Dave Roth


GD.pm - Interface to Gd Graphics Library


The gd Graphics Library

The gd graphics library, written by Thomas Boutell of Cold Spring Harbor Labs (Boutell@netcom.com), is a package of C routines for creating, reading, writing, and manipulating GIF graphics files. It provides graphics primitives for:

  • Reading and writing GIF files.

  • Drawing lines, rectangles, arcs, ellipses and text.

  • Flood fills.

  • Using arbitrary images as tiled patterns for filling shapes.

  • Using arbitrary images as paintbrushes.

  • Stamping a portion of one image onto another, optionally resizing it.

  • Simple text rendering.

gd can be obtained at URL http://siva.cshl.org/gd/gd.html. Directions for installing it, and full documentation are available from that site.

The GD.pm interface

GD.pm is a port of the gd library by Lincoln Stein (lstein@genome.wi.mit.edu). It is similar to the original C library, but contains some modifications to take advantage of perl's object-oriented features. It defines the following three classes:


An image class, which holds the image data and accepts graphic primitive method calls.


A font class, which holds static font information and used for text rendering.


A simple polygon object, used for storing lists of vertices prior to rendering a polygon into an image.

A Simple Example:


        use GD;
        # create a new image
        $im = new GD::Image(100,100);

        # allocate some colors
        $white = $im->colorAllocate(255,255,255);
        $black = $im->colorAllocate(0,0,0);       
        $red = $im->colorAllocate(255,0,0);      
        $blue = $im->colorAllocate(0,0,255);

        # make the background transparent and interlaced

        # Put a black frame around the picture

        # Draw a blue oval

        # And fill it with red

        # Convert the image to GIF and print it on standard output
        print $im->gif;


1. To create a new, empty image, send a new() message to GD::Image, passing it the width and height of the image you want to create. An image object will be returned. Other class methods allow you to initialize an image from a preexisting GIF, GD or XBM file.
2. Next you will ordinarily add colors to the image's color table. colors are added using a colorAllocate() method call. The three parameters in each call are the red, green and blue (rgb) triples for the desired color. The method returns the index of that color in the image's color table. You should store these indexes for later use.
3. Now you can do some drawing! The various graphics primitives are described below. In this example, we do some text drawing, create an oval, and create and draw a polygon.
4. Polygons are created with a new() message to GD::Polygon. You can add points to the returned polygon one at a time using the addPt() method. The polygon can then be passed to an image for rendering.
5. When you're done drawing, you can convert the image into GIF format by sending it a gif() message. It will return a (potentially large) scalar value containing the binary data for the image. Ordinarily you will print it out at this point or write it to a file.

Method Calls

Creating and Saving Images


GD::Image::new(width,height) class method

To create a new, blank image, send a new() message to the GD::Image class. For example:

        $myImage = new GD::Image(100,100) || die;

This will create an image that is 100 x 100 pixels wide. If you don't specify the dimensions, a default of 64 x 64 will be chosen. If something goes wrong (e.g. insufficient memory), this call will return undef.


GD::Image::newFromGif(FILEHANDLE) class method

This will create an image from a GIF file read in through the provided filehandle. The filehandle must previously have been opened on a valid GIF file or pipe. If successful, this call will return an initialized image which you can then manipulate as you please. If it fails, which usually happens if the thing at the other end of the filehandle is not a valid GIF file, the call returns undef. Notice that the call doesn't automatically close the filehandle for you.

To get information about the size and color usage of the information, you can call the image query methods described below.

