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Tk_CanvasDrawableCoords





Tk_CanvasTkwin(3)     Tk Library Procedures                   4.1



_________________________________________________________________

NAME

Tk_CanvasTkwin, Tk_CanvasGetCoord,  Tk_CanvasDrawableCoords,
     Tk_CanvasSetStippleOrigin,            Tk_CanvasWindowCoords,
     Tk_CanvasEventuallyRedraw,  Tk_CanvasTagsOption  -   utility
     procedures for canvas type managers

SYNOPSIS

#include <tk.h>

     Tk_Window
     Tk_CanvasTkwin(canvas)

     int
     Tk_CanvasGetCoord(interp, canvas, string, doublePtr)

     Tk_CanvasDrawableCoords(canvas, x, y, drawableXPtr, drawableYPtr)

     Tk_CanvasSetStippleOrigin(canvas, gc)

     Tk_CanvasWindowCoords(canvas, x, y, screenXPtr, screenYPtr)

     Tk_CanvasEventuallyRedraw(canvas, x1, y1, x2, y2)

     Tk_OptionParseProc *Tk_CanvasTagsParseProc;

     Tk_OptionPrintProc *Tk_CanvasTagsPrintProc;

ARGUMENTS

Tk_Canvas     canvas          (in)      A token that identi-
                                             fies     a    canvas
                                             widget.

     Tcl_Interp    *interp         (in/out)  Interpreter  to  use
                                             for error reporting.

     char          *string         (in)      Textual  description
                                             of  a canvas coordi-
                                             nate.

     double        *doublePtr      (out)     Points to  place  to
                                             store   a  converted
                                             coordinate.

     double        x               (in)      An x  coordinate  in
                                             the   space  of  the
                                             canvas.

     double        y               (in)      A  y  coordinate  in
                                             the   space  of  the
                                             canvas.



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Tk_CanvasTkwin(3)     Tk Library Procedures                   4.1



     short         *drawableXPtr   (out)     Pointer to  a  loca-
                                             tion   in  which  to
                                             store an  x  coordi-
                                             nate in the space of
                                             the         drawable
                                             currently being used
                                             to   redisplay   the
                                             canvas.

     short         *drawableYPtr   (out)     Pointer to  a  loca-
                                             tion   in  which  to
                                             store a y coordinate
                                             in  the space of the
                                             drawable   currently
                                             being     used    to
                                             redisplay  the  can-
                                             vas.

     GC            gc              (out)     Graphics context  to
                                             modify.

     short         *screenXPtr     (out)     Points to a location
                                             in  which  to  store
                                             the  screen  coordi-
                                             nate  in  the canvas
                                             window          that
                                             corresponds to x.

     short         *screenYPtr     (out)     Points to a location
                                             in  which  to  store
                                             the  screen  coordi-
                                             nate  in  the canvas
                                             window          that
                                             corresponds to y.

     int           x1              (in)      Left  edge  of   the
                                             region   that  needs
                                             redisplay.      Only
                                             pixels  at or to the
                                             right of this  coor-
                                             dinate  need  to  be
                                             redisplayed.

     int           y1              (in)      Top  edge   of   the
                                             region   that  needs
                                             redisplay.      Only
                                             pixels  at  or below
                                             this coordinate need
                                             to be redisplayed.

     int           x2              (in)      Right  edge  of  the
                                             region   that  needs



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Tk_CanvasTkwin(3)     Tk Library Procedures                   4.1



                                             redisplay.
                                              Only pixels to  the
                                             left of this coordi-
                                             nate  need   to   be
                                             redisplayed.

     int           y2              (in)      Bottom edge  of  the
                                             region   that  needs
                                             redisplay.      Only
                                             pixels   above  this
                                             coordinate  need  to
                                             be redisplayed.
_________________________________________________________________


DESCRIPTION

These procedures are called by canvas type managers to  per-
     form various utility functions.

     Tk_CanvasTkwin returns the Tk_Window associated with a  par-
     ticular canvas.

