Added a few comments in the script

2pass_encode_test.sh

@@ -2,12 +2,14 @@
 set -e
 set -x
 
+# Directories. Source files go in SOURCE, encoded output is in ENCODED and source files are moved to done after
 SOURCE="source"
 ENCODED="encoded"
 DONE="done"
 
 mkdir -p "$SOURCE" "$ENCODED" "$DONE"
 
+# Define resolutions and bitrates. Number of bitrates and resolutions should match
 res=("3840:-2" "2560:-2" "1920:-2" "1280:-2" "854:-2" "480:-2")
 bitrate=("26000k" "15000k" "7000k" "4000k" "2000k" "1000k")
 
@@ -21,52 +23,59 @@ for INPUT in "$SOURCE"/*; do
 
     mkdir -p "$OUTDIR"
     for i in ${!res[@]}; do
-	    BUFSIZE=$(( 2 * ${bitrate[i]%k} ))k
+	BUFSIZE=$(( 2 * ${bitrate[i]%k} ))k # Sets buffer size
+	# Next command calls ffmpeg for first pass.
+	# -g and keyint_min set the max and min intervals for key frames/i-frames. Should use the same number. Recommended to use fps*segment_length (in seconds)
+	# -force_key_frames sets how often key frames must be forced. Should be set up to force at intervals equal to or less than segment length. If segment length changes, the expr following should change to match
+	# -pass 1 and -passlogfile tell ffmpeg this is the first pass and create a log file to be passed to second pass.
         ffmpeg -y -i "$INPUT" -vf "scale=${res[i]}" -c:v libx264 \
             -b:v ${bitrate[i]} -minrate:v ${bitrate[i]} \
             -maxrate:v ${bitrate[i]} -bufsize:v ${BUFSIZE} \
             -g 120 -keyint_min 120 \
-            -force_key_frames "expr:gte(t,n_forced*4)" \
-	    -pass 1 -passlogfile "$OUTDIR/res_$i.log" -an -f mp4 /dev/null 
-	   
-       	if [ $i -eq 0 ]; then
+	    -force_key_frames "expr:gte(t,n_forced*4)" \
+	    -pass 1 -passlogfile "$OUTDIR/res_$i.log" -an -f mp4 /dev/null
+       	if [ $i -eq 0 ]; then # If first resolution/bitrate, also encodes audio, skips audio if not
+		# -pass 2 and -passlogfile tell ffmpeg this is the second pass and pass the log file from first pass
             ffmpeg -y -i "$INPUT" -vf "scale=${res[i]}" -c:v libx264 \
                 -b:v ${bitrate[i]} -minrate:v ${bitrate[i]} \
                 -maxrate:v ${bitrate[i]} -bufsize:v ${BUFSIZE} \
                 -g 120 -keyint_min 120 \
                 -force_key_frames "expr:gte(t,n_forced*4)" \
                 -pass 2 -passlogfile "$OUTDIR/res_$i.log" \
-		        -c:a aac -b:a 128k -ac 2 "$OUTDIR/res_$i.mp4"
+		-c:a aac -b:a 128k -ac 2 "$OUTDIR/res_$i.mp4"
         else
             ffmpeg -y -i "$INPUT" -vf "scale=${res[i]}" -c:v libx264 \
                 -b:v ${bitrate[i]} -minrate:v ${bitrate[i]} \
                 -maxrate:v ${bitrate[i]} -bufsize:v ${BUFSIZE} \
                 -g 120 -keyint_min 120 \
                 -force_key_frames "expr:gte(t,n_forced*4)" \
-                -pass 2 -passlogfile "$OUTDIR/pass_$i.log" \
-		        -an "$OUTDIR/res_$i.mp4"
+                -pass 2 -passlogfile "$OUTDIR/res_$i.log" \
+		-an "$OUTDIR/res_$i.mp4"
         fi
         rm -f "$OUTDIR/res_$i.log"*
     done
 
+    # makes command for number of resolutions (all of which are encoded as seperate mp4 files)
     DASH_CMD="ffmpeg"
     for i in ${!res[@]}; do
         DASH_CMD+=" -i $OUTDIR/res_$i.mp4"
     done
 
+    # makes command for DASH representation mapping, includes audio mapping
     MAP_ARGS=""
     for i in ${!res[@]}; do
         MAP_ARGS+=" -map $i:v "
     done
     MAP_ARGS+=" -map 0:a"
 
+    # Processes Dash, outputs NAME.mpd. Segment length is defined by -seg_duration
     $DASH_CMD $MAP_ARGS -c copy \
-        -f dash -seg_duration 5 -use_timeline 1 -use_template 1 \
+        -f dash -seg_duration 4 -use_timeline 1 -use_template 1 \
         -init_seg_name "init-$NAME-\$RepresentationID\$.mp4" \
         -media_seg_name "chunk-$NAME-\$RepresentationID\$-\$Number%05d\$.m4s" \
         -adaptation_sets "id=0,streams=v id=1,streams=a" \
         "$OUTDIR/$NAME.mpd"
 
     echo "Done Encoding $BASENAME"
-    mv "$INPUT" "$DONE/"
+    mv "$INPUT" "$DONE/" # Moves source file to done/
 done