The code that implements a new image type is called an image manager. It consists of a collection of procedures plus three different kinds of data structures. The first data structure is a Tk_ImageType structure, which contains the name of the image type and pointers to five procedures provided by the image manager to deal with images of this type:
typedef struct Tk_ImageType {
char *name;
Tk_ImageCreateProc *createProc;
Tk_ImageGetProc *getProc;
Tk_ImageDisplayProc *displayProc;
Tk_ImageFreeProc *freeProc;
Tk_ImageDeleteProc *deleteProc;
} Tk_ImageType;
The fields of this structure will be described in later subsections
of this entry.
The second major data structure manipulated by an image manager is called an image master; it contains overall information about a particular image, such as the values of the configuration options specified in an image create command. There will usually be one of these structures for each invocation of the image create command.
The third data structure related to images is an image instance. There will usually be one of these structures for each usage of an image in a particular widget. It is possible for a single image to appear simultaneously in multiple widgets, or even multiple times in the same widget. Furthermore, different instances may be on different screens or displays. The image instance data structure describes things that may vary from instance to instance, such as colors and graphics contexts for redisplay. There is usually one instance structure for each -image option specified for a widget or canvas item.
The following subsections describe the fields of a Tk_ImageType in more detail.
typePtr->name provides a name for the image type. Once Tk_CreateImageType returns, this name may be used in image create commands to create images of the new type. If there already existed an image type by this name then the new image type replaces the old one.
typedef int Tk_ImageCreateProc( Tcl_Interp *interp, char *name, int argc, char **argv, Tk_ImageType *typePtr, Tk_ImageMaster master, ClientData *masterDataPtr);The interp argument is the interpreter in which the image command was invoked, and name is the name for the new image, which was either specified explicitly in the image command or generated automatically by the image command. The argc and argv arguments describe all the configuration options for the new image (everything after the name argument to image). The master argument is a token that refers to Tk's information about this image; the image manager must return this token to Tk when invoking the Tk_ImageChanged procedure. Typically createProc will parse argc and argv and create an image master data structure for the new image. createProc may store an arbitrary one-word value at *masterDataPtr, which will be passed back to the image manager when other callbacks are invoked. Typically the value is a pointer to the master data structure for the image.
If createProc encounters an error, it should leave an error message in interp->result and return TCL_ERROR; otherwise it should return TCL_OK.
createProc should call Tk_ImageChanged in order to set the size of the image and request an initial redisplay.
typedef ClientData Tk_ImageGetProc( Tk_Window tkwin, ClientData masterData);The tkwin argument identifies the window in which the image will be used and masterData is the value returned by createProc when the image master was created. getProc will usually create a data structure for the new instance, including such things as the resources needed to display the image in the given window. getProc returns a one-word token for the instance, which is typically the address of the instance data structure. Tk will pass this value back to the image manager when invoking its displayProc and freeProc procedures.
typedef void Tk_ImageDisplayProc( ClientData instanceData, Display *display, Drawable drawable, int imageX, int imageY, int width, int height, int drawableX, int drawableY);The instanceData will be the same as the value returned by getProc when the instance was created. display and drawable indicate where to display the image; drawable may be a pixmap rather than the window specified to getProc (this is usually the case, since most widgets double-buffer their redisplay to get smoother visual effects). imageX, imageY, width, and height identify the region of the image that must be redisplayed. This region will always be within the size of the image as specified in the most recent call to Tk_ImageChanged. drawableX and drawableY indicate where in drawable the image should be displayed; displayProc should display the given region of the image so that point (imageX, imageY) in the image appears at (drawableX, drawableY) in drawable.
typedef void Tk_ImageFreeProc( ClientData instanceData, Display *display);The instanceData will be the same as the value returned by getProc when the instance was created, and display is the display containing the window for the instance. freeProc should release any resources associated with the image instance, since the instance will never be used again.
typedef void Tk_ImageDeleteProc( ClientData masterData);The masterData argument will be the same as the value stored in *masterDataPtr by createProc when the image was created. deleteProc should release any resources associated with the image.
Copyright © 1994 The Regents of the University of California. Copyright © 1994-1996 Sun Microsystems, Inc. Copyright © 1995, 1996 Roger E. Critchlow Jr.