Notes

Outline

OpenGL Multisample Sébastien Dominé

Agenda

Getting started... Based upon WGL_ARB_multisample Requires WGL_ARB_extensions_string and WGL_ARB_pixel_format Allows the application to get a multisampled frame buffer with a given number of samples per pixels Multisample filtering taxonomy: Sample : a subpixel frame buffer sample containing color, depth, and stencil information Tap : source of data for filtering Not the same as Supersampling – no notion of Tap

Multisample pixel

Multisample frame buffer It doesn’t happen for free More video memory is required:

How does it happen in OpenGL WGL_ARB_extensions_string WGL_ARB_pixel_format WGL_ARB_multisample GL_ARB_multisample GL_NV_multisample_filter_hint

WGL extension string WGL_ARB_extensions_string Don’t search the GL string for it – it is a WGL extension! Query for the entry point: wglGetEntensionsStringARB = wglGetProcAddress(“wglGetEntensionsStringARB”) wglGetEntensionsStringARB != NULL const char * wgl_ext_string = wglGetEntensionsStringARB(hDC);

New way to query pixel formats - Trick Multisample window in OpenGL: WGL_ARB_pixel_format and WGL_ARB_multisample extensions Need to create a dummy window to collect the extensions and entry points bound to a hardware accelerated context Then you can create the multisample window

New way to query pixel formats Win32 GL function ChoosePixelFormat, DescribePixelFormat are not extensible WGL_ARB_pixel_format Array of paired attribute/value to describe a pixel format Adds wglChoosePixelFormat and wglGetPixelFormatAttrib{fi}vARB – still uses SetPixelFormat Well defined matching comparison function per attribute

WGL ARB pixel format  BOOL wglChoosePixelFormatARB( HDC hdc,                                                               const int *piAttribIList,                                                               const FLOAT *pfAttribFList,                                                               UINT nMaxFormats,                                                               int *piFormats,                                                               UINT *nNumFormats); float fAttributes[] = {0, 0}; int   pixelFormat; UINT  numFormats; int   iAttributes[] = { WGL_DOUBLE_BUFFER_ARB, TRUE,          WGL_ACCELERATION_ARB,  WGL_FULL_ACCELERATION_ARB,          0, 0 }; status = wglChoosePixelFormat(hDC, iAttributes, fAttributes,                         1, &pixelFormat, &numFormats);

WGL ARB pixel format - Trick If wglChoosePixelFormat succeeds, it doesn’t mean it found a matching PFD – need to test numFormats

WGL ARB pixel format BOOL wglGetPixelFormatAttribivARB( HDC hdc,                                                                    int iPixelFormat,                                                                    int iLayerPlane,                                                                    UINT nAttributes,                                                                    const int *piAttributes,                                                                    int *piValues); int iAttributes[3]; int iResults[3]; iAttributes[0] = WGL_DOUBLE_BUFFER_ARB; iAttributes[1] = WGL_ACCELERATION_ARB; iAttributes[2] = 0; status = wglGetPixelFormatAttribivARB(hDC, pxlfmt, 0, 2,                                       iAttributes, iResults);

Query for a multisample pixelformat WGL_ARB_multisample Add the following pixel format attributes: WGL_SAMPLE_BUFFERS_ARB WGL_SAMPLES_ARB  iAttributes[0] = WGL_SAMPLE_BUFFERS_ARB;  iAttributes[1] = 1;  iAttributes[2] = WGL_SAMPLES_ARB;  iAttributes[3] = 2; // could be 4 And call wglChoosePixelFormat...

Enable multisample We now have a multisample window Enabling/Disabling multisample rasterization: glEnable(GL_MULTISAMPLE_ARB) glDisable(GL_MULTISAMPLE_ARB)

GL_ARB_multisample Multisample Fragment Operations: Explicit Fragment Coverage Value void glSampleCoverageARB( GLclampf value, GLboolean invert) Assign Fragment Coverage Value based on Fragment Alpha Sub pixel Screen door transparency Force Fragment Alpha Value to 1

GL_NV_multisample_filter_hint NVIDIA extension explicitly exposes the different filters: glHint(GL_MULTISAMPLE_FILTER_HINT_NV, GL_FASTEST); glHint(GL_MULTISAMPLE_FILTER_HINT_NV, GL_NICEST); GeForce 3/4Ti:

GL_NV_multisample_filter_hint

GL_NV_multisample_filter_hint

GL_NV_multisample_filter_hint

Performance considerations Texture thrashing? If T&L or CPU bound – Multisample can be cheap Scalability forces the video options to enumerate all the modes Test case – Quake III

Performance considerations

Conclusion Easy to support Tremendous visual quality improvement Explicit control of the what should or shouldn’t be antialiased Doesn’t necessarily mean slowdown Much nicer than a Display Driver control panel toggle Demo NV Pixel Format 1.0