xref: /third_party/openGLES/extensions/SGIX/GLX_SGIX_pbuffer.txt

Name

    SGIX_pbuffer

Name Strings

    GLX_SGIX_pbuffer

Version

    Version 1.29, 2009/11/19

Number

    50

Dependencies

    SGIX_FBConfig is required
    SGIS_multisample affects the definition of this extension

Overview

    This extension defines pixel buffers (GLXPbuffers, or pbuffer for
    short). GLXPbuffers are additional non-visible rendering buffers for an
    OpenGL renderer.  GLXPbuffers are equivalent to GLXPixmaps with the
    following exceptions:

    1.	There is no associated X pixmap. Also, since a GLXPbuffer is a GLX
        resource, it may not be possible to render to it using X or an 
   	X extension other than GLX.

    2.	The format of the color buffers and the type and size of any
     	associated ancillary buffers for a GLXPbuffer can only be
        described with a GLXFBConfig -- an X Visual cannot be used.

    3.	It is possible to create a GLXPbuffer whose contents may be 
	asynchronously lost at any time.

    4.  GLXPbuffers can be rendered to using either direct or indirect
        rendering contexts.

    5.  The allocation of a GLXPbuffer can fail if there are insufficient
	resources (i.e., all the pbuffer memory has been allocated and 
	the implementation does not virtualize pbuffer memory.)

    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.  (But they 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, 
    or, in the case of a "unpreserved" pbuffer, as a result of X server 
    activity that changes its frame buffer requirements.


Issues

    *   Should the optimum width and height be fixed sizes or a multiple?
   
    *   Any better names for GLX_BUFFER_CLOBBER_MASK_SGIX, etc?

    UM  Should we add a command so an application can set the behavior
        when a deep window buffer (e.g., depth buffer or multisample buffer)
        is clobbered by a pbuffer? The choices would be "preserved" or
        "unpreserved". 
 
    *   When a pbuffer interferes with a window's ancillary buffer should
        the action -- swapped or saved -- be specified or left as
	implementation dependent?
    

New Procedures and Functions

    GLXPbuffer glXCreateGLXPbufferSGIX(Display *dpy,
				       GLXFBConfig config,
				       unsigned int width,
				       unsigned int height,
				       int *attrib_list);

    void glXDestroyGLXPbufferSGIX(Display *dpy,
			          GLXPbuffer pbuf);

    void glXQueryGLXPbufferSGIX(Display *dpy,
			        GLXPbuffer pbuf,
			        int attribute,
			        unsigned int *value);

    void glXSelectEventSGIX(Display *dpy,
	  		    GLXDrawable drawable,
			    unsigned long mask);

    void glXGetSelectedEventSGIX(Display *dpy,
  		                 GLXDrawable drawable,
			         unsigned long *mask);


New Tokens

    Accepted by the <attribute> parameter of glXGetFBConfigAttribSGIX:

        GLX_MAX_PBUFFER_WIDTH_SGIX		0x8016
        GLX_MAX_PBUFFER_HEIGHT_SGIX		0x8017
        GLX_MAX_PBUFFER_PIXELS_SGIX		0x8018
        GLX_OPTIMAL_PBUFFER_WIDTH_SGIX		0x8019
        GLX_OPTIMAL_PBUFFER_HEIGHT_SGIX		0x801A

    Returned by glXGetFBConfigAttribSGIX (when <attribute> is set to 
    GLX_DRAWABLE_TYPE_SGIX) and accepted by the <attrib_list> parameter of 
    glXChooseFBConfigSGIX (following the GLX_DRAWABLE_TYPE_SGIX token):

	GLX_PBUFFER_BIT_SGIX			0x00000004

    Accepted by the <attrib_list> parameter of glXCreateGLXPbufferSGIX
    and by the <attribute> parameter of glXQueryGLXPbufferSGIX:

        GLX_PRESERVED_CONTENTS_SGIX		0x801B
        GLX_LARGEST_PBUFFER_SGIX		0x801C

    Accepted by the <attribute> parameter of glXQueryGLXPbufferSGIX:

