Expose ffmpeg child-process

README.md

@@ -132,6 +132,9 @@ Edit specs are JavaScript / JSON objects describing the whole edit operation wit
   enableFfmpegLog: false,
   verbose: false,
   fast: false,
+
+  onStart: (cmd) => console.log('Command:', cmd),
+  onProcessStart: (subprocess) => subprocess.stdout.pipe(process.stdout),
 }
 ```
 
@@ -156,6 +159,8 @@ Edit specs are JavaScript / JSON objects describing the whole edit operation wit
 | `clips[].transition` | | Specify transition at the **end** of this clip. See `defaults.transition` | `defaults.transition` | |
 | `clips[].layers[]` | | List of layers within the current clip that will be overlaid in their natural order (last layer on top) | | |
 | `clips[].layers[].type` | | Layer type, see below | | |
+| `onStart` | | Callback function, called before ffmpeg process start with command line as string argument | | |
+| `onProcessStart` | | Callback function, called after ffmpeg process start with subprocess argument | | |
 
 ### Transition types
 

index.js

@@ -39,8 +39,15 @@ module.exports = async (config = {}) => {
 
     ffmpegPath = 'ffmpeg',
     ffprobePath = 'ffprobe',
+
+    onStart,
+    onProcessStart,
+    
   } = config;
 
+  assert(!onStart || typeof onStart === 'function', 'Callback onStart expected to be a function');
+  assert(!onProcessStart || typeof onProcessStart === 'function', 'Callback onProcessStart expected to be a function');
+
   const isGif = outPath.toLowerCase().endsWith('.gif');
 
   const audioFilePath = isGif ? undefined : audioFilePathIn;
@@ -314,6 +321,7 @@ module.exports = async (config = {}) => {
       ...outputArgs,
     ];
     if (verbose) console.log('ffmpeg', args.join(' '));
+    if (onStart) onStart(`ffmpeg ${args.join(' ')}`);
     return execa(ffmpegPath, args, { encoding: null, buffer: false, stdin: 'pipe', stdout: process.stdout, stderr: process.stderr });
   }
 
@@ -323,6 +331,7 @@ module.exports = async (config = {}) => {
 
   try {
     outProcess = startFfmpegWriterProcess();
+    if (onProcessStart) onProcessStart(outProcess);
     let outProcessError;
 
     // If we don't catch it here, the whole process will crash and we cannot process the error