draw a graphic in the memory

OpenGL OpenGL: Advanced Coding
1

I want draw a graphic in the memory, needn’t to display.I use CreateCompatibleDC() to create a hDC, but while I use SetPixelFormat() to set the pixelformat for the hDC, the function fails. what is the problem? Can anyone help me?
My codes are follow:
void CGLTest::Init()
{
PIXELFORMATDESCRIPTOR pfd;
int n;
HGLRC hrc;
//SourceDC is a global variable, type is CDC
//tempHDC is a global variable, type is HDC
SourceDC.CreateCompatibleDC(NULL);
tempHDC=SourceDC.GetSafeHdc();

if(!bSetupPixelFormat())
	return;

n=::GetPixelFormat(tempHDC);
: [img]http://www.opengl.org/discussion_boards/ubb/biggrin.gif[/img]escribePixelFormat(tempHDC, n, sizeof(pfd), &pfd);
hrc=wglCreateContext(tempHDC);
wglMakeCurrent(tempHDC,hrc);

}

BOOL CGLTest::bSetupPixelFormat()
{
static PIXELFORMATDESCRIPTOR pfd=
{
sizeof(PIXELFORMATDESCRIPTOR),
1,
PFD_DRAW_TO_WINDOW|PFD_DRAW_TO_BITMAP|PFD_SUPPORT_OPENGL,
PFD_TYPE_RGBA,
24,
0,0,0,0,0,0,
0,
0,
0,
0,0,0,0,
32,
0,
0,
PFD_MAIN_PLANE,
0,
0,0,0
};
int pixelformat;

if((pixelformat=ChoosePixelFormat(tempHDC,&pfd))==0)
{
	MessageBox("ChoosePixelFormat falled");
	return FALSE;
}
if(SetPixelFormat(tempHDC,pixelformat,&pfd)==FALSE)
{
	MessageBox("SetPixelFormat falled");
	return FALSE;
}
return TRUE;

}

2

Not sure, but ChoosePixelformat sucks.
You’d better enumerate the pixelformats and select one yourself.
What happens if you don’t specify PFD_DRAW_TO_WINDOW?
OpenGL window classes need WS_CLIPCHILDREN and WS_CLIPSIBLINGS set, not sure if that is necessary for memory DCs.

3

Have you tried using only one of the flags PFD_DRAW_TO_WINDOW and PFD_DRAW_TO_BITMAP? I’m not sure, but since these are mutually exclusive, specifying both might be the problem.

Edit: I just noticed you said that it is SetPixelFormat (not ChoosePixelFormat) that fails. Are you sure you’re not setting the pixel format more than once for a particular window? Check GetlastError to see exactly what the problem is.

[This message has been edited by Aaron (edited 10-24-2002).]

4

iirc, only one of this flags can be set.
When PDF_DRAW_TO_BITMAP set, OpenGL work in software emulation mode.

5

Thanks.
I have tried using only PFD_DRAW_TO_BITMAP, but the problem is still there. my programm doesn’t work.
Any other suggestion?

Xue

6

When I need to draw a scene in memory without rendering it (in non extended OpenGL) I use almost the same init sequence as you except that I use PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL| PFD_DOUBLEBUFFER for dwFlags member. Then I draw the scene, don’t do SwapBuffers and use:

glReadBuffer(GL_BACK);
// read the pixels into the image
glReadPixels(0, 0, img->width, img->height,
	GL_BGR_EXT,
	GL_UNSIGNED_BYTE,
	img->data);

to read the image from the backbuffer. Seems to work for me (if you need more speed you can use some specific extensions)…

7

There are annoying problems with the window being occluded though. A screesaver, any window or tooltip will screw up the rendering. I noticed just yesterday that if I minimise the window (even during rendering) the rendering comes out just fine. It’s a bit hacky but I think you should be able to render off-screen by rendering to a minimised window. Perhaps something else along the same lines is possible.

Cheers,
Madoc

8

Draw into a pbuffer and get the bitmap with glReadPixels. All current OpenGL implementations support the pbuffer extention.
There is a very easy to use pbuffer C++ class at http://www.cfxweb.net/~delphigl/downloads.htm