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<title>Creating/Manipulating images with gd LG #91</title>
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<BIG><BIG><STRONG><FONT COLOR="maroon">Creating/Manipulating images with gd</FONT></STRONG></BIG></BIG>
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<STRONG>By <A HREF="../authors/shuveb.html">Shuveb Hussain</A></STRONG>
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<p>
gd is an open source library written for easy
manipulation/creation of images. It lets you open
images in formats like jpeg,png,xpm and a few more.
You can think of gd as something like this: It opens
images in different formats and converts them to
generic bit-mapped images in memory. It then lets you
do graphical operations like drawing lines, arcs,
ellipses or rectangles on that image, and can finally
store the resulting image back in any of the earlier
mentioned formats. For exapmple, you could write a
simple command line program that converts a given file
in JPEG format to PNG using gd. gd can do more. It can
change colors in the image, copy,cut, merge or rotate
it. One more area where gd is useful is when you want
to create images on the fly. With gd, you can
programatically create an image, color it, draw on it
and save it to disk. gd is best known for creating
images on the fly for use in web pages. This is made
possible with the help of PHP.
</p>
<p>
If you got a GNU/Linux system that uses RPM to manage
packages, try out
<pre>
rpm -q gd
</pre>
to find out if gd is installed.
You can download the latest tarball from <a href =
"http://www.boutell.com"> www.boutell.com </a>
</p>
<h2> Creating Images </h2>
<p>
The following program creates a 100 by 100 pixel
black image with a white line running diagonally
across it.
</p>
<p align=center>
<IMG ALT="100x100 image created by gd"
SRC="misc/shuveb/test.jpg" WIDTH="100" HEIGHT="100">
</p>
If you want to save some typing, use <A
HREF="misc/shuveb/gd-eg1.c.txt">this listing </A>
<pre>
/* File : gd-eg1.c */
#include < gd.h >
#include < stdio.h >
int main() {
gdImagePtr im; //declaration of the image
FILE *out; //output file
int black,white;
im = gdImageCreate(100,100); //create an image, 100
by 100 pixels
black = gdImageColorAllocate(im, 0, 0, 0); //
allocate black color
white = gdImageColorAllocate(im, 255, 255, 255); //
allocate white color
gdImageLine(im, 0, 0,100,100, white); // draw a line
using the allocated white color.
out = fopen("test.jpg", "w"); //open a file
gdImageJpeg(im, out, -1); //write the image to the
file using the default quality setting
/* be good, clean up stuff */
fclose(out);
gdImageDestroy(im);
}
</pre>
Compile the program with the following command line:
<pre>
$ gcc gd-eg1.c -lgd
</pre>
Run the resulting a.out file and you should have a
test.jpg file created in the current directory. If you
view it, you'll have a 100 by 100 pixel black image
with a white line cutting across.
The program, I'm sure is simple, butI'll explain the
code a little.
<pre>
gdImagePtr im; //declaration of the image
</pre>
this declares a pointer to a gd image descriptor.
<pre>
im = gdImageCreate(100,100); //create an image, 100
by 100 pixels
</pre>
we now create an image 100 by 100 pixels and store the
reference it returns in the variable im. This is much
like a file handle. All further operations on this
image shall be carried out using this reference.
<pre>
black = gdImageColorAllocate(im, 0, 0, 0); //
allocate black color
white = gdImageColorAllocate(im, 255, 255, 255); //
allocate white color
</pre>
before you can draw anything on to the image, you'll
need to allocate color. Allocating color for the first
time for a newly created image will make it the
background color for that image. The function
gdImageColorAllocate takes four arguments. The first
one is the image pointer and the next three are Red,
Green and Blue values respectively. Thus calling
gdImageColorAllocate(im, 0, 0, 0) for the newly
created image will paint the background of the new
image black.
We store the color indexes in variables because
graphical drawing or font drawing functions take a
'color' argument.
<pre>
gdImageLine(im, 0, 0,100,100, white); // draw a line
using the allocated white color.
</pre>
This function draws a line from the top left
corner(0,0) to the bottom right corner(100,100) using
the color white on to the image pointed to by im.
<pre>
gdImageJpeg(im, out, -1); //write the image to the
file using the default quality setting
</pre>
this is the function call that writes the image on to
a disk file in the JPEG format. The final argument of
this function is the quality setting for JPEG format
images. This can be between 1 and 100, where 100 is
the highest quality. Passing -1 will use the default
quality setting.
Similarly, there are other functions that store images
in different formats
<pre>
GdImagePng(im,out) // store as PNG (note no quality
setting)
GdImageGd and gdImageGd2 are functions that store
images in formats specified by the library.
gdImageDestroy(im);
</pre>
<p>
and you finally release memory allocated to hold the
image data.
</p>
<p>
Please note that the PNG format is now enjoying good
support and uses better compression algorithms. It
also achieves something that the JPEG format does not
: Transparency.
