Custom file processors 

Create a new processor class 

The file must implement the \TYPO3\CMS\Core\Resource\Processing\ProcessorInterface and two required methods.

canProcessTask()

Will decide whether the given file should be handled at all. The expected return type is boolean.

processTask()

Will then do whatever needs to be done to process the given file.

Register the file processor 

To register a new processor, add the following code to ext_localconf.php

<?php

$GLOBALS['TYPO3_CONF_VARS']['SYS']['fal']['processors']['MyNewImageProcessor'] = [
  'className' => \MyVendor\MyExtension\Resource\Processing\MyNewImageProcessor::class,
  'before' => ['LocalImageProcessor'],
];

With the before and after options, priority can be defined.

Processing system resources from EXT: and PKG: 

The processing pipeline also scales and crops images that an extension or a package ships, referenced as EXT: or PKG: system resource. For such a task TaskInterface::getSourceFile() returns a SystemResourceInterface that is not a FAL File . The result is not stored in the database or in a _processed_/ folder. See Processed files.

A processor receives such a task only if it implements the marker interface ResourceAwareProcessorInterface . The Core processors LocalImageProcessor and SvgImageProcessor implement it for the task type Image.CropScaleMask. A subclass of these processors inherits the interface. If the subclass uses methods that only a FAL file has, it must decline other sources in canProcessTask() .

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Resource\Processing;

use TYPO3\CMS\Core\Resource\File;
use TYPO3\CMS\Core\Resource\Processing\LocalImageProcessor;
use TYPO3\CMS\Core\Resource\Processing\TaskInterface;

class FalOnlyImageProcessor extends LocalImageProcessor
{
  public function canProcessTask(TaskInterface $task): bool
  {
    // Leave system resources from EXT: and PKG: to the Core processors
    return $task->getSourceFile() instanceof File
        && parent::canProcessTask($task);
  }
}