How to draw on given bitmap handle (C++ / Win32)?

Question

I'm writing an unmanaged Win32 C++ function that gets a handle to a bitmap, and I need to draw on it.

My problem is that to draw I need to get a device context, but when I do GetDC (NULL), it gives me a device context for the WINDOW! The parameter for GetDC () is a window handle (HWND), but I don't have a window; just a bitmap handle.

How can I draw on this bitmap? Thanks!

Answer 1

CreateCompatibleDC() and SelectObject() your bitmap into it.

However, not every bitmap can be selected into any DC. You might have to play with mapping mode and other options of memory DCs.

Answer 2

In addition to Pavel's answer, the "compatible with the screen" always bugged me too, but, since CreateCompatibleDC(NULL) is universally used for that purpose, I assume it is correct.

I think that the "compatible" thing is related just to DDB (the DC is set up to write on the correct DDB type for the current screen), but does not affect read/writes on DIBs.

So, to be safe, always use DIBs and not DDBs if you need to work on bitmaps that doesn't just have to go temporarily onscreen, nowadays the difference in performance is negligible. See here for more info about DIBs and DDBs.

Answer 3

The basic win32 paradigm for drawing on a bitmap is that you select the bitmap onto a device context, after which, all drawing operations on that device context are stored in the bitmap. You then use one of the various 'blit' operations (e.g. StretchBlt) to transfer this to a display surface, which is just the device context of a window client area.

Others have provided better detail, this is just the high-level view.

Answer 4

Well, this is a bit outside the box.. I guess.. But I do know that Graphics can return a HDC, and Graphics take a Bitmap as an argument to its ctor . A Bitmap in turn can be created from a HBITMAP and a HPALETTE. The only problem here is that I do not know if the HPALETTE argument can be NULL.

Graphics* g;
Bitmap* bitmap;
HBITMAP _bitmap;   // <- this one is yours

bitmap = Bitmap::FromHBITMAP(_bitmap, NULL);
g = new Graphics(bitmap);

HDC hdc = g->GetHDC();

// when done, call g->ReleaseHDC(hdc);

However, I would urge you to receive the HDC as an argument to your function as well.. I do not think that anyone will have a BITMAP and NOT have the DC to it.
If you're having these issues with finding a HDC to a HBITMAP, so will everyone else.