FFmpeg – Change resolution of the video with aspect ratio

aspect-ratioffmpegresolutionvideo

all.

How to change resolution of the video with aspect ratio with FFmpeg?

There are options
http://manpages.ubuntu.com/manpages/oneiric/man1/ffmpeg.1.html

       -s size
       Set frame size. The format is wxh (ffserver default = 160x128,
       ffmpeg default = same as source).  The following abbreviations are
       recognized:

and

       -aspect aspect
       Set the video display aspect ratio specified by aspect.

       aspect can be a floating point number string, or a string of the
       form num:den, where num and den are the numerator and denominator
       of the aspect ratio. For example "4:3", "16:9", "1.3333", and
       "1.7777" are valid argument values.

For example, I have two input videos:

with 200*400 resolution
with 400*700 resolution

I need to make output video with 100*200 resolution.

If I will run ffmpeg with -s 100×200, then second video will have bad aspect ratio.

How can I limit output video by width, with auto aspect ratio by height?

For example, I want specify for the output video only width 100px and ffmpeg must automatically calculate height with right aspect ratio.

For first video it will be:

200/100=2

400/2=200

Ie 100×200

For second video it will be:

400/100=4

700/4=75

Ie 100×75

Is it possible?

Best Answer

ffmpeg -i <input> -vf scale=720x406,setdar=16:9 <output>

Related Solutions

Html – Maintain the aspect ratio of a div with CSS

Just create a wrapper <div> with a percentage value for padding-bottom, like this:

.demoWrapper {
  padding: 10px;
  background: white;
  box-sizing: border-box;
  resize: horizontal;
  border: 1px dashed;
  overflow: auto;
  max-width: 100%;
  height: calc(100vh - 16px);
}

div {
  width: 100%;
  padding-bottom: 75%;
  background: gold; /** <-- For the demo **/
}

<div class="demoWrapper">
  <div></div>
</div>

It will result in a <div> with height equal to 75% of the width of its container (a 4:3 aspect ratio).

This relies on the fact that for padding :

The percentage is calculated with respect to the width of the generated box's containing block [...] (source: w3.org, emphasis mine)

Padding-bottom values for other aspect ratios and 100% width :

aspect ratio  | padding-bottom value
--------------|----------------------
    16:9      |       56.25%
    4:3       |       75%
    3:2       |       66.66%
    8:5       |       62.5%

Placing content in the div :

In order to keep the aspect ratio of the div and prevent its content from stretching it, you need to add an absolutely positioned child and stretch it to the edges of the wrapper with:

div.stretchy-wrapper {
  position: relative;
}

div.stretchy-wrapper > div {
  position: absolute;
  top: 0; bottom: 0; left: 0; right: 0;
}

Here's a demo and another more in depth demo

Maintaining aspect ratio with FFmpeg

-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

Best Answer

Related Solutions

Html – Maintain the aspect ratio of a div with CSS

Maintaining aspect ratio with FFmpeg

Related Topic