        Example usage:

        open (GIF,"barnswallow.gif") || die;
        $myImage = newFromGif GD::Image(GIF) || die;
        close GIF;

GD::Image::newFromXbm(FILEHANDLE) class method

This works in exactly the same way as newFromGif, but reads the contents of an X Bitmap file:

        open (XBM,"coredump.xbm") || die;
        $myImage = newFromXbm GD::Image(XBM) || die;
        close XBM;

GD::Image::newFromGd(FILEHANDLE) class method

This works in exactly the same way as newFromGif, but reads the contents of a GD file. GD is Tom Boutell's disk-based storage format, intended for the rare case when you need to read and write the image to disk quickly. It's not intended for regular use, because, unlike GIF or JPEG, no image compression is performed and these files can become BIG.

        open (GDF,"godzilla.gd") || die;
        $myImage = newFromGd GD::Image(GDF) || die;
        close GDF;

GD::Image::gif object method

This returns the image data in GIF format. You can then print it, pipe it to a display program, or write it to a file. Example:

        $gif_data = $myImage->gif;
        open (DISPLAY,"| display -") || die;
        print DISPLAY $gif_data;
        close DISPLAY;

GD::Image::gd object method

This returns the image data in GD format. You can then print it, pipe it to a display program, or write it to a file. Example:

        print MYOUTFILE $myImage->gd;

Color Control


GD::Image::colorAllocate(red,green,blue) object method

This allocates a color with the specified red, green and blue components and returns its index in the color table, if specified. The first color allocated in this way becomes the image's background color. (255,255,255) is white (all pixels on). (0,0,0) is black (all pixels off). (255,0,0) is fully saturated red. (127,127,127) is 50% gray. You can find plenty of examples in /usr/X11/lib/X11/rgb.txt.

If no colors are allocated, then this function returns -1.


        $white = $myImage->colorAllocate(0,0,0); #background color
        $black = $myImage->colorAllocate(255,255,255);
        $peachpuff = $myImage->colorAllocate(255,218,185);

GD::Image::colorDeallocate(colorIndex) object method

This marks the color at the specified index as being ripe for reallocation. The next time colorAllocate is used, this entry will be replaced. You can call this method several times to deallocate multiple colors. There's no function result from this call.


        $peachy = $myImage->colorAllocate(255,210,185);

GD::Image::colorClosest(red,green,blue) object method

This returns the index of the color closest in the color table to the red green and blue components specified. If no colors have yet been allocated, then this call returns -1.


        $apricot = $myImage->colorClosest(255,200,180);

GD::Image::colorExact(red,green,blue) object method

This returns the index of a color that exactly matches the specified red green and blue components. If such a color is not in the color table, this call returns -1.

        $rosey = $myImage->colorExact(255,100,80);
        warn "Everything's coming up roses.\n" if $rosey >= 0;

GD::Image::colorsTotal) object method

This returns the total number of colors allocated in the object.

        $maxColors = $myImage->colorsTotal;

GD::Image::getPixel(x,y) object method

This returns the color table index underneath the specified point. It can be combined with rgb() to obtain the rgb color underneath the pixel.


        $index = $myImage->getPixel(20,100);
        ($r,$g,$b) = $myImage->rgb($index);

GD::Image::rgb(colorIndex) object method

This returns a list containing the red, green and blue components of the specified color index.


        @RGB = $myImage->rgb($peachy);

GD::Image::transparent(colorIndex) object method

This marks the color at the specified index as being transparent. Portions of the image drawn in this color will be invisible. This is useful for creating paintbrushes of odd shapes, as well as for making GIF backgrounds transparent for displaying on the Web. Only one color can be transparent at any time. To disable transparency, specify -1 for the index.

If you call this method without any parameters, it will return the current index of the transparent color, or -1 if none.


        $im = newFromGif GD::Image(GIF);
        $white = $im->colorClosest(255,255,255); # find white
        print $im->gif;

Special Colors

GD implements a number of special colors that can be used to achieve special effects. They are constants defined in the GD:: namespace, but automatically exported into your namespace when the GD module is loaded.


GD::Image::setBrush( ) and GD::gdBrushed

You can draw lines and shapes using a brush pattern. Brushes are just images that you can create and manipulate in the usual way. When you draw with them, their contents are used for the color and shape of the lines.