        GLX_WIDTH_SGIX 				0x801D
        GLX_HEIGHT_SGIX				0x801E
	GLX_EVENT_MASK_SGIX			0x801F

    Accepted by the <mask> parameter of glXSelectEventSGIX and returned
    in the <mask> parameter of glXGetSelectedEventSGIX:

	GLX_BUFFER_CLOBBER_MASK_SGIX  		0x08000000

    Returned in the <event_type> field of a "buffer clobber" event:

	GLX_DAMAGED_SGIX 			0x8020
	GLX_SAVED_SGIX				0x8021

    Returned in the <draw_type> field of a "buffer clobber" event:

        GLX_WINDOW_SGIX				0x8022
	GLX_PBUFFER_SGIX			0x8023

    Returned in the <mask> field of a "buffer clobber" event:

	GLX_FRONT_LEFT_BUFFER_BIT_SGIX		0x00000001
	GLX_FRONT_RIGHT_BUFFER_BIT_SGIX		0x00000002
	GLX_BACK_LEFT_BUFFER_BIT_SGIX		0x00000004
	GLX_BACK_RIGHT_BUFFER_BIT_SGIX		0x00000008
	GLX_AUX_BUFFERS_BIT_SGIX		0x00000010
	GLX_DEPTH_BUFFER_BIT_SGIX		0x00000020
	GLX_STENCIL_BUFFER_BIT_SGIX		0x00000040
	GLX_ACCUM_BUFFER_BIT_SGIX		0x00000080
	GLX_SAMPLE_BUFFERS_BIT_SGIX		0x00000100

Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.0 Specification (Rasterization)

    None

Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame buffer)

    None

Additions to Chapter 5 of the 1.0 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.0 Specification (State and State Requests)

    None

Additions to the GLX Specification

    [Add the following to section 3.2.2 on Configuration Management]

    [Add to glXGetFBConfigAttribSGIX]:

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

    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 GLX_OPTIMAL_PBUFFER_WIDTH_SGIX and
    GLX_OPTIMAL_PBUFFER_HEIGHT_SGIX to determine this width and height. If the
    values are zero then there is no optimal value.

    [Add to glXChooseFBConfigSGIX]:

    If GLX_OPTIMAL_PBUFFER_WIDTH_SGIX and/or GLX_OPTIMAL_PBUFFER_HEIGHT_SGIX 
    are specified in <attrib_list> then they are ignored.

    [Additions to tables 2 and 3, given in SGIX_FBConfig]

