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 {
const 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 model; 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.
typedef int Tk_ImageCreateProc(
Tcl_Interp *interp,
const char *name,
int objc,
Tcl_Obj *const objv[],
const Tk_ImageType *typePtr,
Tk_ImageModel model,
void **modelDataPtr);
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 objc and objv arguments describe all the configuration
options for the new image (everything after the name argument to
image).
The model 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 objc and objv
and create an image model data structure for the new image.
createProc may store an arbitrary one-word value at
*modelDataPtr, which will be passed back to the
image manager when other callbacks are invoked.
Typically the value is a pointer to the model data
structure for the image.
If createProc encounters an error, it should leave an error message in the interpreter 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 void *Tk_ImageGetProc(
Tk_Window tkwin,
void *modelData);
The tkwin argument identifies the window in which the
image will be used and modelData is the value
returned by createProc when the image model 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(
void *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(
void *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(
void *modelData);
The modelData argument will be the same as the value
stored in *modelDataPtr by createProc when the
image was created.
deleteProc should release any resources associated with
the image.