PRB: Antialiased Polygons Not Drawn in OpenGL Antipoly Sample

 Last reviewed: November 16, 1995
Article ID: Q139653
The information in this article applies to:
  • Microsoft Win32 Software Development Kit (SDK) for: 

        - Microsoft Windows NT version 3.51
    - Microsoft Windows 95 version 4.0

SYMPTOMS

The antipoly sample in OpenGL SDK BOOK directory is unable to draw antialised polygons with the generic implementation of Windows NT and Windows 95 OpenGL.

CAUSE

The technique used in this sample uses alpha values rather than depth values to determine what to write to the color buffer. You need to first turn off the depth buffer, choose the (GL_SRC_ALPHA_SATURATE, GL_ONE) blend function, and then draw polygons from front to back. But the generic Windows NT and Windows 95 OpenGL implementation does not provide alpha buffer. The cAlphaBits field of the pixel format descriptor for all generic pixel formats is 0.

RESOLUTION

The sample will work if an OpenGL accelerator card is present and additional device pixel formats provided by the OpenGL accelerator card support alpha buffer.

STATUS

This behavior is by design.

Additional reference words: 4.00 Windows 95
KBCategory: kbprg kbprb
KBSubcategory: GdiOpenGL

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Last reviewed: November 16, 1995
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