Mastering Perl/Tk

The Bitmap Image Type

The Bitmap command can read XBM data from a file or directly from data embedded in your Perl/Tk program. Suppose we have a bitmap of a circle. The following code creates the bitmap image from the XBM file and gives it a black foreground (the circle) and a white background. The image is placed inside a Label with a gray background, and then it’s packed:

my $b = $mw->Bitmap(-file       => 'circle.xbm',
                    -foreground => 'black',
                    -background => 'white',
);

my $l = $mw->Label(-image => $b, -background => 'gray')->pack;

Since the Label shrink-wraps around the image, all we see is the picture shown in Figure 17-5. Notice also that we’ve omitted any mask file.

Figure 17-5.  A bitmap of a circle without a mask

Suppose we copy the original bitmap and invert it (that is, toggle all the bits so ones become zeros and zeros become ones) and save the result as a mask file. This statement reconfigures the Bitmap image and adds a -maskfile option, producing a transparent area where the circle used to be (Figure 17-6), allowing the Label’s gray color to show through:

$b->configure(-maskfile => 'images/circle.msk');

Notice the bitmap’s background color appears wherever the mask has an on bit and the original source bit is off.

Figure 17-6.  Mask that’s an inverted version of bitmap makes ...