compress videos and transcode

ffmpeg_compress.sh

# http://ruby-china.org/topics/10987
# ffmpeg_compress.sh

# $1是源文件，$2是码率，网络传输建议码率是1024k~2048k，$3是分辨率，720p是-1:720，-1表示根据高720保持比例缩放，$4是视频比例，看源视频了，比如4:3, 3:2, 16:9等，$5是输出文件名

# 1st pass:

ffmpeg -i $1 -vcodec libx264 -vprofile high -preset slow -b:v $2 -maxrate $2 -bufsize 4000k -vf scale=$3 -aspect $4 -threads 0 -pass 1 -an -f mp4 /dev/null

# 2nd pass:

ffmpeg -i $1 -vcodec libx264 -vprofile high -preset slow -b:v $2 -maxrate $2 -bufsize 4000k -vf scale=$3 -aspect $4 -threads 0 -pass 2 -acodec libfaac -b:a $5 -f mp4 $6

# 具体使用方法实例：
# sh ffmpeg_compress.sh input.avi 2000k -1:720 16:9 output.mp4