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Transcoding

The Transcoding component is a library for abstracting file transcoding tool usage, and the presets that configure them. If you want a quick way to use it directly without implementing it in code elsewhere, check out the Mutate CLI App or the Symfony2 Transcoding Bundle.

Implementation Details & Example Usage

The Transcoding component consists of several parts.

1. The first is the Transcoder class which unifies the transcode process. It provides the glue through which various adapters, presets and transcoding jobs are registered and can interact in order to transcode files consistently and safely. It also dispatches pre/post/error events when any file is transcoded.

2. Second, there are Adapter classes which are plugins that receive a standard input file, provide some logic to transcode the file, and return a standard output file. They can put restrictions on the types of input/output files they are allowed to handle.

3. Third are the Preset classes, which provide groupings of options for an Adapter to use when implementing its transcode logic. They can also put restrictions on the types of input/output files they are allowed to handle.

4. Fourth are File instances. They are a thin extension of PHP's standard SplFileObject class. These, in conjunction with Preset instances, are what Adapters take as input. If the Adapter returns a file, it should also be an instance of AC\Transcoding\File.

5. Fifth are FileHandlerDefinition instances. These can be specified by Adapters, as well as Presets, and define what types of files are allowable as both input and output. These instances are used internally by the Transcoder to ensure valid input/output and to assist in building a valid output file path if none is specified, or to catch an invalid path if provided, before it gets to the adapter for the transcode process.

Below you will see basic example usage and implementation of each the items mentioned above.

Transcoder

The Transcoder does the work of standardizing the transcoding input and output. What exactly it does when transcoding a file is determined by the registered presets, and adapters.

Usage

Using the Transcoder by its self is simple, as it has no dependencies. It can accept presets/adapters from anywhere, some of which may have their own dependencies if necessary.

$transcoder = new AC\Transcoding\Transcoder;

// ... register presets & adapters ... 
$transcoder->registerAdapter(new MyCustomFFmpegAdapter("/path/to/ffmpeg"));
$transcoder->registerPreset(new FFmpeg/WebmHDPreset);

//transcode one file using a preset
$newFile = $transcoder->transcodeWithPreset($inputFilePath, 'webm-hd', $outputFilePath);

//transcode a file with a specific adaptor and options
$newFile = $transcoder->transcodeWithAdapter($inputFilePath, 'ffmpeg', array(
    /* options */
));

Adapters

Adapters are wrappers for a pre-existing toolset which does the real work for any file conversion/manipulation. Technically these adapters can be anything. Common examples are ffmpeg for audio/video manipulation and ImageMagick for image manipulation in PHP. By default, the library provides Adapter implementations for several commonly used tools, including those just mentioned.

Registering an adapter

Adapters can be fairly simple, or quite complex. The adapters included in the library do not have external dependencies which aren't provided by the library (aside from requiring certain tools be installed on the system). However, other adapters may require external PHP dependencies and special set-up. It is beyond the scope of the library to handle this.

//build your custom adapter
$adapter = new MyCoolAdapter(/* inject any dependencies */);
$transcoder->registerAdapter($adapter);

//register adapters provided with the library
$transcoder->registerAdapter(new FFmpegAdapter);
$transcoder->registerAdapter(new ImageFormatConverterAdapter);
$transcoder->registerAdapter(new ImageEffectsAdapter);

Writing an adapter

All adapters receive input in the same way - they simply take an input file object, a string output path, and a Preset instance for use during the transcode process. Generally, adapters aren't used directly, but the Transcoder will call passing along registered presets, and testing for valid input/output based on the preset definition. Below is an example template for a very simple custom adapter. For more detailed documentation on writing an adapter, see the README.md in adapters/.

<?php
use AC\Transcoding\Adapter;
use AC\Transcoding\File;
use AC\Transcoding\Preset;

class FooAdapter extends Adapter {
    protected $name = 'foo';
    protected $description = "A made-up adapter for documentation purposes.";

    public function transcodeFile(File $inFile, Preset $preset, $outFilePath) {

        //do actual transcode process, however that needs to happen for this adapter

        //return a new file instance for the created file
        return new File($outFilePath);
    }
}

Implementing command-line tools

Many file conversion tools are available as command line executables. Writing code to make executing command line processes safe and consistent accross environments has already been done well with the Symfony\Component\Process component, which is provided with this library. If you want to implement a tool that requires using the command line, we highly recommend using this library rather than writing custom code. Read more on the Symfony\Component\Process component here.

