Generating Images with PHP

A set of php functions exists for generation of image files. These were developed by Thomas Boutell and are called GD (Gif Draw). When PHP is compiled with GD, these functions are available to create or modify images.

A function called gd_info() gives a modicum of information about the capabilities of the GD/php build in use. Click here for a demonstration.

Other information on the GD functions is available at php.net or Zend or Boutell.com

Overview of Steps in Image Generation

1. Create the image.
2. Allocate the colors to be used.
3. Draw something.
4. Set the mime type.
5. Output the image.

Here is an example:

<?php
//----------- create image --------------
$im=imageCreate(250,250);
//----------allocate colors -------------
$white=imageColorAllocate($im,0xff,0xff,0xff);
$gray=imageColorAllocate($im,0xcc,0xcc,0xcc);
$black=imageColorAllocate($im,0x00,0x00,0x00);
//--- draw 2 circles, a dot, and some text ---
imagefilledEllipse($im,51,125,10,10,$gray);
imageEllipse($im,125,125,148,148,$black);
imageSetPixel($im,125,125,$black);
imagestring($im, 5, 10,230, 'hydrogen', $black); 
//----------- set mime type -------------
header('Content-Type: image/gif');
//------------ output image -------------
imageGIF($im);
?>

This image was sent directly to the browser, as a php file with the code:

<img src="H.php">

The image file must contain no errors. If it does, the image will not display. No other warning will be given. Parsing errors will not be detected.

Image Generation Considered in More Detail

Image Creation

All the imagecreate() functions take two integer arguments, x and y, representing the image width and height in pixels.

All return a resource representing the image.

Color Allocation

image is an image resource returned from a call to one of the imagecreate() functions. r,g,b are bytes, denoting red, green, blue color.

The first call to imagecolorallocate() sets the background color for the image.

Drawing and Adding Text

The top left corner is (0,0).

Adding Text:

Other Fonts for Adding Text

The function imagestring() uses a single font. This is available in 5 sizes, denoted by the integers, 1 to 5. However, it can use other fonts, if they are first introduced with the function imageloadfont('fontfile'). These fonts must be gdf fonts.

I have found two sources for gdf fonts.

1. Dave Wedwick has created a routine which converts Windows ttf fonts to gdf fonts. It can be found in the User Contributed Notes for imageloadfont() at php.net.
2. Some gdf fonts are available for free download at Widgnet.com.

<?php
$im=imageCreate(400,400);
$blue=imageColorAllocate($im,0xee,0xee,0xff);
$black=imageColorAllocate($im,0x00,0x00,0x00);
imageEllipse($im,200,200,212,212,$black);
imageEllipse($im,94,200,10,10,$black);
imageSetPixel($im,200,200,$black);
$fontfile="./fonts/arial14.gdf";
$font=imageloadfont($fontfile);

imagestring($im, $font, 10, 380, "Hydrogen", $black);
header('Content-Type: image/gif');
imageGIF($im);
?>

Set the Mime Type

The header function is used to set the mime type. For example, the header for a gif image would be:

header('Content-Type: image/gif');

The mime types needed for the other image types are:

A complete list of mime types and subtypes is available at W3C Mime Reference

The mime type must agree with the output function.

Output the Image

Output functions:

Output an image to a File rather than the Browser

The earlier example sent an image directly to the browser. No image file was created. Although the above method is simple, it is sometimes advantageous to actually create an image file, since image files are downloaded faster than the php file can be downloaded and interpreted. This is done by using the optional argument in the output function to name a file for the output.

The image must be called twice. The first time it is called as 'filename.php' since the php code must be invoked. The documentation says that an image will be seen in the browser, but I do not get one. Thereafter it can be called as 'filename.gif' (or whatever). An image will appear, and a '.gif' file will actually appear on the server.

More Useful PHP GD Functions

Determining Image Properties

• getimagesize('filename') -- returns an array where arr[0],arr[1] are the image width and height, arr[2] is an integer, denoting the image type, and arr[3] is a string giving height and width.
• imagesx -- Get image width
• imagesy -- Get image height
• imagefontheight -- Get font height
• imagefontwidth -- Get font width
• imagecolorstotal($im) -- returns number of colors in an image
• imagecolorsforindex($im, i) -- returns an array of red, green, blue, alpha
• imagecolorset($im, i, red, green, blue) -- nothing is returned
• imagecolorat($im,x,y)-- if palette based, returns the index of the color at point x,y

In order to print out the results of these functions, you should open the image with one of the image create functions, but not use the header or output function.

<?php
 $filename='../snowflake/02c.gif';
 $im=imageCreateFromgif($filename);
 $t=imagecolorstotal($im);
 for ($i=0;$i<$t;$i++){
   $arr= imagecolorsforindex($im,$i);
   print_r($arr);
 };
?>

The file can then be called with an anchor link.

<pre>
<a href="exam.php">Find Image Colors</a>
</pre>

Find Image Colors

Copy an Image to another Image

imagecopy(dest, src, dx, dy, sx, sy, sw, sh)

• where dest and src are resources representing the destination and source images.
• dx and dy are the point of insertion in the destination image
• sx and sy are the left, top corner of the source image.
• sw and sh are the width and height of the portion of source image to be copied.

Note: The entire color palette of the source image is copied as well.

Change an Image's Color

• image is an image resource
• dither is boolean
• n is the number of colors to allow in the palette

changes truecolor image to palette based.

Apply a Filter to an Image

 imagefilter(image, filter [,arg1,arg2,arg3])

The argument filter can take on any of the values:

Rotate Image

I've only been able to use this function to rotate an image in a file, not one I'm drawing.

<?php
$filename = 'elip.gif';
$degrees = 45;
header('Content-type: image/gif');
$source = imagecreatefromgif($filename);
$rotate = imagerotate($source, $degrees, 0);
imagegif($rotate,'rotate.gif',10);
?>

Make image transparent

imagecolortransparent(image, i)
Defines color i to be transparent.

Note from cinymini at php.net cinymini 14-Dec-2006 11:17 When you use palette images (created with imagecreate()), the first color allocated is the background color. This color cannot be used for transparency. So if you want to make the background transparent, first allocate a dummy background color, then allocate the real background color and declare this is as transparent.

Overlapping colors using imagecolorallocatealpha(). The code for this image is at Zend.