Obtaining HBitmap from HDC

[Andonic]

Hello all,
I'm trying to resize the image of a CStatic to the size of the CStatic itself, because usually the CStatic resizes automatically to the size of the picture, and I want just the opposite. For that purpose, I have tried to use the CDC function StretchBlt to resize the containing HBITMAP from the original size to the size of the CStatic.

This works well but when trying to obtain the HBITMAP from the resized CDC i just get a black image but with the correct size.

void WriteBmp(char* name,int W,int H,int Bpp,int* data)
{
  BITMAPINFO Bmi={0};
  Bmi.bmiHeader.biSize      = sizeof(BITMAPINFOHEADER);
  Bmi.bmiHeader.biWidth      =  W;
  Bmi.bmiHeader.biHeight      = H;
  Bmi.bmiHeader.biPlanes      = 1;
  Bmi.bmiHeader.biBitCount    = Bpp; 
  Bmi.bmiHeader.biCompression  = BI_RGB;
  Bmi.bmiHeader.biSizeImage    = W*H*Bpp/8; 


  FILE* image = fopen (name,"wb");
  if(image==0)
    return;
  int h = abs(Bmi.bmiHeader.biHeight);
  int w = abs(Bmi.bmiHeader.biWidth);
  Bmi.bmiHeader.biHeight=-h;
  Bmi.bmiHeader.biWidth=w;
  int sz = Bmi.bmiHeader.biSizeImage;

  BITMAPFILEHEADER bfh;
  bfh.bfType=('M'<<8)+'B'; 
  bfh.bfOffBits=sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER); 
  bfh.bfSize=sz+bfh.bfOffBits; 
  bfh.bfReserved1=0; 
  bfh.bfReserved2=0; 
   
  fwrite(&bfh,sizeof(bfh),1,image);
  fwrite(&Bmi.bmiHeader,sizeof(BITMAPINFOHEADER),1,image);
  fwrite(data,sz,1,image);
  fclose(image);
}

int GuardarHBITMAPenFichero(HBITMAP hbmp,CString nombreFichero)
{
        /// Save HBITMAP in the file nombreFichero
	
	BITMAP bm;
	
	GetObject(hbmp,sizeof(BITMAP),&bm);

	
	int Bps = bm.bmBitsPixel;
	int Width  = bm.bmWidth;
	int Height  = bm.bmHeight;

	int size = Bps/8 * ( Width * Height );
	BYTE *lpBits = new BYTE[size];

	BITMAPINFOHEADER bInfoHeader; 
	BITMAPINFO bInfo;
	::ZeroMemory(&bInfoHeader,sizeof(BITMAPINFOHEADER));
	bInfoHeader.biSize = sizeof(BITMAPINFOHEADER);     
	bInfoHeader.biWidth = Width;
	bInfoHeader.biHeight = -Height;// si negativo origen en la parte superior izquierda, si positivo en la parte inferior izquierda
	bInfoHeader.biPlanes = 1;
	bInfoHeader.biBitCount = 24 /*bit per pixel 24,32 */ ;
	bInfoHeader.biCompression = 0;  //RGB
	bInfoHeader.biSizeImage = bInfoHeader.biWidth*bInfoHeader.biHeight*bInfoHeader.biBitCount/8;

	bInfo.bmiHeader=bInfoHeader;
	HDC memDC = ::GetDC(0);
	HBITMAP OldBM = (HBITMAP)::SelectObject(memDC, hbmp );

	::GetDIBits(
		memDC,           // manipulador de contexto de dispositivo
		hbmp,      // manipulador de mapa de bits
		0,   // primera lÃ*nea a colocar en el mapa de bits de destino
		Height,   // número de lÃ*neas a copiar
		lpBits,    // dirección del array de bits del mapa de bits
		&bInfo, // dirección de estructura con datos del mapa de bits
		DIB_RGB_COLORS    // RGB o Ã*ndices de paleta
   );
	// lpbits es un puntero a byte a la ventana que quieres capturar
	
	
	WriteBmp((char*)(LPCSTR)nombreFichero,Width,Height,24,(int*)lpBits);

	delete [] lpBits;
	::DeleteDC(memDC);
	
	return 0;
}




HBITMAP Asignar(CStatic *panel,HBITMAP imagen)
{
      /// This function draws the HBITMAP in the panel with the size of the panel
	RECT r;
	BITMAP bm;

	GetObject(imagen,sizeof(BITMAP),&bm);

	int Width  = bm.bmWidth;  // width of the original image
	int Height  = bm.bmHeight; // height of the original image
	
	HBITMAP hResized_image;

		
	panel->GetWindowRect(&r); // size of the panel where we'll draw the image
	panel->ScreenToClient(&r);
	
	HDC panel_hdc;
	panel_hdc=panel->GetDC()->GetSafeHdc(); // HDC of the panel

	HDC DCOrigen = ::CreateCompatibleDC ( panel_hdc ); // We create a compatible HDC to draw the image in, this DCs' dimensions will be the image's ones.