     Tk_CanvasGetCoord translates a  string  specification  of  a
     coordinate (such as 2p or 1.6c) into a double-precision can-
     vas coordinate.  If string is a valid coordinate description
     then Tk_CanvasGetCoord stores the corresponding canvas coor-
     dinate at  *doublePtr  and  returns  TCL_OK.   Otherwise  it
     stores  an  error  message  in  interp->result  and  returns
     TCL_ERROR.

     Tk_CanvasDrawableCoords is called by  type  managers  during
     redisplay  to  compute  where to draw things.  Given x and y
     coordinates    in    the    space     of     the     canvas,
     Tk_CanvasDrawableCoords  computes the corresponding pixel in
     the drawable that is currently being used for redisplay;  it
     returns    those    coordinates    in    *drawableXPtr   and
     *drawableYPtr.  This procedure should not be invoked  except
     during redisplay.

     Tk_CanvasSetStippleOrigin is also used during redisplay.  It
     sets the stipple origin in gc so that stipples drawn with gc
     in the current offscreen pixmap will line up  with  stipples
     drawn  with  origin  (0,0)  in  the  canvas's actual window.
     Tk_CanvasSetStippleOrigin is needed in  order  to  guarantee
     that  stipple  patterns  line up properly when the canvas is
     redisplayed in small pieces.  Redisplays are carried out  in
     double-buffered  fashion  where  a  piece  of  the canvas is
     redrawn in an offscreen pixmap and then copied back onto the
     screen.   In  this  approach the stipple origins in graphics
     contexts need to be adjusted during each redisplay  to  com-
     pensate  for  the position of the off-screen pixmap relative
     to the window.  If an item is being drawn with stipples, its



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Tk_CanvasTkwin(3)     Tk Library Procedures                   4.1



     type  manager typically calls Tk_CanvasSetStippleOrigin just
     before using gc to draw something;   after  it  is  finished
     drawing,  the type manager calls XSetTSOrigin to restore the
     origin in gc back to (0,0) (the restore  is  needed  because
     graphics  contexts  are  shared,  so they cannot be modified
     permanently).

     Tk_CanvasWindowCoords is similar to  Tk_CanvasDrawableCoords
     except that it returns coordinates in the canvas's window on
     the screen, instead of coordinates in an off-screen pixmap.

     Tk_CanvasEventuallyRedraw may be invoked by a  type  manager
     to inform Tk that a portion of a canvas needs to be redrawn.
     The x1, y1, x2, and y2 arguments  specify  the  region  that
     needs  to  be redrawn, in canvas coordinates.  Type managers
     rarely need to invoke  Tk_CanvasEventuallyRedraw,  since  Tk
     can  normally  figure  out when an item has changed and make
     the redisplay request on its behalf (this happens, for exam-
     ple  whenever  Tk  calls a configureProc or scaleProc).  The
     only   time   that   a   type   manager   needs   to    call
     Tk_CanvasEventuallyRedraw  is  if an item has changed on its
     own without being invoked through one of the  procedures  in
     its Tk_ItemType; this could happen, for example, in an image
     item if the image is modified using image commands.

     Tk_CanvasTagsParseProc and Tk_CanvasTagsPrintProc  are  pro-
     cedures  that handle the -tags option for canvas items.  The
     code of a canvas type manager won't  call  these  procedures
     directly,   but   will  use  their  addresses  to  create  a
     Tk_CustomOption structure for the -tags option.    The  code
     typically looks like this:
          static Tk_CustomOption tagsOption = {Tk_CanvasTagsParseProc,
            Tk_CanvasTagsPrintProc, (ClientData) NULL
          };

          static Tk_ConfigSpec configSpecs[] = {
            ...
            {TK_CONFIG_CUSTOM, "-tags", (char *) NULL, (char *) NULL,
               (char *) NULL, 0, TK_CONFIG_NULL_OK, &tagsOption},
            ...
          };


KEYWORDS

canvas, focus, item type, redisplay, selection, type manager










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