      Attribute                       Type        Description
      ---------                       ----        -----------
      GLX_BUFFER_SIZE                 integer     depth of the color buffer
      GLX_LEVEL                       integer     frame buffer level
      GLX_DOUBLEBUFFER                boolean     True if color buffers have
						  front/back pairs
      GLX_STEREO                      boolean     True if color buffers have
				      		  left/right pairs
      GLX_AUX_BUFFERS                 integer     number of auxiliary color
						  buffers 
      GLX_RED_SIZE                    integer     number of bits of Red if in
      						  RGB mode
      GLX_GREEN_SIZE                  integer     number of bits of Green if in
      						  RGB mode
      GLX_BLUE_SIZE                   integer     number of bits of Blue if in
      						  RGB mode
      GLX_ALPHA_SIZE                  integer     number of bits of Alpha if in
      						  RGB mode
      GLX_DEPTH_SIZE                  integer     number of bits in the depth
      						  buffer
      GLX_STENCIL_SIZE                integer     number of bits in the stencil
      						  buffer
      GLX_ACCUM_RED_SIZE              integer     number of bits of Red in the
       						  accumulation buffer
      GLX_ACCUM_GREEN_SIZE            integer     number of bits of Green in the
      						  accumulation buffer
      GLX_ACCUM_BLUE_SIZE             integer     number of bits of Blue in the
      accumulation buffer
      GLX_ACCUM_ALPHA_SIZE            integer     number of bits of Alpha in the
      						  accumulation buffer
      GLX_SAMPLE_BUFFERS_SGIS         integer     number of multisample buffers
      GLX_SAMPLES_SGIS                integer     number of samples stored in
						  each multisample buffer
      GLX_X_VISUAL_TYPE_EXT           integer     X visual type of the
						  associated visual
      GLX_TRANSPARENT_TYPE_EXT        enum        GLX_NONE_EXT,
                                                  TRANSPARENT_RGB_EXT, or 
                                                  TRANSPARENT_INDEX_EXT
      GLX_TRANSPARENT_INDEX_VALUE_EXT integer     transparent index value.
      GLX_TRANSPARENT_RED_VALUE_EXT   color       transparent color value.
      GLX_TRANSPARENT_GREEN_VALUE_EXT color       transparent color value.
      GLX_TRANSPARENT_BLUE_VALUE_EXT  color       transparent color value.
      GLX_TRANSPARENT_ALPHA_VALUE_EXT color       transparent color value.
      GLX_VISUAL_CAVEAT_EXT           enum        GLX_NONE_EXT or
      						  GLX_SLOW_VISUAL_EXT
      GLX_DRAWABLE_TYPE_SGIX          bitmask     mask indicating which GLX
      						  drawables are supported. Valid
						  bits are GLX_WINDOW_BIT_SGIX and
                                                  GLX_PIXMAP_BIT_SGIX
      GLX_RENDER_TYPE_SGIX            bitmask     mask indicating which OpenGL
      						  rendering modes are supported.
						  Valid bits are GLX_RGBA_BIT_SGIX
						  and GLX_COLOR_INDEX_SGIX.
      GLX_X_RENDERABLE_SGIX           boolean     True if X can render to drawable
      GLX_MAX_PBUFFER_WIDTH_SGIX      integer	  maximum width of GLXPbuffer
      GLX_MAX_PBUFFER_HEIGHT_SGIX     integer	  maximum height of GLXPbuffer
      GLX_MAX_PBUFFER_PIXELS_SGIX     integer	  maximum size of GLXPbuffer
      GLX_OPTIMAL_PBUFFER_WIDTH_SGIX  integer     best width to use when
						  creating pbuffer, or zero if
						  all widths are equally good.
      GLX_OPTIMAL_PBUFFER_HEIGHT_SGIX integer     best height to use when creating 
						  pbuffer, or zero if all
						  heights are equally good.

      Table 3: GLX configuration attributes for GLXFBConfigs (Note that
               GLX_RGBA and GLX_USE_GL are not supported)

      Attribute                               Default             Match Criteria
      ---------                               -------             --------------
      GLX_BUFFER_SIZE                         0                   minimum
      GLX_LEVEL                               0                   exact
      GLX_DOUBLEBUFFER                        don't care          exact
      GLX_STEREO                              False               exact
      GLX_AUX_BUFFERS                         0                   minimum
      GLX_RED_SIZE                            0                   minimum
      GLX_GREEN_SIZE                          0                   minimum
      GLX_BLUE_SIZE                           0                   minimum
      GLX_ALPHA_SIZE                          0                   minimum
      GLX_DEPTH_SIZE                          0                   minimum
      GLX_STENCIL_SIZE                        0                   minimum
      GLX_ACCUM_RED_SIZE                      0                   minimum
      GLX_ACCUM_GREEN_SIZE                    0                   minimum
      GLX_ACCUM_BLUE_SIZE                     0                   minimum
      GLX_ACCUM_ALPHA_SIZE                    0                   minimum
      GLX_SAMPLE_BUFFERS_SGIS                 0                   minimum
      GLX_SAMPLES_SGIS                        0                   minimum
      GLX_X_VISUAL_TYPE_EXT                   don't care          exact
      GLX_TRANSPARENT_PIXEL_EXT               GLX_NONE_EXT        exact
      GLX_TRANSPARENT_INDEX_VALUE_EXT         don't care          exact
      GLX_TRANSPARENT_RED_VALUE_EXT           don't care          exact
      GLX_TRANSPARENT_GREEN_VALUE_EXT         don't care          exact
      GLX_TRANSPARENT_BLUE_VALUE_EXT          don't care          exact
      GLX_TRANSPARENT_ALPHA_VALUE_EXT         don't care          exact
      GLX_VISUAL_CAVEAT_EXT                   don't care          exact
      GLX_DRAWABLE_TYPE_SGIX                  GLX_WINDOW_BIT_SGIX minimum
      GLX_RENDER_TYPE_SGIX                    GLX_RGBA_BIT_SGIX   minimum
      GLX_X_RENDERABLE_SGIX                   don't care          exact
      GLX_FBCONFIG_ID_SGIX                    don't care          exact
      GLX_MAX_PBUFFER_WIDTH_SGIX      	      0 		  minimum
      GLX_MAX_PBUFFER_HEIGHT_SGIX    	      0			  minimum
      GLX_MAX_PBUFFER_PIXELS_SGIX             0 		  minimum
      GLX_OPTIMAL_PBUFFER_WIDTH_SGIX          ignored		  ignored
      GLX_OPTIMAL_PBUFFER_HEIGHT_SGIX         ignored		  ignored