To make a brushed line, you must create or load the brush first, then assign it to the image using setBrush. You can then draw in that with that brush using the gdBrushed special color. It's often useful to set the background of the brush to transparent so that the non-colored parts don't overwrite other parts of your image.


        # Create a brush at an angle
        $diagonal_brush = new GD::Image(5,5);
        $white = $diagonal_brush->allocateColor(255,255,255);
        $black = $diagonal_brush->allocateColor(0,0,0);
        $diagonal_brush->line(0,4,4,0,$black); # NE diagonal

        # Set the brush
        # Draw a circle using the brush

GD::Image::setStyle(@colors) and GD::gdStyled

Styled lines consist of an arbitrary series of repeated colors and are useful for generating dotted and dashed lines. To create a styled line, use setStyle to specify a repeating series of colors. It accepts an array consisting of one or more color indexes. Then draw using the gdStyled special color. Another special color, gdTransparent can be used to introduce holes in the line, as the example shows.


        # Set a style consisting of 4 pixels of yellow,
        # 4 pixels of blue, and a 2 pixel gap

To combine the gdStyled and gdBrushed behaviors, you can specify gdStyledBrushed. In this case, a pixel from the current brush pattern is rendered wherever the color specified in setStyle() is neither gdTransparent nor 0.


Draw filled shapes and flood fills using a pattern. The pattern is just another image. The image will be tiled multiple times in order to fill the required space, creating wallpaper effects. You must call setTile in order to define the particular tile pattern you'll use for drawing when you specify the gdTiled color. details.


The gdStyled color is used for creating dashed and dotted lines. A styled line can contain any series of colors and is created using the setStyled command.

Drawing Commands


GD::Image::setPixel(x,y,color) object method

This sets the pixel at (x,y) to the specified color index. No value is returned from this method. The coordinate system starts at the upper left at (0,0) and gets larger as you go down and to the right. You can use a real color, or one of the special colors gdBrushed, gdStyled and gdStyledBrushed can be specified.


        # This assumes $peach already allocated

GD::Image::line(x1,y1,x2,y2,color) object method

This draws a line from (x1,y1) to (x2,y2) of the specified color. You can use a real color, or one of the special colors gdBrushed, gdStyled and gdStyledBrushed.


        # Draw a diagonal line using the currently defind
        # paintbrush pattern.

GD::Image::dashedLine(x1,y1,x2,y2,color) object method

This draws a dashed line from (x1,y1) to (x2,y2) in the specified color. A more powerful way to generate arbitrary dashed and dotted lines is to use the setStyle() method described below and to draw with the special color gdStyled.



GD::Image::rectangle(x1,y1,x2,y2,color) object method

This draws a rectangle with the specified color. (x1,y1) and (x2,y2) are the upper left and lower right corners respectively. Both real color indexes and the special colors gdBrushed, gdStyled and gdStyledBrushed are accepted.



GD::Image::filledRectangle(x1,y1,x2,y2,color) object method

This draws a rectangle filed with the specified color. You can use a real color, or the special fill color gdTiled to fill the polygon with a pattern.


        # read in a fill pattern and set it
        open(GIF,"happyface.gif") || die;
        $tile = newFromGif GD::Image(GIF);

        # draw the rectangle, filling it with the pattern

GD::Image::polygon(polygon,color) object method

This draws a polygon with the specified color. The polygon must be created first (see below). The polygon must have at least three vertices. If the last vertex doesn't close the polygon, the method will close it for you. Both real color indexes and the special colors gdBrushed, gdStyled and gdStyledBrushed can be specified.


        $poly = new GD::Polygon;

GD::Image::filledPolygon(poly,color) object method

This draws a polygon filled with the specified color. You can use a real color, or the special fill color gdTiled to fill the polygon with a pattern.


        # make a polygon
        $poly = new GD::Polygon;

        # draw the polygon, filling it with a color

GD::Image::arc(cx,cy,width,height,start,end,color) object method

This draws arcs and ellipses. (cx,cy) are the center of the arc, and (width,height) specify the width and height, respectively. The portion of the ellipse covered by the arc are controlled by start and end, both of which are given in degrees from 0 to 360. Zero is at the top of the ellipse, and angles increase clockwise. To specify a complete ellipse, use 0 and 360 as the starting and ending angles. To draw a circle, use the same value for width and height.

