMem = CreateCompatibleDC(ps->hdc);
    HBITMAP hbm = CreateCompatibleBitmap(ps->hdc,m_cx,m_cy);
    HBITMAP hbmOld;

    hbmOld = (HBITMAP)SelectObject(hdcMem,hbm);

    Rectangle(hdcMem,0,0,m_cx,m_cy);

    //DrawGradient(hdcMem);
    //LightenEdges(hdcMem);

    BitBlt(m_hdc,
        0,
        0,
        m_cx,
        m_cy,
        hdcMem,
        m_cx,
        m_cy,
        SRCCOPY
    );

    SelectObject(hdcMem,hbmOld);

    DeleteObject(hbm);
    DeleteDC(hdcMem);
}

Are you sure you've got the correct pen selected into the memory DC?

Originally Posted by brewbuck

Are you sure you've got the correct pen selected into the memory DC?

I did not explicitly select a pen into the DC, though I tried that now when you mentioned it, but still the same result. I am also drawing a gradient in the DC (the commented out function) but this doesn't show up when I blit into the control's DC either... The exact same code works fine when drawing directly into the control's DC.

Originally Posted by Eirik

Code:

BitBlt(m_hdc,
        0,
        0,
        m_cx,
        m_cy,
        hdcMem,
        m_cx,   // x-coordinate of source upper-left corner
        m_cy,   // y-coordinate of source upper-left corner
        SRCCOPY
    );

Looks like you're trying to blit pixels m_cx -> m_cx * 2 and m_cy -> m_cy * 2 from the source dc instead of pixels 0 -> m_cx and 0 -> m_cy. The commented lines should be both be 0.

Originally Posted by adeyblue

Looks like you're trying to blit pixels m_cx -> m_cx * 2 and m_cy -> m_cy * 2 from the source dc instead of pixels 0 -> m_cx and 0 -> m_cy. The commented lines should be both be 0.

Hmm. I believe you're right, but if that's true, why did he get a black rectangle when using a BLACKNESS rop? The blackness should have been confined to the destination rectangle, which as you point out, is off the edge of the bitmap... Weird.

EDIT: Nevermind.. It's the source which is clipped, not the dest. I think you've basically explained the problem.

Originally Posted by adeyblue

Looks like you're trying to blit pixels m_cx -> m_cx * 2 and m_cy -> m_cy * 2 from the source dc instead of pixels 0 -> m_cx and 0 -> m_cy. The commented lines should be both be 0.

Oh my! Thanks for spotting that adeyblue! It always helps to have a second pair of eyes to look at the code!