      Table 4: Default values and match criteria for GLX configuration
               attributes for GLXFBConfigs 


    [Add the following to section 3.2.3 on Offscreen Rendering]

    To create a GLXPbuffer call:

        GLXPbuffer glXCreateGLXPbufferSGIX(Display *dpy,
				           GLXFBConfig config,
				           unsigned int width,
				           unsigned int height,
				           int *attrib_list);

    This creates a single GLXPbuffer and returns its XID. <width> and <height>
    specify the pixel width and height of the rectangular pbuffer and
    <attrib_list> specifies a list of attributes for the pbuffer. Currently
    only two attributes can be specified in <attrib_list>:
    GLX_PRESERVED_CONTENTS_SGIX and GLX_LARGEST_PBUFFER_SGIX.

    <attrib_list> can be either NULL, in which case all the attributes assume
    their default values as described below.  If it not NULL then its format is
    similar to the attribute list paramter of glXChooseFBConfigSGIX: each
    attribute is immediately followed by the corresponding desired value and the
    list is terminated with None.

    Use GLX_LARGEST_PBUFFER_SGIX to get the largest available pbuffer when
    the allocation of the pbuffer would otherwise fail.  The width or height
    of the allocated pbuffer never exceed <width> and <height>,
    respectively. Use glXQueryGLXPbufferSGIX to retrieve the dimensions of the
    allocated pbuffer.  By default, GLX_LARGEST_PBUFFER_SGIX is False.
    
    If the GLX_PRESERVED_CONTENTS_SGIX attribute is set to False in
    <attrib_list>, then an "unpreserved" pbuffer is created and the contents
    of the pbuffer may be lost at any time. If this attribute is not
    specified, or if it is specified as True in <attrib_list>, then when a
    resource conflict occurs the contents of the pbuffer will be preserved
    (most likely by swapping out portions of the buffer to main memory).  In
    either case, the client can register to receive a "buffer clobber" event
    which is generated when the pbuffer contents have been preserved or have
    been damaged. (See the event description.)

    The resulting pbuffer will contain color buffers and ancillary as
    specified by <config>.  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 glXSwapBuffers.  The contents
    of the back buffers are undefined after such a swap.  (Pbuffers are
    the same as windows in this respect.)

    Any GLX rendering context created with a GLXFBConfig or X Visual that is
    "compatible" with the <config> may be used to render into the pbuffer. (See
    description of glXCreateContextWithConfigSGIX, glXMakeCurrent and
    glXMakeCurrentReadSGI for definition of "compatible".)  If a pbuffer is
    created with GLX_PRESERVED_CONTENTS_SGIX set to False, then portion of the
    buffer contents may be lost at any time due to frame buffer resource
    conflicts.  Once the contents of a "non preserved" pbuffer has been lost it
    is considered to be in a "damaged" state.  It is not an error to render to a
    pbuffer that is in this state but the effect of rendering to it is
    undefined. It is also not an error to query the pixel contents of such a
    pbuffer, but the values of the returned pixels are undefined.  Note, that
    while this specification allows for non preserved pbuffers to be damaged as
    a result of other pbuffer activity, the intent is to only have visible
    windows activity "damage" pbuffers.

    Since the contents of a "unpreserved" pbuffer can be lost at anytime 
    with only asynchronous notification (via the "buffer clobber" event), the 
    only way a client can guarantee that valid pixels are read back with 
    glReadPixels is by grabbing the X server. (Note that this operation is 
    potentially expensive and should not be done frequently. Also, since this 
    locks out other X clients, it should only be done for short periods of 
    time.) Clients that don't wish to do this can check if the data returned 
    by glReadPixels is valid by calling XSync and then checking the event 
    queue for "buffer clobber" events (assuming that these events had been 
    pulled off of the queue prior to the glReadPixels call). 
    
