patch MediaWiki extension TimedMediaHandler enable alpha transcodes REL1_39

ext-TimedMediaHandler-alphawebm-REL1_39.patch

diff --git a/includes/WebVideoTranscode/WebVideoTranscodeJob.php b/includes/WebVideoTranscode/WebVideoTranscodeJob.php
index df24861a..6849a0e9 100644
--- a/includes/WebVideoTranscode/WebVideoTranscodeJob.php
+++ b/includes/WebVideoTranscode/WebVideoTranscodeJob.php
@@ -14,6 +14,7 @@ use FSFile;
 use Job;
 use MediaWiki\Logger\LoggerFactory;
 use MediaWiki\MediaWikiServices;
+use MediaWiki\TimedMediaHandler\Handlers\WebMHandler\WebMHandler;
 use MediaWiki\TimedMediaHandler\TimedMediaHandler;
 use TempFSFile;
 use Title;
@@ -389,6 +390,24 @@ class WebVideoTranscodeJob extends Job {
 		}
 	}
 
+	/**
+	 * Adds input codec information for more accurate input codec selection
+	 */
+	private function getInputCodec() {
+		$file = $this->getFile();
+		$handler = $file->getHandler();
+		if ( $handler instanceof WebMHandler ) {
+			$streams = $handler->getStreamTypes( $file );
+			if ( in_array( 'VP9', $streams ) ) {
+				// In order to carry out vp9 alpha channel to transcodes,
+				// it's necessary to specify the input codec as vp9,
+				// otherwise the default vp8 gets selected
+				return ' -vcodec libvpx-vp9';
+			}
+		}
+		return '';
+	}
+
 	/**
 	 * Utility helper for ffmpeg mapping
 	 * @param array $options
@@ -402,10 +421,12 @@ class WebVideoTranscodeJob extends Job {
 			return "source file is missing, " . $this->getSourceFilePath() . ". Encoding failed.";
 		}
 
+		$inputCodec = $this->getInputCodec();
+
 		// Set up the base command
 		$cmd = wfEscapeShellArg(
 			$wgFFmpegLocation
-		) . ' -nostdin -y -i ' . wfEscapeShellArg( $this->getSourceFilePath() );
+		) . " -nostdin -y $inputCodec -i " . wfEscapeShellArg( $this->getSourceFilePath() );
 
 		if ( isset( $options['vpre'] ) ) {
 			$cmd .= ' -vpre ' . wfEscapeShellArg( $options['vpre'] );
@@ -588,7 +609,8 @@ class WebVideoTranscodeJob extends Job {
 
 		// Force to 4:2:0 chroma subsampling. Others are supported in Theora
 		// and in VP9 profile 1, but Chrome and Edge don't grok them.
-		$cmd .= ' -pix_fmt yuv420p';
+		// yuva420p allows alpha channel
+		$cmd .= ' -pix_fmt yuva420p';
 
 		// Check for video quality:
 		if ( isset( $options['videoQuality'] ) && $options['videoQuality'] >= 0 ) {