Using OpenGL to draw directly to a .NET Bitmap

I'd like to use OpenGL to draw directly to a .NET System::Drawing::Bitmap (C++/CLI). It seems like it should work like this:

char buf[48];
ZeroMemory( &buf, sizeof(buf));
System::Drawing::Bitmap bmp(
    4,
    4,
    12,
    Imaging::PixelFormat::Format24bppRgb,
    (System::IntPtr)buf);
Graphics^ g = Graphics::FromImage(%bmp);
HDC local_hdc = (HDC)((void*)g->GetHdc());
HGLRC local_hrc = wglCreateContext( local_hdc );
if(!wglMakeCurrent( local_hdc, local_hrc ))
    ShowError();

//Draw something with OpenGL ...

g->ReleaseHdc((IntPtr)((void*)local_hdc));

//Do something with the bitmap ...

But wglMakeCurrent fails with "The pixel format is invalid".

"Of course!" I think. "I just need to set the pixel format of the bitmap in such a way that OpenGL can render to it!"

Sadly, if I use the above Bitmap constructor, I'm limited to an enum of canned pixelformats, rather than a struct holding a bunch of attributes that I can OR together. And I don't think it's possible to set the pixel format after the fact.

Anyway, this way of doing things would be prettier than using a bunch of Win32 calls to make a Device-Independent Bitmap, but the canned 开发者_如何学Gopixel formats seem to omit any sort of GL-enabled anything, and I just fail. Can you succeed?

You probably don't want to draw directly into your buffer anyway, you'd lose all hardware acceleration which is the advantage of OpenGL to begin with. Instead:

• Set up an OpenGL context as usual, using video RAM.
• Draw into it, fully accelerated.
• Create a Bitmap.
• Call Bitmap::LockBits to get the pointer to the raw pixel memory.
• Use glCopyImage to read the results from video RAM into the Bitmap.

The PixelFormat passed to LockBits and glCopyImage in the last two steps need to match of course.