Tk_CanvasTkwin(3) Tk Library Procedures Tk_CanvasTkwin(3)______________________________________________________________________________NAME
Tk_CanvasTkwin, Tk_CanvasGetCoord, Tk_CanvasDrawableCoords, Tk_Canvas‐
SetStippleOrigin, 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 can‐
vas widget.
Tcl_Interp *interp (in/out) Interpreter to use for error
reporting.
char *string (in) Textual description of a canvas
coordinate.
double *doublePtr (out) Points to place to store a con‐
verted 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 cor‐
responds to x.
short *screenYPtr (out) Points to a location in which
to store the screen coordinate
in the canvas window that cor‐
responds 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 redis‐
played.
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 can‐
vas.
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 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 off‐
screen 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 rela‐
tive 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 coor‐
dinates. Type managers rarely need to invoke Tk_CanvasEventuallyRe‐
draw, 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 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
Tk 4.1 Tk_CanvasTkwin(3)
