HBITMAP unscaledBitmap(void *i, intptr_t dx, intptr_t dy)
{
BITMAPINFO bi;
VOID *ppvBits;
HBITMAP bitmap;
ZeroMemory(&bi, sizeof (BITMAPINFO));
bi.bmiHeader.biSize = sizeof (BITMAPINFOHEADER);
bi.bmiHeader.biWidth = (LONG) dx;
bi.bmiHeader.biHeight = -((LONG) dy); // negative height to force top-down drawing;
bi.bmiHeader.biPlanes = 1;
bi.bmiHeader.biBitCount = 32;
bi.bmiHeader.biCompression = BI_RGB;
bi.bmiHeader.biSizeImage = (DWORD) (dx * dy * 4);
bitmap = CreateDIBSection(NULL, &bi, DIB_RGB_COLORS, &ppvBits, 0, 0);
if (bitmap == NULL)
xpanic("error creating HBITMAP for unscaled ImageList image copy", GetLastError());
dotoARGB(i, (void *) ppvBits);
return bitmap;
}
static void paintArea(HWND hwnd, void *data)
{
RECT xrect;
PAINTSTRUCT ps;
HDC hdc;
HDC rdc;
HBITMAP rbitmap, prevrbitmap;
RECT rrect;
BITMAPINFO bi;
VOID *ppvBits;
HBITMAP ibitmap;
HDC idc;
HBITMAP previbitmap;
BLENDFUNCTION blendfunc;
void *i;
intptr_t dx, dy;
int hscroll, vscroll;
// FALSE here indicates don't send WM_ERASEBKGND
if (GetUpdateRect(hwnd, &xrect, FALSE) == 0)
return; // no update rect; do nothing
getScrollPos(hwnd, &hscroll, &vscroll);
hdc = BeginPaint(hwnd, &ps);
if (hdc == NULL)
xpanic("error beginning Area repaint", GetLastError());
// very big thanks to Ninjifox for suggesting this technique and helping me go through it
// first let's create the destination image, which we fill with the windows background color
// this is how we fake drawing the background; see also http://msdn.microsoft.com/en-us/library/ms969905.aspx
rdc = CreateCompatibleDC(hdc);
if (rdc == NULL)
xpanic("error creating off-screen rendering DC", GetLastError());
// the bitmap has to be compatible with the window
// if we create a bitmap compatible with the DC we just created, it'll be monochrome
// thanks to David Heffernan in http://stackoverflow.com/questions/23033636/winapi-gdi-fillrectcolor-btnface-fills-with-strange-grid-like-brush-on-window
rbitmap = CreateCompatibleBitmap(hdc, xrect.right - xrect.left, xrect.bottom - xrect.top);
if (rbitmap == NULL)
xpanic("error creating off-screen rendering bitmap", GetLastError());
prevrbitmap = (HBITMAP) SelectObject(rdc, rbitmap);
if (prevrbitmap == NULL)
xpanic("error connecting off-screen rendering bitmap to off-screen rendering DC", GetLastError());
rrect.left = 0;
rrect.right = xrect.right - xrect.left;
rrect.top = 0;
rrect.bottom = xrect.bottom - xrect.top;
if (FillRect(rdc, &rrect, areaBackgroundBrush) == 0)
xpanic("error filling off-screen rendering bitmap with the system background color", GetLastError());
i = doPaint(&xrect, hscroll, vscroll, data, &dx, &dy);
if (i == NULL) // cliprect empty
goto nobitmap; // we need to blit the background no matter what
// now we need to shove realbits into a bitmap
// technically bitmaps don't know about alpha; they just ignore the alpha byte
// AlphaBlend(), however, sees it - see http://msdn.microsoft.com/en-us/library/windows/desktop/dd183352%28v=vs.85%29.aspx
ZeroMemory(&bi, sizeof (BITMAPINFO));
bi.bmiHeader.biSize = sizeof (BITMAPINFOHEADER);
bi.bmiHeader.biWidth = (LONG) dx;
bi.bmiHeader.biHeight = -((LONG) dy); // negative height to force top-down drawing
bi.bmiHeader.biPlanes = 1;
bi.bmiHeader.biBitCount = 32;
bi.bmiHeader.biCompression = BI_RGB;
bi.bmiHeader.biSizeImage = (DWORD) (dx * dy * 4);
// this is all we need, but because this confused me at first, I will say the two pixels-per-meter fields are unused (see http://blogs.msdn.com/b/oldnewthing/archive/2013/05/15/10418646.aspx and page 581 of Charles Petzold's Programming Windows, Fifth Edition)
// now for the trouble: CreateDIBSection() allocates the memory for us...