    When glXCreateGLXPbufferSGIX fails to create a GLXPbuffer due to
    insufficient resources, a BadAlloc error is generated and None is
    returned. If <config> is not a valid GLXFBConfig then a GLXBadFBConfigSGIX
    error is generated; if <config> does not support GLXPbuffers then a BadMatch
    error is generated.

    A GLXPbuffer is destroyed by calling:

        void glXDestroyGLXPbufferSGIX(Display *dpy,
			              GLXPbuffer pbuf);

    The GLXPbuffer will be destroyed once it is no longer current to any
    client. When a GLXPbuffer is destroyed, any memory resources that are
    attached to it are freed, and its XID is made available for reuse.

    If <pbuf> is not a valid GLXPbuffer then a GLXBadPbufferSGIX error
    is generated.

    To query an attribute associated with a GLXPbuffer call

        void glXQueryGLXPbufferSGIX(Display* dpy,
			            GLXPbuffer pbuf,
			            int attribute,
			            unsigned int *value);

    <attribute> must be set to one of GLX_WIDTH_SGIX, GLX_HEIGHT_SGIX,
    GLX_PRESERVED_CONTENTS_SGIX, GLX_LARGEST_PBUFFER_SGIX, or
    GLX_FBCONFIG_ID_SGIX.

    To get the GLXFBConfig for a GLXPbuffer, first retrieve the i.d.  for the
    FBConfig and then call glXChooseFBConfigSGIX.

    If <pbuf> is not a valid GLXPbuffer then a GLXBadPbufferSGIX error is
    generated.


    [Add new section, Events]

    A client can ask to receive GLX events on a window or GLXPbuffer.

      void glXSelectEventSGIX(Display *dpy,
	  		      GLXDrawable drawable,
			      unsigned long mask);

    Currently only one GLX event, GLX_BUFFER_CLOBBER_MASK_SGIX, can be selected:

      typdef struct {
	int event_type;	      /* GLX_DAMAGED_SGIX or GLX_SAVED_SGIX */
        int draw_type;        /* GLX_WINDOW_SGIX or GLX_PBUFFER_SGIX */
	unsigned long serial; /* # of last request processed by server */
	Bool send_event;      /* event was generated by a SendEvent request */
        Display *display;     /* display the event was read from */
        GLXDrawable drawable; /* i.d. of Drawable */
        unsigned int mask;    /* mask indicating which buffers are affected*/
        int x, y;
        int width, height;
        int count;            /* if nonzero, at least this many more */
      } GLXBufferClobberEventSGIX;

    A single X server operation can cause several "buffer clobber" events to be
    sent. (e.g., a single pbuffer may be damaged and cause multiple "buffer
    clobber" events to be generated). Each event specifies one region of the
    GLXDrawable that was affected by the X Server operation. <mask> indicates
    which color or ancillary buffers were affected. All the "buffer clobber"
    events generated by a single X server action are guaranteed to be contiguous
    in the event queue. The conditions under which this event is generated and
    the event <type> varies, depending on the type of the GLXDrawable.

    For "preserved" pbuffers, a "buffer clobber" event, with <type>
    GLX_SAVED_SGIX, is generated whenever the contents of a pbuffer has to be
    moved to avoid being damaged.  The event(s) describes which portions of the
    pbuffer were affected. Clients who receive many "buffer clobber" events,
    referring to different save actions, should consider freeing the pbuffer
    resource in order to prevent the system from thrashing due to insufficient
    resources.

    For an "unpreserved" pbuffer a "buffer clobber" event, with <type>
    GLX_DAMAGED_SGIX, is generated whenever a portion of the pbuffer becomes
    invalid.

    For Windows, "buffer clobber" events, with <type> GLX_DAMAGED_SGIX or
    GLX_SAVED_SGIX, occur whenever an ancillary buffer, associated with the
    window, gets clobbered or moved out of offscreen memory. The event contains
    information indicating which color or ancillary buffers, and which
    portions of those buffers, were affected.

