extensions/EXT/WGL_EXT_pbuffer.txt

Name

    EXT_pbuffer

Name Strings

    WGL_EXT_pbuffer

Version

    Date: 1/27/1999   Version 1.6

Number

    171

Dependencies

    WGL_EXT_extensions_string is required.
    WGL_EXT_pixel_format is required.
    WGL_EXT_make_current_read affects the definition of this extension.

Overview

    This extension defines pixel buffers (pbuffer for short). Pbuffers are
    additional non-visible rendering buffers for an OpenGL renderer.
    Pbuffers are equivalent to a window that has the same pixel format
    descriptor with the following exceptions:

    1.  There is no rendering to a pbuffer by GDI.

    2.  The pixel format descriptors used for a pbuffer can only be those
        that are supported by the ICD.  Generic formats are not valid.

    4.  The allocation of a pbuffer can fail if there are insufficient
        resources (i.e., all the pbuffer memory has been allocated).

    The intent of the pbuffer semantics is to enable implementations to
    allocate pbuffers in non-visible frame buffer memory.  These
    pbuffers are intended to be "static" resources in that a program
    will typically allocate them only once rather than as a part of its
    rendering loop.  (Pbuffers should be deallocated when the program is
    no longer using them -- for example, if the program is iconified.)

    The frame buffer resources that are associated with a pbuffer are
    also static and are deallocated only when the pbuffer is destroyed.

New Procedures and Functions

    HPBUFFEREXT wglCreatePbufferEXT(HDC hDC,
                                    int iPixelFormat,
                                    int iWidth,
                                    int iHeight,
                                    const int *piAttribList);

    HDC wglGetPbufferDCEXT(HPBUFFEREXT hPbuffer);

    int wglReleasePbufferDCEXT(HPBUFFEREXT hPbuffer,
                               HDC hDC);

    BOOL wglDestroyPbufferEXT(HPBUFFEREXT hPbuffer);

    BOOL wglQueryPbufferEXT(HPBUFFEREXT hPbuffer,
                            int iAttribute,
                            int *piValue);

New Tokens

    Accepted by the <attribute> parameter of wglGetPixelFormatAttribivEXT,
    wglGetPixelFormatAttribfvEXT, and wglChoosePixelFormatEXT:

      WGL_DRAW_TO_PBUFFER_EXT              0x202D
      WGL_MAX_PBUFFER_PIXELS_EXT           0x202E
      WGL_MAX_PBUFFER_WIDTH_EXT            0x202F
      WGL_MAX_PBUFFER_HEIGHT_EXT           0x2030
      WGL_OPTIMAL_PBUFFER_WIDTH_EXT        0x2031
      WGL_OPTIMAL_PBUFFER_HEIGHT_EXT       0x2032

    Accepted by the <piAttribList> parameter of wglCreatePbufferEXT:

      WGL_PBUFFER_LARGEST_EXT              0x2033

    Accepted by the <iAttribute> parameter of wglQueryPbufferEXT:

      WGL_PBUFFER_WIDTH_EXT                0x2034
      WGL_PBUFFER_HEIGHT_EXT               0x2035

Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.2 Specification (Rasterization)

    None

Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations
and the Frame buffer)

    None

Additions to Chapter 5 of the 1.2 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.2 Specification (State and State Requests)

    None

Additions to the WGL Specification

    A pixel buffer (pbuffer) can be created with wglCreatePbufferEXT
    which returns a handle associated with the pbuffer.

      HPBUFFEREXT wglCreatePbufferEXT(HDC hDC,
                                      int iPixelFormat,
                                      int iWidth,
                                      int iHeight,
                                      const int *piAttribList);

    <hDC> specifies a device context for the device on which the pbuffer
    is created. <iPixelFormat> specifies a non-generic pixel format
    descriptor index.  Support for pbuffers may be restricted to
    specific pixel formats.  Use wglGetPixelFormatAttribivEXT or
    wglGetPixelFormatAttribfvEXT to query the WGL_DRAW_TO_PBUFFER_EXT
    attribute to determine which pixel formats support the creation of
    pbuffers.

    <iWidth> and <iHeight> specify the pixel width and height of the
    rectangular pbuffer.

    <piAttribList> is a list of attributes {type, value} pairs containing
    integer attribute values.  All of the attributes in the <piAttribList>
    are followed by the corresponding required value.  The list is
    terminated with a value of 0.

    The following attributes are supported by wglCreatePbufferEXT:

      WGL_PBUFFER_LARGEST_EXT     If this attribute is set to a
                                  non-zero value, the largest
                                  available pbuffer is allocated
                                  when the allocation of the pbuffer
                                  would otherwise fail due to
                                  insufficient resources.  The width
                                  or height of the allocated pbuffer
                                  never exceeds <iWidth> and <iHeight>,
                                  respectively.  Use wglQueryPbufferEXT
                                  to retrieve the dimensions of the
                                  allocated pbuffer.

    The resulting pbuffer will contain color buffers and ancillary
    buffers as specified by <iPixelFormat>.  Note that pbuffers use
    framebuffer resources so applications should consider deallocating
    them when they are not in use.