You can specify a normal color or one of the special colors gdBrushed, gdStyled, or gdStyledBrushed.


        # draw a semicircle centered at 100,100

GD::Image::fill(x,y,color) object method

This method flood-fills regions with the specified color. The color will spread through the image, starting at point (x,y), until it is stopped by a pixel of a different color from the starting pixel (this is similar to the "paintbucket" in many popular drawing toys). You can specify a normal color, or the special color gdTiled, to flood-fill with patterns.


        # Draw a rectangle, and then make its interior blue
GD::Image::fillToBorder(x,y,bordercolor,color) object method

Like fill, this method flood-fills regions with the specified color, starting at position (x,y). However, instead of stopping when it hits a pixel of a different color than the starting pixel, flooding will only stop when it hits the color specified by bordercolor. You must specify a normal indexed color for the bordercolor. However, you are free to use the gdTiled color for the fill.


        # This has the same effect as the previous example

Image Copying Commands

Two methods are provided for copying a rectangular region from one image to another. One method copies a region without resizing it. The other allows you to stretch the region during the copy operation.

With either of these methods it is important to know that the routines will attempt to flesh out the destination image's color table to match the colors that are being copied from the source. If the destination's color table is already full, then the routines will attempt to find the best match, with varying results.


GD::Image::copy(sourceImage,dstX,dstY,srcX,srcY,width,height) object method

This is the simpler of the two copy operations, copying the specified region from the source image to the destination image (the one performing the method call). (srcX,srcY) specify the upper left corner of a rectangle in the source image, and (width,height) give the width and height of the region to copy. (dstX,dstY) control where in the destination image to stamp the copy. You can use the same image for both the source and the destination, but the source and destination regions must not overlap or strange things will happen.


        $myImage = new GD::Image(100,100);
        ... various drawing stuff ...
        $srcImage = new GD::Image(50,50);
        ... more drawing stuff ...
        # copy a 25x25 pixel region from $srcImage to
        # the rectangle starting at (10,10) in $myImage

GD::Image::copyResized(sourceImage,dstX,dstY,srcX,srcY,destW,destH,srcW,srcH) object method

This method is similar to copy() but allows you to choose different sizes for the source and destination rectangles. The source and destination rectangle's are specified independently by (srcW,srcH) and (destW,destH) respectively. copyResized() will stretch or shrink the image to accomodate the size requirements.


        $myImage = new GD::Image(100,100);
        ... various drawing stuff ...
        $srcImage = new GD::Image(50,50);
        ... more drawing stuff ...
        # copy a 25x25 pixel region from $srcImage to
        # a larger rectangle starting at (10,10) in $myImage

Character and String Drawing

Gd allows you to draw characters and strings, either in normal horizontal orientation or rotated 90 degrees. These routines use a GD::Font object, described in more detail below. There are four built-in fonts, available in global variables gdLargeFont, gdMediumBoldFont, gdSmallFont and gdTinyFont. Currently there is no way of dynamically creating your own fonts.


GD::Image::string(font,x,y,string,color) Object Method

This method draws a string startin at position (x,y) in the specified font and color. Your choices of fonts are gdSmallFont, gdMediumBoldFont, gdTinyFont and gdLargeFont.


        $myImage->string(gdSmallFont,2,10,"Peachy Keen",$peach);

GD::Image::stringUp(font,x,y,string,color) Object Method

Just like the previous call, but draws the text rotated counterclockwise 90 degrees.


GD::Image::char(font,x,y,char,color) Object Method GD::Image::charUp(font,x,y,char,color) Object Method

These methods draw single characters at position (x,y) in the specified font and color. They're carry-overs from the C interface, where there is a distinction between characters and strings. Perl is insensible to such subtle distinctions.

