Maintaining aspect ratio with FFmpeg

Question

-vf "scale=640:-1"

works great until you will encounter error

[libx264 @ 0x2f08120] height not divisible by 2 (640x853)

So most generic approach is use filter expressions:

scale=640:trunc(ow/a/2)*2

It takes output width (ow), divides it by aspect ratio (a), divides by 2, truncates digits after decimal point and multiplies by 2. It guarantees that resulting height is divisible by 2.

Credits to ffmpeg trac

UPDATE

As comments pointed out simpler way would be to use -vf "scale=640:-2". Credits to @BradWerth for elegant solution

For example:

1920x1080 aspect ratio 16:9 => 640x480 aspect 4:3:

ffmpeg -y -i import.media -aspect 16:9 scale=640x360,pad=640:480:0:60:black output.media

aspect ratio 16:9 , size width 640pixel => height 360pixel:
With final output size 640x480, and pad 60pixel black image (top and bottom):

"-vf scale=640x360,pad=640:480:0:60:black"

I've asked this a long time ago, but I've actually got a solution which was not known to me at the time -- in order to keep the aspect ratio, you should use the video filter scale, which is a very powerful filter.

You can simply use it like this:

-vf "scale=640:-1" 

Which will fix the width and supply the height required to keep the aspect ratio. But you can also use many other options and even mathematical functions, check the documentation here - http://ffmpeg.org/ffmpeg.html#scale

Although most of these answers are great, I was looking for a command that could resize to a target dimension (width or height) while maintaining aspect ratio. I was able to accomplish this using ffmpeg's Expression Evaluation.

Here's the relevant video filter, with a target dimension of 512:

-vf "thumbnail,scale='if(gt(iw,ih),512,trunc(oh*a/2)*2)':'if(gt(iw,ih),trunc(ow/a/2)*2,512)'"

---

For the output width:

'if(gt(iw,ih),512,trunc(oh*a/2)*2)'

If width is greater than height, return the target, otherwise, return the proportional width.

---

For the output height:

'if(gt(iw,ih),trunc(ow/a/2)*2,512)'

If width is greater than height, return the proportional height, otherwise, return the target.

Use force_original_aspect_ratio, from the ffmpeg trac:

ffmpeg -i input.mp4 -vf scale=720:400:force_original_aspect_ratio=decrease output.mp4