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 stroke grestore % Restore the box path clip % Clip to the boxThe 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) showOnce 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 pathOnce 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:
174 72 translate % Set our origin to middle
0 2 360 { % For every second degree of circle
newpath
gsave
rotate % Rotate to angle
0 0 moveto % From new origin
300 0 rlineto % Setup a 300 point long line
stroke % ... and draw it
grestore
} for
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.
