Clipping for Effect

Within the graphics state of a PostScript system is a special path called the clipping path. Every bit of ink to be placed on the page is checked against this path. If PostScript determines that the ink would go outside the current clipping path, that portion of ink is ignored. If the ink would be within the clipping path, it is actually placed on the page. For the mathematically inclined, the clipping process is intersection: the set of pixels to be painted is intersected with the set of pixels within the current clipping path to get the set of pixels to paint. For objects that are partly inside and partly outside the clipping path, the natural implication is that only the part that is within the clipping path is drawn.

By default, the clipping path is defined to be a rectangle just within the boundary of the page (usually it is set to about a quarter of an inch). You can set your own clip path by constructing the path with the normal path construction operators and invoking the clip operator. There is only one difficulty: once you reduce the size of the current clipping path, there is no way to expand the size of the clipping path with clip. The only way to go back to a larger clipping path is to save the one you would like to restore with gsave and restore it later with grestore. In fact, it is always good policy to only set a clipping path withing a bracketing gsave/grestore pair. You will always be safe if you do this.

Clipping a Simple Path

As a simple example of clipping, let us say that we want to draw a box and fill it with text in such a way that some text is cut off. The effect we are wanting is that of a hole in a piece of paper over some newsprint, say. This can be done quite simply.

First, we set up the box to act as our window. We can set up the path, stroke it if we want to see it, and then clip to it:

        gsave                      % Save the old clip path
          72 72 box                % Set up our box
          gsave                    % Don't allow box to be lost after stroke
          grestore                 % Restore the box path
          clip                     % Clip to the box

The clip path is now established, and we can now go on to draw the text that should be clipped (note that there is a leading gsave... this is to keep us from loosing our old clip path which covered the whole page).

          60 60 moveto
          (This is Times-Roman clipped to a box) show
          70 90 moveto
          (This is Times-Roman clipped to a box) show
          50 120 moveto
          (This is Times-Roman clipped to a box) show

Once we have finished, we can just do a grestore to clean up after ourselves.

While there are some implementation limitations on the complexity of the clip path, in general you can have very complex paths... not just squares. Arcs, lines, even text can be used to create the clip path.

Clipping to Text

There may come a time when you will want to do some special effects with text. For example, you might want to print out "July 4" using letters that look like the flag. This is fairly easy to do using clipping. A somewhat simpler problem would be to draw text that looks like a sunburst (that is, the text is filled with a sunburst pattern). This is also fairly easy to do once you know how to clip to text. The secret is an operator called charpath. This operator takes a string and a boolean and builds the path at the current point that would trace out the text of the string. The path, once created, can be stroked, filled, clipped, or any other combination of things; it is, after all, just a path. The boolean which charpath requires is for handling special kinds of fonts, and it is generally left true.

As before, the steps to this example are to build the path, clip to it, and draw the image needing to be clipped.

Here we build the path by setting up our current point and string, and then invoking charpath:

      gsave                                      % Save old clip path
        /Times-Roman findfont 60 scalefont setfont
        72 72 moveto (Clipping) true charpath    % Set up the text's path

Once we have the path, we can invoke clip to establish the complex path of the text as the current clip path. With the clip path established, we can draw our sunburst, which will be confined to the area inside the text:

Again, because of our judicious use of gsave and grestore, a simple grestore cleans up the graphics state when we're done.

As you might imagine, this sort of effect is very powerful and can make it very easy for you to create some stunning images.

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pjw 12/21/96