    It is possible to create a pbuffer with back buffers and to swap
    the front and back buffers by calling wglSwapLayerBuffers.  The
    contents of the back buffers after the swap depends on the
    <iPixelFormat>.  (Pbuffers are the same as windows in this respect.)

    When wglCreatePbufferEXT fails to create a pbuffer, NULL is returned.
    To get extended error information, call GetLastError.  Possible errors
    are as follows:

      ERROR_INVALID_PIXEL_FORMAT     Pixel format is not valid.
      ERROR_NO_SYSTEM_RESOURCES      Insufficient resources exist.
      ERROR_INVALID_DATA             <iWidth> or <iHeight> is negative
                                     or zero.
      ERROR_INVALID_DATA             <piAttribList> is not a valid attribute.

    To create a device context for the pbuffer, call

      HDC wglGetPbufferDCEXT(HPBUFFEREXT hPbuffer);

    where <hPbuffer> is a handle returned from a previous call to
    wglCreatePbufferEXT.  A device context is returned by
    wglGetPbufferDCEXT which can be used to associate a rendering
    context with the pbuffer.  Any rendering context created with
    a wglCreateContext that is "compatible" with the <iPixelFormat> may be
    used to render into the pbuffer. (See the description of
    wglCreateContext, wglMakeCurrent, and wglMakeCurrentReadEXT for a
    definition of "compatible".)

    When wglGetPbufferDCEXT fails, NULL is returned.  To get extended
    error information, call GetLastError. Possible errors are as follows:

      ERROR_INVALID_HANDLE           <hPbuffer> is not a valid handle.

    To release a device context obtained from a previous call to
    wglGetPbufferDCEXT, call

        int wglReleasePbufferDCEXT(HPBUFFEREXT hPbuffer,
                                   HDC hDC);

    If the return value is a value of 1, the device context was released.
    If the device context was not released, the return value is 0.  To get
    extended error information, call GetLastError. Possible errors are
    as follows:

      ERROR_INVALID_HANDLE           <hPbuffer> is not a valid handle.
      ERROR_DC_NOT_FOUND             <hDC> is not a valid DC.

    A pbuffer is destroyed by calling

      BOOL wglDestroyPbufferEXT(HPBUFFEREXT hPbuffer);

    The pbuffer is destroyed once it is no longer current to any rendering
    context.  When a pbuffer is destroyed, any memory resources that are
    attached to it are freed and its handle is no longer valid.

    If wglDestroyPbufferEXT fails, FALSE is returned.  To get extended
    error information, call GetLastError. Possible errors are as follows:

      ERROR_INVALID_HANDLE             <hPbuffer> is not a valid handle.

    To query the maximum width, height, or number of pixels in any given
    pbuffer for a specific pixel format, use wglGetPixelFormatAttribivEXT
    or wglGetPixelFormatAttribfvEXT with <attribute> set to one of
    WGL_MAX_PBUFFER_WIDTH_EXT, WGL_MAX_PBUFFER_HEIGHT_EXT, or
    WGL_MAX_PBUFFER_PIXELS_EXT.

    WGL_MAX_PBUFFER_WIDTH_EXT and WGL_MAX_PBUFFER_HEIGHT_EXT indicate the
    maximum width and height that can be passed into wglCreatePbufferEXT
    and WGL_MAX_PBUFFER_PIXELS_EXT indicates the maximum number of pixels
    (width x height) for a pbuffer.  Note that an implementation may
    return a value for WGL_MAX_PBUFFER_PIXELS_EXT that is less than the
    maximum width times the maximum height.  Also, the value for
    WGL_MAX_PBUFFER_PIXELS_EXT is static and assumes that no other
    pbuffers are contending for the framebuffer memory.  Thus it may
    not be possible to allocate a pbuffer of the size given by
    WGL_MAX_PBUFFER_PIXELS_EXT.

    On some implementations, there may be an optimum width and height
    to use when allocating a pbuffer. (For example, the implementation
    may use fixed size tiles to allocate pbuffers.)  Use
    WGL_OPTIMAL_PBUFFER_WIDTH_EXT and WGL_OPTIMAL_PBUFFER_HEIGHT to
    determine this width and height.  If the value returned is zero,
    there is no optimal width or height value.

    To query an attribute associated with a specific pbuffer, call

      BOOL wglQueryPbufferEXT(HPBUFFEREXT hPbuffer,
                              int iAttribute,
                              int *piValue);

    with <hPbuffer> set to a previously returned pbuffer handle.
    <iAttribute> must be set to one of WGL_PBUFFER_WIDTH_EXT or
    WGL_PBUFFER_HEIGHT_EXT.

    If wglQueryPbufferEXT fails, FALSE is returned.  To get extended
    error information, call GetLastError. Possible errors are as follows:

      ERROR_INVALID_HANDLE           <hPbuffer> is not a valid handle.
      ERROR_INVALID_DATA             <iAttribute> is not a valid attribute.

    Because there is no way to extend wgl, these calls are defined in the
    ICD and can be called by obtaining the address with wglGetProcAddress.
    Because this is not a GL extension, it is not included in the
    GL_EXTENSIONS string.

New State

    None

New Implementation Dependent State

    None