Creating a HBITMAP from glReadPixels

Question

I need to create a HBITMAP from data returned by a glReadPixels() call:

HDC hCompDC = CreateCompatibleDC(NULL);
HDC hDC = GetDC();

m_hClipboardBitmap = CreateCompatibleBitmap(hDC, size.cx, size.cy);

if ( m_hClipboardBitmap == NULL )
{
    throw runtime_error( "Unable to create bitmap." );
}

HBITMAP hOldBm = (HBITMAP) SelectObject( hCompDC, m_hClipboardBitmap );

int numberOfBytes = 4 * size.cx * size.cy; 
unsigned char *pPixelData = new unsigned char[numberOfBytes];

::glReadPixels(minimum.x, minimum.y, size.cx, size.cy, GL_BGRA, GL_UNSIGNED_BYTE, pPixelData);

I tried using:

BITMAPINFOHEADER header; 
header.biWidth = size.cx; 
header.biHeight = size.cy; 
header.biSizeImage = numberOfBytes; 
header.biSize = sizeof(BITMAPINFOHEADER); 
header.biPlanes = 1; 
header.biBitCount =  4 * 8; // RGBA 
header.biCompression = 0; 
header.biXPelsPerMeter = 0; 
header.biYPelsPerMeter = 0; 
header.biClrUsed = 0; 
header.biClrImportant = 0;

HANDLE handle = (HANDLE)::GlobalAlloc (GHND, sizeof(BITMAPINFOHEADER) + numberOfBytes); 

if(handle != NULL) 
{ 
    char *pData = (char *) ::GlobalLock((HGLOBAL)handle); 

    memcpy(pData,&header,sizeof(BITMAPINFOHEADER)); 
    memcpy(pData + sizeof(BITMAPINFOHEADER), pPixelData, numberOfBytes); 

    ::GlobalUnlock((HGLOBAL)handle);

    OpenClipboard(); 
    EmptyClipboard(); 
    SetClipboardData(CF_DIB, handle); 
    CloseClipboard();
}

And that pastes into mspaint OK (so the data is good) but how on earth do I get it into a HBITMAP?!?!

My GDI32 skills are a little rusty, but have you looked at [CreateDIBitmap](http://msdn.microsoft.com/en-us/library/dd183491\(v=vs.85\).aspx)? I think it should just be `CreateDIBitmap(hDC,&header,CBM_INIT,pPixelData,&info,DIB_RGB_COLORS)` — user786653, Aug 24 '11 at 14:55
I already made an attempt at using CreateDIBitmap but it didn't work for me. I ended up using BITMAPFILEHEADER and BITMAPINFOHEADER and writing it to a .bmp directly. Would've been nicer had I been able to obtain a HBITMAP but nevermind. Back to C# it is... — user907285, Aug 25 '11 at 09:11

score 1 · Answer 1 · answered May 01 '14 at 11:11

Very old thread, but I wanted to give an answer, at least to keep it as a repository.

void WriteOpenGLPixelsToHBITMAP( HBITMAP dstHBITMAP, HDC dstDC, SIZE dims )
{

    BITMAPINFO bitmapInfo;
    {
        ::memset( &bitmapInfo, 0, sizeof( BITMAPINFO ) );
        bitmapInfo.bmiHeader.biSize        = sizeof( BITMAPINFOHEADER );
        bitmapInfo.bmiHeader.biPlanes      = 1;
        bitmapInfo.bmiHeader.biBitCount    = 32;
        bitmapInfo.bmiHeader.biCompression = BI_RGB;
        bitmapInfo.bmiHeader.biWidth       = dims.cx;
        bitmapInfo.bmiHeader.biHeight      = dims.cy;
        bitmapInfo.bmiHeader.biSizeImage   = dims.cx * dims.cy * 4; // Size 4, assuming RGBA from OpenGL
    }

    void    *bmBits = NULL;
    HDC     memDC   = ::CreateCompatibleDC( dstDC );
    HBITMAP memBM   = ::CreateDIBSection( NULL, &bitmapInfo, DIB_RGB_COLORS, &bmBits, NULL, 0 );


    ::glReadPixels( 0,
                    0,
                    dims.cx,
                    dims.cy,
                    GL_BGRA_EXT,
                    GL_UNSIGNED_BYTE,
                    bmBits );
    HGDIOBJ prevBitmap = ::SelectObject( memDC, memBM );
    HGDIOBJ obj        = ::SelectObject( dstDC, dstHBITMAP );

    // Remember that OpenGL origin is at bottom, left, but bitmaps are top, left
    if ( false == BitBlt( dstDC, 0 /*left*/, dims.cy /*top*/, dims.cx, dims.cy, memDC, 0, 0, SRCCOPY ) )
    {
        assert( false && "Failed to write pixels to HBitmap from OpenGL glReadPixels" );
    }

    ::SelectObject( memDC, prevBitmap );
    ::DeleteObject( memBM );
    ::DeleteDC( memDC );                    
}

As mentioned, be aware of image being inverted. You can swap SRCCOPY for SRCINVERT. Also you might want to make sure you are copying regions. The code above assumes that the region matches the viewport.

score 0 · Answer 2 · answered Aug 23 '11 at 09:43

0

Are you calling the function with the correct parameters. Check the documentation of the function: http://msdn.microsoft.com/en-us/library/dd183491(v=vs.85).aspx. Seems like you have swapped the parameter order and are passing a pointer to a pointer to the data.

-Timo

answered Aug 23 '11 at 09:43

Timo Heinäpurola

431
4
7

No that's not it, even swapping the params and just passing the pointer gives me nothing. I found a way to get a HANDLE (updated the example) but now don't know how to get that into a HBITMAP – user907285 Aug 24 '11 at 09:54

Creating a HBITMAP from glReadPixels

2 Answers2

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