[osg-users] floating point pbuffers - not supported by current PixelBufferWin32 implementation

[Christian Buchner]

Hi all,

I spent the last 3 hours trying to coerce OSG to give me a floating point
pbuffer. Just setting the required bits for color components to 32 bits in
the graphicscontext traits isn't working.

Turns out, on nVidia cards you also have to give the
WGL_FLOAT_COMPONENTS_NV flag as "true" to get a valid pixel format on
Windows. The following code does this:

    std::vector<int> fAttribList;

    fAttribList.push_back(WGL_SUPPORT_OPENGL_ARB);
    fAttribList.push_back(true);
    fAttribList.push_back(WGL_PIXEL_TYPE_ARB);
    fAttribList.push_back(WGL_TYPE_RGBA_ARB);

    fAttribList.push_back(WGL_RED_BITS_ARB);
    fAttribList.push_back(32);
    fAttribList.push_back(WGL_GREEN_BITS_ARB);
    fAttribList.push_back(32);
    fAttribList.push_back(WGL_BLUE_BITS_ARB);
    fAttribList.push_back(32);
    fAttribList.push_back(WGL_ALPHA_BITS_ARB);
    fAttribList.push_back(32);
    fAttribList.push_back(WGL_STENCIL_BITS_ARB);
    fAttribList.push_back(8);
    fAttribList.push_back(WGL_DEPTH_BITS_ARB);
    fAttribList.push_back(24);
    fAttribList.push_back(WGL_FLOAT_COMPONENTS_NV);
    fAttribList.push_back(true);
    fAttribList.push_back(WGL_DRAW_TO_PBUFFER_ARB);
    fAttribList.push_back(true);
    fAttribList.push_back(WGL_DOUBLE_BUFFER_ARB);
    fAttribList.push_back(false);

    fAttribList.push_back(0);

    unsigned int nformats = 0;
    int format;
    WGLExtensions* wgle = WGLExtensions::instance();
    wgle->wglChoosePixelFormatARB(hdc, &fAttribList[0], NULL, 1, &format,
&nformats);
    std::cout << "Suitable pixel formats: " << nformats << std::endl;

On my GTX 970 card here this returns exactly one suitable pixel format (3
if you drop the DOUBLE_BUFFER_ARB requirement even)..

It seems that the implementation of PixelBufferWin32 cannot currently be
given any user-defined attributes to the wglChoosePixelFormatARB function.
Is this a capability that we should consider adding? Or should we
automatically sneak in this vendor specific flag if the color components
the traits specify have 32 bits and a previous call to
wglChoosePixelFormatARB returned 0 matches?

I am leaving this up for debate.

Is there a vendor-neutral alternative to the WGL_FLOAT_COMPONENTS_NV flag?

For now, I can simply patch my local copy of the OSG libraries to support
floating point pbuffers on nVidia cards.

Christian
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