	::SelectObject(DCOrigen,imagen); //We select the image into the HDC DCOrigen

/// here the image is resized
	StretchBlt(
			panel_hdc,      // manipulador del contexto de dispositivo de destino
			0, // coordenada x de la esquina superior izquierda del rectángulo de destino
			0, // coordenada y de la esquina superior izquierda del rectángulo de destino
			r.right-r.left,   // anchura del rectángulo de destino
			r.bottom-r.top,  // altura del rectángulo de destino
			DCOrigen,       // manipulador del contexto de dispositivo de origen
			0,  // coordenada x de la esquina superior izquierda del rectángulo de origen 
			0,  // coordenada y de la esquina superior izquierda del rectángulo de origen
			Width ,    // anchura del rectángulo de origen
			Height,   // altura del rectángulo de origen
			SRCCOPY       // código de operación de rastreo
	);

// I'm trying to get the HBITMAP of the resized HDC

	hResized_image=(HBITMAP)::GetCurrentObject(panel_hdc,           // handle to DC
	//											OBJ_BITMAP   // object type
	//											);

/// Save into a file  , save a black image
GuardarHBITMAPenFichero(hResized_image,"c:\\resized_HBITMAP.bmp");

/////
// Free resources ...
 
/////
	return resized_image;

}

The problem is that i haven't been able to get the correct HBITMAP of the resized HDC , when i save it in a file the result is a black image. Why doesn't this work? is there another way that works to get the HBITMAP from a HDC?

[zerver]

You must select a bitmap into the DC before calling StretchBlt.

1. Create a new DC compatible with panel_hdc. (CreateCompatibleDC)
2. Create a bitmap compatible with panel_hdc. (CreateCompatibleBitmap)
3. Select this bitmap into the new DC.
4. Do a StretchBlt of DCOrigen onto the new DC. (this will draw on your bitmap)
5. Do a BitBlt the of the new DC onto panel_dc. (this will copy the resized bitmap to screen)

If any problems, read here:

http://www.codeguru.com/forum/showthread.php?t=410645

[Andonic]

Thanks a lot. I have added the Bitblt and the new HDC and finally i have obtained the HBITMAP for the CStatic::SetBitmap function, so the refresh works well.

HBITMAP Asignar(CStatic *panel,HBITMAP imagen)
{
        // Draw a HBITMAP into a CStatic
	RECT r;
	BITMAP bm;
	GetObject(imagen,sizeof(BITMAP),&bm);
	int Bps = bm.bmBitsPixel;
	int Width  = bm.bmWidth;
	int Height  = bm.bmHeight;
	
	HBITMAP hResized_image;

	panel->GetWindowRect(&r);
	panel->ScreenToClient(&r);
	
	HDC panel_hdc;
	panel_hdc=panel->GetDC()->GetSafeHdc();
	
	HDC DCOrigen = ::CreateCompatibleDC ( panel_hdc ); // DC for the image with the original size
	HBITMAP hbitmap_anterior=(HBITMAP)::SelectObject(DCOrigen,imagen);

	HDC hDC_new = ::CreateCompatibleDC ( panel_hdc );
	hImagen_escalada = ::CreateCompatibleBitmap ( panel_hdc, r.right-r.left, r.bottom-r.top );
	HGDIOBJ hOld = SelectObject(hDC_new, hResized_image);
	
	StretchBlt(
			hDC_new,      // manipulador del contexto de dispositivo de destino
			0, // coordenada x de la esquina superior izquierda del rectángulo de destino
			0, // coordenada y de la esquina superior izquierda del rectángulo de destino
			r.right-r.left,   // anchura del rectángulo de destino
			r.bottom-r.top,  // altura del rectángulo de destino
			DCOrigen,       // manipulador del contexto de dispositivo de origen
			0,  // coordenada x de la esquina superior izquierda del rectángulo de origen 
			0,  // coordenada y de la esquina superior izquierda del rectángulo de origen
			Width ,    // anchura del rectángulo de origen
			Height,   // altura del rectángulo de origen
			SRCCOPY       // código de operación de rastreo
	);

	BitBlt(panel_hdc, 0, 0, r.right - r.left, r.bottom - r.top,hDC_new, 0, 0, SRCCOPY);
	hResized_image= (HBITMAP)::SelectObject(hDC_new, hOld);

 	panel->SetBitmap(hResized_image);  // it manage the image to be refreshed 

	::SelectObject(DCOrigen, hbitmap_anterior);
	::DeleteDC(DCOrigen); // Destructora para CreateCompatibleDC
	
	return hResized_image;
}

Only two more questions, why is neccesary other HDC? Is Bitblt really neccesary if i use SetBitmap? Well, thank you again :-) as i have said already works.

[zerver]

Thanks a lot. I have added the Bitblt and the new HDC and finally i have obtained the HBITMAP for the CStatic::SetBitmap function, so the refresh works well.

Only two more questions, why is neccesary other HDC? Is Bitblt really neccesary if i use SetBitmap? Well, thank you again :-) as i have said already works.

A DC obtained using GetDC() is a so called "window DC" because it draws directly onto a window. You should not select a bitmap into a window DC.
Instead you create a compatible DC. It will be called a "memory DC" after you have selected the bitmap into it because it then draws onto memory.

If your only goal is to obtain a bitmap for use with SetBitmap, BitBlt is not necessary. I got the impression that the bitmap should be drawn onto screen as well.