## Tcl8.6.0/Tk8.6.0 Documentation > Tk C API, version 8.6.0 > CanvTkwin

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  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
  DESCRIPTION
  KEYWORDS

### 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 identifies a canvas widget.

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

const char *string (in)
  Textual description of a canvas coordinate.

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.

short *drawableXPtr (out)
  Pointer to a location in which to store an x coordinate in the space of the drawable currently being used to redisplay the canvas.

short *drawableYPtr (out)
  Pointer to a location in which to store a y coordinate in the space of the drawable currently being used to redisplay the canvas.

GC gc (out)
  Graphics context to modify.

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

short *screenYPtr (out)
  Points to a location in which to store the screen coordinate 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 coordinate 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 redisplay. Only pixels to the left of this coordinate 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 perform various utility functions.

Tk_CanvasTkwin returns the Tk_Window associated with a particular canvas.

Tk_CanvasGetCoord translates a string specification of a coordinate (such as 2p or 1.6c) into a double-precision canvas coordinate. If string is a valid coordinate description then Tk_CanvasGetCoord stores the corresponding canvas coordinate at *doublePtr and returns TCL_OK. Otherwise it stores an error message in the interpreter 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 compensate for the position of the off-screen pixmap relative to the window. If an item is being drawn with stipples, its 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 example 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 procedures that handle the -tags option for canvas items. The code of a canvas type manager will not 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 const Tk_CustomOption tagsOption = {Tk_CanvasTagsParseProc,
        Tk_CanvasTagsPrintProc, (ClientData) NULL
    };

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

### KEYWORDS

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