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DBE(3)                            X FUNCTIONS                           DBE(3)

       DBE - Double Buffer Extension

       The  Double  Buffer Extension (DBE) provides a standard way to utilize double-buffering within the framework of
       the X Window System.  Double-buffering uses two buffers, called front and back, which hold images.   The  front
       buffer is visible to the user; the back buffer is not.  Successive frames of an animation are rendered into the
       back buffer while the previously rendered frame is displayed in the front buffer.  When a new frame  is  ready,
       the  back and front buffers swap roles, making the new frame visible.  Ideally, this exchange appears to happen
       instantaneously to the user, with no visual artifacts.  Thus, only completely rendered images are presented  to
       the  user,  and remain visible during the entire time it takes to render a new frame.  The result is a flicker-
       free animation.

              Normal windows are created using XCreateWindow() or XCreateSimpleWindow(), which allocate a set of  win-
              dow attributes and, for InputOutput windows, a front buffer, into which an image can be drawn.  The con-
              tents of this buffer will be displayed when the window is visible.

              This extension enables applications to use double-buffering with a window.   This  involves  creating  a
              second  buffer, called a back buffer, and associating one or more back buffer names (XIDs) with the win-
              dow, for use when referring to (i.e., drawing to or reading from) the window's back  buffer.   The  back
              buffer name is a drawable of type XdbeBackBuffer.

              DBE provides a relative double-buffering model.  One XID, the window, always refers to the front buffer.
              One or more other XIDs, the back buffer names, always refer to the back buffer.  After  a  buffer  swap,
              the  window continues to refer to the (new) front buffer, and the back buffer name continues to refer to
              the (new) back buffer.  Thus, applications and toolkits that want to just  render  to  the  back  buffer
              always use the back buffer name for all drawing requests to the window.  Portions of an application that
              want to render to the front buffer always use the window XID for all drawing requests to the window.

              Multiple clients and toolkits can all use double-buffering on the same window.  DBE does not  provide  a
              request for querying whether a window has double-buffering support, and if so, what the back buffer name
              is.  Given the asynchronous nature of the X Window System, this would cause race  conditions.   Instead,
              DBE  allows multiple back buffer names to exist for the same window; they all refer to the same physical
              back buffer.  The first time a back buffer name is allocated for a window, the  window  becomes  double-
              buffered  and the back buffer name is associated with the window.  Subsequently, the window already is a
              double-buffered window, and nothing about the window changes when a new back buffer name  is  allocated,
              except  that the new back buffer name is associated with the window.  The window remains double-buffered
              until either the window is destroyed, or until all of the back buffer names for the window  are  deallo-

              In general, both the front and back buffers ae treated the same.  In particular, here are some important

                     Only one buffer per window can be visible at a time (the front buffer).

                     Both buffers associated with a window have the same visual type, depth, width, height, and  shape
                     as the window.

                     Both  buffers  associated  with  a window are "visible" (or "obscured") in the same way.  When an
                     Expose event is generated for a window, this  event  is  considered  to  apply  to  both  buffers
                     equally.   When a double-buffered window is exposed, both buffers are tiled with the window back-
                     ground.  Even though the back buffer is not visible, terms such as  obscure  apply  to  the  back
                     buffer as well as to the front buffer.

                     It  is  acceptable at any time to pass an XdbeBackBuffer in any function that expects a drawable.
                     This enables an application to draw directly into XdbeBackBuffer in the same fashion as it  would
                     draw into any other drawable.

                     It is an error (Window) to pass an XdbeBackBuffer in a function that expects a Window.

                     An XdbeBackBuffer will never be sent in a reply, event, or error where a Window is specified.

                     If  backing-store  and save-under applies to a double-buffered window, it applies to both buffers

                     If the XClearArea() or XClearWindow() function is executed on a double-buffered window, the  same
                     area in both the front and back buffers is cleared.

              The  effect  of  passing  a window to a function that accepts a drawable is unchanged by this extension.
              The window and front buffer are synonymous with each other.  This includes obeying the  XGetImage()  and
              XGetSubImage() semantics and the subwindow-mode semantics if a graphics context is involved.  Regardless
              of whether the window was explicitly passed in an XGetImage() or XGetSubImage() call, or implicitly ref-
              erenced  (i.e.,  one  of  the  window's  ancestors was passed in the function), the front (i.e. visible)
              buffer is always referenced.  Thus, DBE-naive screen dump clients will  always  get  the  front  buffer.
              XGetImage() and XGetSubImage() on a back buffer return undefined image contents for any obscured regions
              of the back buffer that fall within the image.

              Drawing to a back buffer always uses the clip region that would be used to draw to the front buffer with
              a GC subwindow-mode of ClipByChildren.  If an ancestor of a double-buffered window is drawn to with a GC
              having a subwindow-mode of IncludeInferiors, the effect on  the  double-buffered  window's  back  buffer
              depends  on  the  depth of the double-buffered window and the ancestor.  If the depths are the same, the
              contents of the back buffer of the double-buffered window are not changed.  If the depths are different,
              the  contents  of  the  back  buffer of the double-buffered window are undefined for the pixels that the
              IncludeInferiors drawing touched.

