# User:Mjb/MP4Box

## Contents

## MP4Box download

This is the GPAC installer. When running the installer, untick every box except the one for MP4Box. On Windows, it will put mp4box.exe and supporting files in your `"%ProgramFiles(x86)%\GPAC"`

folder.

## Command-line examples

### Rewrite the PAR (pixel aspect ratio)

My input video was a YouTube rip at 1920×1080, and was 4:3 NTSC TV content that was playing back horizontally stretched. FFmpeg reports it has SAR 1:1 DAR 16:9, i.e. 1920×1080 stored image size @ 1:1 SAR = 1920×1080 displayed image size = 16:9 DAR (display aspect ratio). So I ran it through MP4Box like this:

`mp4box -add "input.mp4" -par 1=3:4 -new "output.mp4"`

FFmpeg reports output.mp4 has SAR 3:4 DAR 4:3, and indeed it plays back correctly as 4:3. Hooray!

FFmpeg numbers its streams starting with 0, but MP4Box starts at 1, hence the "1=" to say that the new PAR applies to stream 1 of the input.

### PAR calculation

The "3:4" in the example above is calculated thusly:

Given the DAR of 16:9, we need to solve for *x* and *y* (the PAR) in this equation:

- ( 16 ⁄ 9 ) × (
*x*⁄*y*) = 4:3

So divide both sides by 16:9:

- (
*x*⁄*y*) = ( 4 ⁄ 3 ) ÷ ( 16 ⁄ 9 )

Division by a fraction is the same thing as multiplication by the inverse of that fraction:

- (
*x*⁄*y*) = ( 4 ⁄ 3 ) × ( 9 ⁄ 16 )

Multiplication of two fractions is a new fraction where the top is the product of the tops, and the bottom is the product of the bottoms:

- (
*x*⁄*y*) = ( 4 × 9 ) ⁄ ( 3 × 16 ) - (
*x*⁄*y*) = 36 ⁄ 48

So 36:48 is the answer!

Now you can use the Euclidian algorithm to get the greatest common divisor so you can reduce 36:48 to an irreducible fraction (optional, but ideal):

- Divide larger number by smaller one, and look at remainder: 48 ÷ 36 = 1 R 12.
- As long as remainder is not zero, divide last divisor (bottom number) by it: 36 ÷ 12 = 3 R 0.
- We're already done; the common divisor is 12.

Dividing 36 and 48 by 12 gives us 3 and 4, the irreducible form of 36:48. So **3:4** is the PAR we want!

(If remainder had been, say, 5, we would be dividing 12 by 5 in the next step.)