How to construct a GDI+ Bitmap object from a Device-Dependent HBITMAP

Question

I want to use GDI+ method Image::Save() to save a DDB to a file in the following scenario:

HBITMAP hBitmap = CreateCompatibleBitmap(hDC, 200, 200) ;

...

//hBitmap is a DDB so I need to pass an HPALETTE
Gdiplus::Bitmap(hBitmap,  ???HPALETTE???  ).Save(L"file.png", ...) ;

The problem is that Bitmap constructor asks for an HPALETTE when the bitmap is not a device-independent bitmap.

Where do I get the necessary HPALETTE from?

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FOLLOWUP:
One of the answers suggests passing NULL as the HPALETTE parameter.
Here is a working example that does so. The result is a purely black and white image where all colors are lost.

#include <windows.h>
#include <gdiplus.h>

int main(){
    using namespace Gdiplus ;

    GdiplusStartupInput gdiplusStartupInput ;
    ULONG_PTR gdiplusToken ;
    GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL) ;

    CLSID pngEncoder = {0x557cf406, 0x1a04, 0x11d3, {0x9a, 0x73, 0x00, 0x00, 0xf8, 0x1e, 0xf3, 0x2e} } ;

    HDC dcHndl = CreateCompatibleDC(NULL) ;

    HBITMAP hBitmap = CreateCompatibleBitmap(dcHndl, 200, 200) ;

    SelectObject(dcHndl, hBitmap) ;

    BitBlt(dcHndl, 0,0, 200,200, GetDC(NULL), 0,0, SRCCOPY|CAPTUREBLT) ;

    Bitmap(hBitmap, NULL).Save(L"file.png", &pngEncoder) ;
}

Answer 1

First (and this is unrelated to your main question):

When creating a bitmap for screen shot, don't use a memory dc because that creates a monochrome bitmap. That's the main reason you are getting a black and white image (on my computer I just get a black image).

Don't use GetDC(0) inside another function. Every call to GetDC match have a matching ReleaseDC to avoid resource leak.

After calling BitBlt it is good practice to select hbitmap out of dc because you are basically finished drawing on dc.

The following code will work on Windows 10

int w = 800;
int h = 600;

HDC hdc = GetDC(HWND_DESKTOP);
HDC memdc = CreateCompatibleDC(hdc);
HBITMAP hbitmap = CreateCompatibleBitmap(hdc, w, h);
HBITMAP oldbmp = (HBITMAP)SelectObject(memdc, hbitmap);
BitBlt(memdc, 0, 0, w, h, hdc, 0, 0, SRCCOPY | CAPTUREBLT);
SelectObject(memdc, oldbmp);

Bitmap(hbitmap, NULL).Save(filename, &pngEncoder);

DeleteObject(hbitmap);
DeleteDC(memdc);
ReleaseDC(HWND_DESKTOP, hdc);

Back to your question regarding the documentation:

Type: HPALETTE
Handle to a GDI palette used to define the bitmap colors if hbm is not a device-independent bitmap (DIB).

In addition,

Do not pass to the Bitmap::FromHBITMAP method a GDI bitmap or a GDI palette that is currently (or was previously) selected into a device context.

The code I posted obeys only one rule, that GDI bitmap is not currently selected in to a device context (but it was previously selected).

The documentation may apply to older versions of Windows. As far as I can see MFC's CImage class does not follow all these rules. New computer displays are all 24 or 32 bit, I don't know how you would get a palette for it.

To follow the documentation to the letter, you can convert DDB to DIB section, using CreateDIBSection and GetDIBits. Use the new DIB section hbitmap_dib in Bitmap::FromHBITMAP. This will satisfy all of the conditions: hbitmap is dib, it is not (and was not) selected in to a device context.

Or, Gdiplus::Bitmap has another method Bitmap::FromBITMAPINFO. If there is no palette, you can use this code instead:

HDC hdc = GetDC(HWND_DESKTOP);
HDC memdc = CreateCompatibleDC(hdc);
HBITMAP hbitmap = CreateCompatibleBitmap(hdc, w, h);
HBITMAP oldbmp = (HBITMAP)SelectObject(memdc, hbitmap);
BitBlt(memdc, 0, 0, 800, 600, hdc, 0, 0, SRCCOPY | CAPTUREBLT);
SelectObject(memdc, oldbmp);

BITMAP bm;
GetObject(hbitmap, sizeof(bm), &bm);
int size = ((bm.bmWidth * bm.bmBitsPixel + 31) / 32) * 4 * bm.bmHeight;
BITMAPINFO info{ sizeof(info), bm.bmWidth, bm.bmHeight, 1, bm.bmBitsPixel, BI_RGB, size };
std::vector<char> bits(size);
GetDIBits(memdc, hbitmap, 0, bm.bmHeight, &bits[0], &info, DIB_RGB_COLORS);

Bitmap *bitmap = Bitmap::FromBITMAPINFO(&info, &bits[0]);
bitmap->Save(filename, &pngEncoder);
delete bitmap;

DeleteObject(hbitmap);
DeleteDC(memdc);
ReleaseDC(HWND_DESKTOP, hdc);