              DBE adds no new events.  DBE does not extend the semantics of any existing events with the exception  of
              adding a new drawable type called XdbeBackBuffer.

              If  events,  replies, or errors that contain a drawable (e.g., GraphicsExpose) are generated in response
              to a request, the drawable returned will be the one specified in the request.

              DBE advertises which visuals support double buffering.

              DBE does not include any timing or synchronization facilities.  Applications that need  such  facilities
              (e.g., to maintain a constant frame rate) should investigate the Synchronization Extension, an X Consor-
              tium standard.

       Window Management Operations

              The basic philosophy of DBE is that both buffers are treated the same by X window management operations.

              When  a  double-buffered window is destroyed, both buffers associated with the window are destroyed, and
              all back buffer names associated with the window are freed.

              If the size of a double-buffered window changes, both buffers assume the new size.  If the window's size
              increases,  the  effect  on  the  buffers  depends  on whether the implementation honors bit gravity for
              buffers.  If bit gravity is implemented, then the contents of both buffers are moved in accordance  with
              the  window's bit gravity, and the remaining areas are tiled with the window background.  If bit gravity
              is not implemented, then the entire unobscured region of both buffers is tiled  with  the  window  back-
              ground.   In either case, Expose events are generated for the region that is tiled with the window back-

              If the XGetGeometry() function is executed on an XdbeBackBuffer, the returned  x,  y,  and  border-width
              will be zero.

              If the Shape extension ShapeRectangles, ShapeMask, ShapeCombine, or ShapeOffset request is executed on a
              double-buffered window, both buffers are reshaped to match the new window shape.  The region  difference
              D  =  new  shape  - old shape is tiled with the window background in both buffers, and Expose events are
              generated for D.

       Complex Swap Actions

              DBE has no explicit knowledge of ancillary buffers (e.g. depth buffers or alpha buffers), and only has a
              limited  set  of defined swap actions.  Some applications may need a richer set of swap actions than DBE
              provides.  Some DBE implementations have knowledge of ancillary buffers, and/or can provide a  rich  set
              of  swap actions. Instead of continually extending DBE to increase its set of swap actions, DBE provides
              a flexible "idiom" mechanism.  If an applications's needs are served by the  defined  swap  actions,  it
              should use them; otherwise, it should use the following method of expressing a complex swap action as an
              idiom.  Following this policy will ensure the best possible performance across a wide variety of  imple-

              As  suggested  by  the  term  "idiom,"  a  complex  swap action should be expressed as a group/series of
              requests.  Taken together, this group of requests may be combined into an atomic operation by the imple-
              mentation,  in order to maximize performance.  The set of idioms actually recognized for optimization is
              implementation dependent.  To help with idiom expression and interpretation, an idiom must be surrounded
              by  two  function  calls: XdbeBeginIdiom() and XdbeEndIdiom().  Unless this begin-end pair surrounds the
              idiom, it may not be recognized by a given implementation, and performance will suffer.

              For example, if an application wants to swap buffers for two windows, and use X to  clear  only  certain
              planes  of  the back buffers, the application would make the following calls as a group, and in the fol-
              lowing order:


                     XdbeSwapBuffers() with XIDs for two windows, each of which uses a swap action of Untouched.

                     XFillRectangle() to the back buffer of one window.

                     XFillRectangle() to the back buffer of the other window.


              The XdbeBeginIdiom() and XdbeEndIdiom() functions do not  perform  any  actions  themselves.   They  are
              treated  as markers by implementations that can combine certain groups/series of requests as idioms, and
              are ignored by other implementations or for non-recognized groups/series of requests.  If these function
              calls  are  made  out of order, or are mismatched, no errors are sent, and the functions are executed as
              usual, though performance may suffer.

              XdbeSwapBuffers() need not be included in an idiom.  For example, if a swap action of Copied is desired,
              but  only some of the planes should be copied, XCopyArea() may be used instead of XdbeSwapBuffers().  If
              XdbeSwapBuffers() is included in an idiom, it  should  immediately  follow  the  XdbeBeginIdiom()  call.
              Also,  when  the  XdbeSwapBuffers()  is  included  in an idiom, that request's swap action will still be
              valid, and if the swap action might overlap with another request, then the final  result  of  the  idiom
              must  be  as if the separate requests were executed serially.  For example, if the specified swap action
              is Untouched, and if a XFillRectangle() using a client clip rectangle  is  done  to  the  window's  back
              buffer after the XdbeSwapBuffers() call, then the contents of the new back buffer (after the idiom) will
              be the same as if the idiom was not recognized by the implementation.

              It is highly recommended that API providers define, and application developers use, "convenience"  func-
              tions that allow client applications to call one procedure that encapsulates common idioms.  These func-
              tions will generate the XdbeBeginIdiom(), idiom, and XdbeEndIdiom() calls.   Usage  of  these  functions
              will ensure best possible performance across a wide variety of implementations.

       XdbeAllocateBackBufferName(), XdbeBeginIdiom(), XdbeDeallocateBackBufferName(), XdbeEndIdiom(), XdbeFreeVisual-
       Info(), XdbeGetBackBufferAttributes(), XdbeGetVisualInfo(), XdbeQueryExtension(), XdbeSwapBuffers().

X Version 11                     libXext 1.3.3                          DBE(3)