Tcl/Tk with C for Image Processing

Let's make a small distinction between two kinds of C-Tcl/Tk applications: those which act like a shell (wish, for example) and those which use the Tcl/Tk extension in a predetermined way.

If you want to create another “instance” of wish with some extra commands you have created, you should read the man pages concerning Tcl_Main and Tcl_AppInit.

If your program uses Tcl/Tk only for the interface, and it is not intended to be used in a “shell-like” fashion, the approach is slightly different. I recommend you grab the nice demo program tkHelloWorld.tar.gz (see Sidebar) to use as an example.

Basically, your program has to implement the following four steps:

In the C code shown in Listing 1, the comments identify exactly which of the four steps is being done.

Listing 1

From this point on, we wish to use C programming only for some critical functions, since the main flow and control of our application is handled by Tk.

If you are interested in the myriad ways you can call a C routine, read TCL and the TK Toolkit by John K. Ousterhout, Addison-Wesley, 1994.

Essentially your C function must have a prototype like the following:

int
C_func_name (ClientData cd, Tcl_Interp *interp,
             int argc, char **argv);

and you must register it by:

Tcl_CreateCommand (interp, "Tcl_func_name",
        C_func_name, (ClientData *) NULL,
        (Tcl_CmdDeleteProc *) NULL);

We want our C routine to process an image called image_name under Tk. The immediate solution would be to pass the color of each pixel (the photo widget has this option) again and again until the image is complete. While this program was running, we could go out for lunch, visit a few friends, have dinner and see a movie. However, there is a better way to accomplish the goal. From C, we ask Tk to take care of it. First, we have to define:

Tk_PhotoHandle      image;
Tk_PhotoImageBlock  blimage;

Then call the following functions in sequence:

image = Tk_FindPhoto ("image_name");
Tk_PhotoGetImage (image, &blimage);

typedef struct {
  unsigned char *pixelPtr;
  int width;
  int height;
  int pitch;
  int pixelSize;
  int offset[3];
} Tk_PhotoImageBlock;

Using the above definitions allows different representations of the image; for example:

The colors of a given pixel can be obtained with three simple C macros:

#define RED(p,x,y)  ((p)->pixelPtr[(y)*(p)->
pitch + (x)*(p)->pixelSize + (p)->offset[0]] )
#define GREEN(p,x,y) ((p)->pixelPtr[(y)*(p)->
pitch + (x)*(p)->pixelSize + (p)->offset[1]])
#define BLUE(p,x,y)  ((p)->pixelPtr[(y)*(p)->
pitch + (x)*(p)->pixelSize + (p)->offset[2]])

You call the macros giving the address of the block structure explained above as the first parameter, and the x and y coordinates (where 0,0 is the upper-left corner) of the pixel as the second and third. For an optimized program, it would be much faster to use address differences to determine the position of the next pixel from the current pixel, i.e., its neighbor.