ibitmap = CreateDIBSection(NULL, // Ninjifox does this, so do some wine tests (http://source.winehq.org/source/dlls/gdi32/tests/bitmap.c#L725, thanks vpovirk in irc.freenode.net/#winehackers) and even Raymond Chen (http://blogs.msdn.com/b/oldnewthing/archive/2006/11/16/1086835.aspx), so.
&bi, DIB_RGB_COLORS, &ppvBits, 0, 0);
if (ibitmap == NULL)
xpanic("error creating HBITMAP for image returned by AreaHandler.Paint()", GetLastError());
// now we have to do TWO MORE things before we can finally do alpha blending
// first, we need to load the bitmap memory, because Windows makes it for us
// the pixels are arranged in RGBA order, but GDI requires BGRA
// this turns out to be just ARGB in little endian; let's convert into this memory
dotoARGB(i, (void *) ppvBits, FALSE); // FALSE = not NRGBA
// the second thing is... make a device context for the bitmap :|
// Ninjifox just makes another compatible DC; we'll do the same
idc = CreateCompatibleDC(hdc);
if (idc == NULL)
xpanic("error creating HDC for image returned by AreaHandler.Paint()", GetLastError());
previbitmap = (HBITMAP) SelectObject(idc, ibitmap);
if (previbitmap == NULL)
xpanic("error connecting HBITMAP for image returned by AreaHandler.Paint() to its HDC", GetLastError());
// AND FINALLY WE CAN DO THE ALPHA BLENDING!!!!!!111
blendfunc.BlendOp = AC_SRC_OVER;
blendfunc.BlendFlags = 0;
blendfunc.SourceConstantAlpha = 255; // only use per-pixel alphas
blendfunc.AlphaFormat = AC_SRC_ALPHA; // premultiplied
if (AlphaBlend(rdc, 0, 0, (int) dx, (int) dy, // destination
idc, 0, 0, (int) dx, (int)dy, // source
blendfunc) == FALSE)
xpanic("error alpha-blending image returned by AreaHandler.Paint() onto background", GetLastError());
// clean up after idc/ibitmap here because of the goto nobitmap
if (SelectObject(idc, previbitmap) != ibitmap)
xpanic("error reverting HDC for image returned by AreaHandler.Paint() to original HBITMAP", GetLastError());
if (DeleteObject(ibitmap) == 0)
xpanic("error deleting HBITMAP for image returned by AreaHandler.Paint()", GetLastError());
if (DeleteDC(idc) == 0)
xpanic("error deleting HDC for image returned by AreaHandler.Paint()", GetLastError());
nobitmap:
// and finally we can just blit that into the window
if (BitBlt(hdc, xrect.left, xrect.top, xrect.right - xrect.left, xrect.bottom - xrect.top,
rdc, 0, 0, // from the rdc's origin
SRCCOPY) == 0)
xpanic("error blitting Area image to Area", GetLastError());
// now to clean up
if (SelectObject(rdc, prevrbitmap) != rbitmap)
xpanic("error reverting HDC for off-screen rendering to original HBITMAP", GetLastError());
if (DeleteObject(rbitmap) == 0)
xpanic("error deleting HBITMAP for off-screen rendering", GetLastError());
if (DeleteDC(rdc) == 0)
xpanic("error deleting HDC for off-screen rendering", GetLastError());
EndPaint(hwnd, &ps);
}
