PHP Programming/Images

Introduction

PHP can create and modify dynamically some images, for example with the GD Graphics Library, included by default since PHP 3.0.

A new image creation must follow with a few steps:

1. Memory loading of a new or an existing image.
2. Optional colors loading to add.
3. Optional components modifications (lines creation, points, fillings, texts addition...).
4. Image restitution by posting its type into the header.
5. Memory release.

Create a new image

To create an image ex nihilo, use the function:

imagecreatetruecolor($height, $width)

which creates in memory a new image, its height and width are defined in pixel, and restitutes a reference to the new image.

There is also another function for this, but it's not recommended as its colors amplitude is worst:

imagecreate($height, $width)

To load in memory an image which had been saved on the disk:

imagecreatefrom<type>($path)

Example:

$img = imagecreatefrompng('image.png');

Other function:

imagecreatefromstring($text)

which creates an image from its text format, specified in parameter.

If an error occurs, theses functions return false.

Work with the colors

To allocate a color, the RGB parameters must be defined:

$color = imagecolorallocate($image,$r,$g,$b)

To define a transparency into a PNG:

imagecolortransparent($image,$color)

where $color is the result of imagecolorallocate.

It's also possible possible to determine the transparency, between 0 and 127 (which represents the total transparency) with the function:

imagecolorallocatealpha($image,$r,$g,$b,$transparency)

Remark: the first allocated color defines the whole image background.

Once the image is created and colored, it becomes possible to apply the following operations to it:

• Draw some pixels (eg: create lines).
• Work on the existing pixels by designating zones.

Draw shapes

To draw a pixel, we use its coordinates (x, y below):

imagesetpixel(image, x, y, color)

To draw a line between two points:

imageline(image, x1, y1, x2, y2, color)

To create a rectangle from its diagonal:

imagerectangle(image, x1, y1, x2, y2, color)

To represent an ellipse from its center, its height and width:

imageellipse(image, x, y, h, l, color)

or by precising its arc by its angles in gradient (clockwise numbered):

imagearc(image, x, y, h, l, angle1, angle2, color)

Rework the existing pixels

The most used functions to rework images like photos, is certainly imagecopyresized, which allows to copy a rectangular zone to paste it in another image. Example:

imagecopyresized(dst_image, src_image, dst_x, dst_y, src_x, src_y, dst_w, dst_h, src_w, src_h);

where:

• src_image is the source image;
• dst_image is the destination image;
• dst_x, dst_y are the coordinates of dst_image;
• src_x, src_y are the coordinates of src_image, beginning at the top left;
• dst_w, dst_h, src_w, src_h are the source and destination rectangles widths and heights.

It becomes understandable after, that if dst_w is equal to src_w, and dst_h to src_h, the image rectangular portion will remain the same size. On the contrary we lengthen or enlarge.

The function imagecopyresampled receives the same parameters as imagecopyresized, but in case of resizing, the quality est improved.

Then it exists the function imagefilter which allow numerous effects, such as grayscale, relief, or recoloration.

Print the output

The obtained image format ("png", "jpeg" or "gif") can be specified by the function called header, via the content-type (by default text/html) with:

header("Content-type: image/<type>");

To visualize the image after, place it in parameter in a function depending on its type: imagepng, imagejpeg or imagegif.

Then, liberate the memory with imagedestroy($image). This stage is optional but strongly recommended for huge images.

Example

The following code displays in the navigator, a 50 pixels red square into a 100 black one.

$image = imagecreatetruecolor(100, 100);
$color = imagecolorallocate($image, 255, 0, 0);
imagefilledrectangle($image,0,0,50,50,$color);
header("Content-type: image/png");
imagepng($image);
imagedestroy($image);