Automating Image Manipulation with GD - Page 14

The GD library

We will be utilizing the GD image manipulation library, which was written in C by Thomas Boutell. The Perl adaptation was written by Lincoln Stein.

The GD library is capable of drawing primitive shapes such as circles, squares, and lines. It's also capable of drawing text strings, as well as performing image manipulation such as combining and resizing images. The library can read and write JPEG, PNG, and XPM graphic formats. It supported GIF until Unisys, who owns the patent for the format, decided to start charging for the format's use. Because of that incident, Thomas Boutell decided to remove GIF support from the library.

Drawing borders

The first thing we'll learn to do is draw several different types of borders around an existing image. These exercises will help you become more familiar with the GD library syntax.

Drawing a 1-pixel border with rectangle()

In this first example, we will draw a black border around an image. With the GD library, we do this with the rectangle() function, which draws a rectangle given the starting x and y coordinates, the ending x and y coordinates, and the color of the border.

1  use GD;
2  my $image = GD::Image->newFromJpeg("66044.jpg");
3  my $black = $image->colorResolve(0,0,0);
4  my ($width,$height) = $image->getBounds();
5  $image->rectangle(0,0,($width-1),($height-1),$black);
6  open(FILE, ">border1.jpg") || die "Cannot open border1.jpg: $!\n";
7  print FILE $image->jpeg;

The first line loads the JPEG file 66044.jpg into a GD object. The second line allocates an index in the color map for black. Images can contain a maximum of 256 colors, so when modifying an existing image, you will either need to create a new mapping for a color, use an existing color in the map, or delete an existing color to make room for the new color. The colorResolve() function on the second line looks for the existence of a color in the color map (0,0,0 in this case which are the red, green, and blue values). If the color does not exist in the map, it will create it if there is room. You can check the number of colors in the map by calling the colorsTotal() function. The getBounds() function returns the width and height of the image that was just loaded. These values will be used to calculate the coordinates for the rectangle that is drawn on the next line.

The beginning coordinates for an image are 0,0. To draw the border, we call the rectangle() function. The first two arguments are the x,y coordinates where the rectangle will be drawn from. The second two parameters specify the ending two coordinates for the rectangle. The last parameter is the color map index for the color of the rectangle. The rectangle will be 1 pixel wide. The last two lines of the script write the new image with the border out to the border1.jpg file.

Drawing wide borders

Since the border is only 1 pixel wide, it may be a bit narrow to be noticeable. We can however draw a wider border by drawing multiple rectangles next to each other. The for loop below draws three consecutive rectangles, one inside of the other from the outer edge of the image.

1  use GD;
2  my $image = GD::Image->newFromJpeg("66044.jpg");
3  my $black = $image->colorResolve(0,0,0);
4  my ($width,$height) = $image->getBounds();
5  for (my $i=0; $i <= 3; $i++) {
6      $image->rectangle($i,$i,($width-1-$i),($height-1-$i),$black);
7  }
8  open(FILE, ">border2.jpg") || die "Cannot open border2.jpg: $!\n";
9  print FILE $image->jpeg;

The difference between the previous example and this one is the for loop on lines 5 through 7. The loop iterates 3 times, drawing a black rectangle one pixel wide in each direction for each iteration.

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