    Calling glXSelectEventSGIX overrides any previous event mask that was set by
    the client for <drawable>. Note that a separate event mask is maintained for
    each client that requested "clobber events" for <drawable>.

    If <drawable> is not a valid GLXPbuffer or a valid Window, a GLXBadDrawable
    error is generated.

    To find out which GLX events are selected for a window or GLXPbuffer call

      void glXGetSelectedEventSGIX(Display *dpy,
	  		           GLXDrawable drawable,
			           unsigned long *mask);


GLX Protocol

	Four new GLX protocol commands are added.

	CreateGLXPbufferSGIX
	    1		CARD8		opcode (X assigned)
	    1		17		GLX opcode (glXVendorPrivateWithReply)
	    2		8 + 2n		request length
	    4		65543		vendor specific opcode
	    4				unused
	    4		CARD32		screen
	    4		GLX_FBCONFIG	fbconfig
	    4		GLX_PBUFFER	pbuffer
	    4		CARD32		width
	    4		CARD32		height
	    8 * n	LISTofATTRIB	properties
	   
	     Where n is the number of token/value pairs.  Each token value is
	     either a CARD32, BOOL32, or INT32, followed by an attribute
	     value which is also either a CARD32, BOOL32, or INT32.


	DestroyGLXPbufferSGIX
	    1		CARD8		opcode (X assigned)
	    1		17		GLX opcode (glXVendorPrivateWithReply)
	    2		4		request length
	    4		65544		vendor specific opcode
	    4				unused
	    4		GLX_PBUFFER	pbuffer

	
	ChangeDrawableAttributesSGIX
	    1		CARD8		opcode (X assigned)
	    1		17		GLX opcode (glXVendorPrivateWithReply)
	    2		4 + 2*n		request length
	    4		65545		vendor specific opcode
	    4				unused
	    4		GLX_DRAWABLE	drawable
	    8 * n	LISTofATTRIB	properties

	     Where n is the number of token/value pairs.  Each token value is
	     either a CARD32, BOOL32, or INT32, followed by an attribute
	     value which is also either a CARD32, BOOL32, or INT32.


	GetDrawableAttributesSGIX
	    1		CARD8		opcode (X assigned)
	    1		17		GLX opcode (glXVendorPrivateWithReply)
	    2		4 + 2n		request length
	    4		65546		vendor specific opcode
	    4				unused
	    4		GLX_DRAWABLE	drawable
	  =>
	    1		1		reply
	    1				unused
	    2		CARD16		sequence number
	    4		2*n		length
	    4		CARD32		numAttribs
	    20				unused
	    8 * n	LISTofATTRIB	properties
	
	     Where n is the number of token/value pairs.  Each token value is
	     either a CARD32, BOOL32, or INT32, followed by an attribute
	     value which is also either a CARD32, BOOL32, or INT32.

    One new event is added:

	BEC is the base event code for the extension, as returned by
	XQueryExtension.

	GLX_BUFFER_CLOBBER_MASK_SGIX
	    1		BEC+16		code
	    1				unused
	    2		CARD16		sequence number
	    2		CARD16		event_type
			0x8017		GLX_DAMAGED_SGIX
			0x8018		GLX_SAVED_SGIX
	    2		CARD16		draw_type
			0x8019		GLX_WINDOW_SGIX
                        0x801A		GLX_PBUFFER_SGIX
	    4		GLX_DRAWABLE    drawable
            4		BITFIELD	mask
	    2		CARD16		x
	    2		CARD16		y
	    2		CARD16		width
	    2		CARD16		height
	    2		CARD16		count
	    6				unused

Dependencies on SGIS_multisample

    If SGIS_multisample is not supported, references to
    GLX_SAMPLE_BUFFERS_BIT_SGIX in this document are invalid and should be
    ignored.

Errors

    One new GLX errors is introduced:

        GLXBadPbufferSGIX

New State

    None

New Implementation Dependent State

    None

Revision History

    Version 1.29, 2009/11/19 - clarify that BEC in the
    GLX_BUFFER_CLOBBER_MASK_SGIX event protocol is returned from
    XQueryExtension, matching the GLX_ARB_create_context spec language.

    Version 1.28, 1997/03/20 - final spec as shipped by SGI.