GIF format images, though good enough, use the LZW
compression algorithm patented by Unisys when using
full compression. The GIF format support in gd was
thus dropped recently. And you must have read about
the hue and cry against software patents..... Some web
sites even claim to be GIF free, like body sprays that
claim to be "free of CFC, that damages friendly Ozone"
More on this at <a href
http://www.burnallgifs.org>www.burnallgifs.org</a>
</p>
<h2>Manipulating Images</h2>
<p>
gd also allows you to open and manipulate existing
images apart from creating new ones from scratch. To
illustrate this, the following program will open an
image of Tux, enlarge it a little and write a string
"Tux, the Linux Penguin" on to the image. Apart from
drawing text on to the image, this program is intended
to explain a few more functions that will be of use.
</p>
Save some typing, use <A
HREF="misc/shuveb/gd-eg2.c.txt">this listing </A>
<table align=center border=1>
<tr>
<td align=center>Before</td><td
align=center>After</td>
</tr>
<tr><td><IMG ALT="Before manipulation"
SRC="misc/shuveb/tuxin.jpg"></td><td><IMG ALT="After
manipulation" SRC="misc/shuveb/tuxout.jpg"></td></tr>
</tr>
</table>
<pre>
/* File : gd-eg2.c */
#include < gd.h >
#include < stdio.h >
int main()
{
gdImagePtr oldtux, newtux; //declaration of the image
pointers
FILE *out, *in;
int red,white;
int brect[8];
char *err;
in = fopen("tuxin.jpg","r");
oldtux = gdImageCreateFromJpeg(in);
newtux = gdImageCreate(150,165); //create an image,
150 by 165 pixels
white = gdImageColorAllocate(newtux, 255, 255,
255);// allocate white color
red = gdImageColorAllocate(newtux, 255, 0, 0); //
allocate black color
gdImageCopyResized(newtux,oldtux,0,0,0,0,150,150,oldtux->sx,oldtux->sy);
err=gdImageStringFT(newtux,brect,
red,"/usr/X11R6/lib/X11/fonts/TTF/luxisr.ttf",10,0,0,160,"Tux
,The Linux Penguin");
if(err) fprintf(stderr,"Error : %s\n",err);
out = fopen("tuxout.jpg", "w"); //open a file
gdImagePng(newtux, out); //write the image to the
file in the PNG format
/* be good, clean up stuff */
fclose(out);
fclose(in);
gdImageDestroy(oldtux);
gdImageDestroy(newtux);
}
</pre>
As you can see, this program uses a few more function
calls. The functions are described below
<pre>
<h3>gdImageCopyResized</h3>
</pre>
<p>
This gd function copies rectangular parts of one
image to another. In the process of copying, it can
also resize the image. The function prototype is:
</p>
<pre>
void gdImageCopyResized(gdImagePtr dst, gdImagePtr
src, int dstX, int dstY, int srcX, int srcY, int
destW, int destH, int srcW, int srcH);
</pre>
<p>
The sx and sy members of the gdImagePtr structure hold
the width and height of the image respectively.
</p>
<p>
You might have noticed that the image becomes tagged
as a result of stretching. If you have gd version 2.0
or better, you could rather use gdImageCopyResampled,
which smoothens rough edges formed as a result of
stretching or shrinking. If you want to copy portions
of the image with no resizing involved, then try
gdImageCopy. To rotate the image as you copy, try the
new gdImageCopyRotated function.
</p>
<pre>
<h3>gdImageStringFT</h3>
</pre>
<p>
This function writes text on to the image using the
freetype library, thus the trailing "FT" in the
function name. You should have freetype installed and
your gd library should have been complied with
freetype support.
</p>
The prototype is:
<pre>
char *gdImageStringFT(gdImagePtr im, int *brect, int
fg, char *fontname, double ptsize, double angle, int
x, int y, char *string)
</pre>
<p>
In difficulty, this function returns a char pointer
that points to an error message else returns 0. The
brect array is filled up with the size of the bounding
rectangle of the printed string. You can also
determine the size of the bounding rectagle without
actually printing a string. To do that pass NULL in
the place of the image pointer argument. For some
strange reason you need to pass the absolute path of
the font file to this function. So, even if you have a
font file in the current directory, you need to
provide the whole path. Only TTF fonts in this
function.
If your needs are simple, you can use the function
gdImageString. Freetype is not needed for this
function to work properly, it uses any one of the five
built in gd fonts.
</p>
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<img ALIGN="LEFT" ALT="[BIO]" SRC="../gx/2002/note.png">
<em>
Shuveb is a pervert by social compulsion sitting in a
small but historical city in southern India. He thinks
life is neither a Midsummer Night's Dream nor a
Tempest, it's simply a Comedy Of Errors, to be lived As
You Like It. Apart from being a part time philosopher,
he is a seasoned C programmer who is often in
confusion about what the * does to a pointer
variable.... APR Bristol is the company that pays him
for learning Linux.
</em>
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<CENTER><SMALL><STRONG>
Copyright © 2003, Shuveb Hussain.
Copying license <A HREF="../copying.html">http://www.linuxgazette.com/copying.html</A><BR>
Published in Issue 91 of <i>Linux Gazette</i>, June 2003
</STRONG></SMALL></CENTER>
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