For example, the FFmpeg and Handbrake adapters use the Symfony\Component\Process component to actually execute its command line process. The general flow goes something like the following:

//method of an adapter class
public function transcodeFile(File $inFile, Preset $preset, $outFilePath) {
    // ... parse the $preset object to assemble a command string in $commandString ...

    //use the Process component to build a process instance with the command string
    $process = new \Symfony\Component\Process\Process($commandString);

    //if this could be a long-running process, be sure to increase the timeout limit accordingly
    $process->setTimeout(3600);

    //pass an anonymous function to the process so the adapter can get output as it occurs
    $result = $process->run();
    $output = $process->getOutput();

    //check for error status return
    if(!$process->isSuccessful()) {
        throw new \RuntimeException($process->getExitCodeText());
    }

    return new File($outputFilePath);
}

Presets

Presets help streamline the transcode process by bundling together common options and requirements into one package. Several presets are provided with the library for common types of file conversions using popular tools.

For more specific documentation and a usable template, see the README.md in presets/.

Registering a preset

Presets shouldn't have dependencies, since they are really just a mechanism for bundling options which will be passed to an adapter. You can declare/register presets in two ways:

//instantiate inline preset
$transcoder->registerPreset(new \AC\Transcoding\Preset('preset_name', 'required_adapter_name', array(/* preset options */), array(/* FileHandlerDefinition options */)));

//pre-defined preset which extends the Preset class above and defines it's settings internally
$transcoder->registerPreset(new Mp4_HD_720Preset);

Writing a preset

A preset can be declared in two ways. You may create one by instantiating the preset class, passing it the required options, or you could extend the base Preset class. The library provides many presets which extend the base Preset class, to make them easy to work with. Presets require two main parts, the first is the actual preset options, which will be passed to the adapter, and the second is two FileHandlerDefinition instances, which standardize what the accepted input/output formats can be. For example, check out this Handbrake preset for generating video for iOS devices:

<?php

namespace AC\Transcoding\Preset\Handbrake;

use AC\Transcoding\Preset;

/**
 * For more information on this preset please visit this link: https://trac.handbrake.fr/wiki/BuiltInPresets#classic
 */
class ClassicPreset extends BasePreset
{
    protected $key = "handbrake.classic";
    protected $name = "Classic Preset";
    protected $description = "HandBrake's traditional, faster, lower-quality settings. ";

    /**
     * Specify the options for this specific preset
     */
    public function configure()
    {
        $this->setOptions(array(
            'video-library-encoder' => 'x264',
            'video-bitrate' => '1000',
            'select-audio-tracks' => '1',
            'audio-encoder' => 'faac',
            'audio-bitrate' => '160',
            'surround-sound-downmixing' => 'dp12',
            'audio-samplerate' => 'Auto',
            'dynamic-range-compression' => '0.0',
            'format' => 'mp4',
        ));
    }
}   

FileHandlerDefinition instances

Both Adapters and Presets can specify FileHandlerDefintion instances to restrict accepted types of input and output files. The Transcoder uses the FileHandlerDefinition instances to handle input and output in a standardized manner. FileHandlerDefinition instances can set restrictions on allowed or rejected input extensions, mime types, mime encodings, and other properties.

The FileHandlerDefinition instances are also used by the Transcoder to assemble an output file path, which will be passed to an adapter, if none was provided to the transcoder when running a job.

By default, all Adapter and Preset classes will return FileHandlerDefinition instances for both input and output files which will accept files of any format.

Events

The Transcoder is also an instance of of the Symfony EventDispatcher. It fires events for pretty much everything that can happen during a transcode process, for example, before and after the process, if an error occurs, and any time a file or directory is modified. You can register listeners for these events to implement other important features, such as logging.

TODO: define/explain all the events