Miscellaneous Image Methods


GD::Image::interlaced( ) GD::Image::interlaced(1) Object method

This method sets or queries the image's interlaced setting. Interlace produces a cool venetian blinds effect on certain viewers. Provide a true parameter to set the interlace attribute. Provide undef to disable it. Call the method without parameters to find out the current setting.


GD::Image::getBounds( ) Object method

This method will return a two-member list containing the width and height of the image. You query but not not change the size of the image once it's created.

Polygon Methods

A few primitive polygon creation and manipulation methods are provided. They aren't part of the Gd library, but I thought they might be handy to have around (they're borrowed from my qd.pl Quickdraw library).


GD::Polygon::new class method

Create an empty polygon with no vertices.

        $poly = new GD::Polygon;

GD::Polygon::addPt(x,y) object method

Add point (x,y) to the polygon.


GD::Polygon::getPt(index) object method

Retrieve the point at the specified vertex.

        ($x,$y) = $poly->getPt(2);

GD::Polygon::setPt(index,x,y) object method

Change the value of an already existing vertex. It is an error to set a vertex that isn't already defined.


GD::Polygon:deletePt(index) object method

Delete the specified vertex, returning its value.

        ($x,$y) = $poly->deletePt(1); 

GD::Polygon::length object method

Return the number of vertices in the polygon.

        $points = $poly->length;

GD::Polygon::vertices object method

Return a list of all the verticies in the polygon object. Each membver of the list is a reference to an (x,y) array.

        @vertices = $poly->vertices;
        foreach $v (@vertices)
           print join(",",@$v),"\n";

GD::Polygon::bounds object method

Return the smallest rectangle that completely encloses the polygon. The return value is an array containing the (left,top,right,bottom) of the rectangle.

        ($left,$top,$right,$bottom) = $poly->bounds;

GD::Polygon::offset(dx,dy) object method

Offset all the vertices of the polygon by the specified horizontal (dh) and vertical (dy) amounts. Positive numbers move the polygon down and to the right.


GD::Polygon::map(srcL,srcT,srcR,srcB,destL,dstT,dstR,dstB) object method

Map the polygon from a source rectangle to an equivalent position in a destination rectangle, moving it and resizing it as necessary. See polys.pl for an example of how this works. Both the source and destination rectangles are given in (left,top,right,bottom) coordinates. For convenience, you can use the polygon's own bounding box as the source rectangle.

        # Make the polygon really tall

Font Utilities

Gd's support for fonts is minimal. Basically you have access to gdSmallFont and gdLargeFont for drawing, and not much else. However, for future compatibility, I've made the fonts into perl objects of type GD::Font that you can query and, perhaps someday manipulate.


GD::Font::gdSmallFont constant

This is the basic small font, "borrowed" from a well known public domain 6x12 font.


GD::Font::gdLargeFont constant

This is the basic large font, "borrowed" from a well known public domain 8x16 font.


GD::Font::gdMediumBoldFont constant

This is a bold font intermediate in size between the small and large fonts, borrowed from a public domain 7x13 font;


GD::Font::gdTinyFont constant

This is a tiny, almost unreadable font, 5x8 pixels wide.


GD::Font::nchars object method

This returns the number of characters in the font.

        print "The large font contains ",gdLargeFont->nchars," characters\n";

GD::Font::offset object method

This returns the ASCII value of the first character in the font


GD::Font::width GD::Font::height object methods

These return the width and height of the font.

        ($w,$h) = (gdLargeFont->width,gdLargeFont->height);

Copyright Information

The GD.pm interface is copyright 1995, Lincoln D. Stein. You are free to use it for any purpose, commercial or noncommercial, provided that if you redistribute the source code this statement of copyright remains attached. The gd library is covered separately under a 1994 copyright by Quest Protein Database Center, Cold Spring Harbor Labs. For usage information see the gd documentation at URL


The latest versions of the perl interface are available at