Using ImageMagick

Basic image editing via SSH

The central tool of ImageMagick is theconvert command. You can use this command to edit images directly in the terminal.

Here are some practical application examples:
 

Convert image format

ImageMagick can read and write images in over 200 formats, including PNG, JPEG, JPEG 2000, GIF, TIFF, DPX, EXR, WebP, PostScript, PDF, and SVG. 

To convert an image to another format, use the convert command according to the scheme convert <source file> <destination file>. The format to which the file is converted is determined by the file extension of the target file. 

For example, to copy the image example.jpg in JPEG format to PNG format, enter the command as follows:

convert example.jpg example.png

It works the same way the other way round (PNG → JPEG):

convert example.png example.jpg

Scale image (change resolution)

You can also use ImageMagick to change the resolution and therefore the size of an image. To do this, use the convert command together with the resize parameter.

For example, to halve the resolution of the image file example.png, use the following command:

convert -resize 50% example.png

The following command scales the image so that it fits into a rectangle of 480x95 pixels, whereby the original aspect ratio is retained. The result is therefore a maximum width of 480 pixels and a maximum height of 95 pixels.

convert -resize 480x95 example.png

By default, ImageMagick retains the aspect ratio of the image to avoid distortion. However, if you want to force the image to the exact target size, even if this leads to distortions, add an exclamation mark (!) directly after the size specification.

convert -resize 480x95! example.png

It is also possible to scale images directly when converting them to another format. Here is an example of the combination of format conversion (JPEG → PNG) and scaling by 50%.

convert example.jpg -resize 50% example.png

Using PDF conversion in PHP scripts

If you want to call ImageMagick from a PHP script to process PDFs, an additional step is required. PHP scripts do not know the path to your policy.xml file by default. You must make it known via the HOME environment variable.

The absolute path to your webspace directory (document root) is required. You can find out how to determine this in the article Determining the absolute path (document root) of your webspace.

The following sample PHP script shows how to set the environment variable, and then convert a PDF file into a JPG file.

<?php

// Replace this path with the absolute path to your webspace directory.
$home_path = "/homepages/12/d123456789/htdocs";

// Sets the HOME environment variable so that ImageMagick finds the policy.xml.
putenv("HOME=" . $home_path);

// Define the full path to the files and the convert tool.
$pdf_file = $home_path . "/document.pdf";
$jpg_file = $home_path . "/preview.jpg";
$convert_path = "/usr/bin/convert";

// Assemble the command and execute it.
// The addition [0] ensures that only the first page of the PDF is converted.
$command = $convert_path . " " . escapeshellarg($pdf_file . "[0]") . " " . escapeshellarg($jpg_file);
$output = shell_exec($command);

// Optional: Display output or error (for debugging)
if ($output) {
   echo "Error output: " . $output;
} else {
   echo "Conversion started successfully.";
}

?>