I have a video stream with the following properties:

Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 720x416 [SAR 1:1 DAR 45:26], 1908 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc

When I run it in VLC, I have to press "A" to change aspect ratio to "4:3" to make the video show with the correct aspect ratio.

Looking at the video facts, Is the error that a) the actual video has been incorrectly stretched in the pixel data, or b) there is simply some metadata value that has been incorrectly set?

If the former, I know I can re-encode the video and change the width and height. But if the latter, what ffmpeg command to I run to fix the metadata without re-encoding the video itself?

6 Answers

There is a difference between Sample Aspect Ratio (SAR) and Display Aspect Ratio (DAR). If you want to change the video to display at 4:3, you will either need to change the actual pixels in the image (by scaling the pixels and changing SAR), or by setting a metadata flag that at the container level that tells external media players to stretch the image to your desired DAR.

You will not be able to scale the pixels and change SAR without applying a video filter. If you choose this method, you will be required to transcode the file - since you cannot "stream copy" the video stream while applying a video filter.

To scale the image and change SAR (while transcoding), try:

ffmpeg -i <INPUT_FILE> -vf scale=720:540 -c:v <Video_Codec> <OUTPUT_FILE>

On the other hand, if you just want to change the metadata flag and adjust the DAR, you will be able to stream copy the video. To do this, try:

ffmpeg -i <INPUT_FILE> -aspect 720:540 -c copy [OUTPUT_FILE]

11

Changing the SAR without reencoding also works with ffmpeg on .mp4 using the h264_metadata as Gyan pointed out here:

ffmpeg -i in.mp4 -c copy -bsf:v "h264_metadata=sample_aspect_ratio=4/3" out.mp4

4

Delgado's answer is correct that MP4Box can do this, but the -par option doesn't work quite as described. With an -out parameter (so as not to disturb your original file):

mp4box source.mp4 -out target.mp4 -par stream-number=width:height

When you use -par stream-number=width:height, you define the pixel aspect ratio – that is, the result of dividing the device aspect ratio by the storage aspect ratio. (Equivalently, you're describing the aspect ratio of a source pixel.) For example, suppose you have a DVD source that's 720×480, and the correct display aspect ratio is 4:3. For this case, you need:

mp4box source.mp4 -out target.mp4 -par 1=8:9

because (4/3) / (720/480) = 8/9.

If the source represents true SD NTSC pixels (in which case only the central 704×480 pixels are supposed to map to a 4×3 screen, with 8 pixels overscan on either side), the correct command would be:

mp4box source.mp4 -out target.mp4 -par 1=10:11

because (4/3) / (704/480) = 10/11 – exactly the reference pixel aspect ratio for standard definition NTSC video.

For the case given in the question, if it's really 4:3, that gives a very odd pixel aspect ratio: (4/3)/(720/416) = 104/135. It's 720 wide, which suggests a DVD source; it's a 25 fps video, suggesting PAL, but the PAR works out to less than 1, suggesting NTSC. It could be 4:5, I suppose (very close to 104:135), but I don't know of anything that produces that pixel aspect ratio; maybe try that first, and then try 3:4 if it still looks a little too stretched horizontally. If you're certain it's exactly 4:3, of course, just use 104:135.

ffmpeg can't change parameters of a video stream without re-encoding (edit: or let's say does it in a strange way by adding a 2nd pair of SAR/DAR, instead of overwriting them), MP4Box (part of gpac) and mkvmerge can. In case of one (edit: 16:9 stretched) video stream and a real/correct aspect ratio of 4:3, you may want to try:

MP4Box -par 1=3:4 VideoFile.mp4 "-par" : PixelAspectRatio (adjusts DAR + SAR with respect to the video resolution) "1"= : stream number "3:4" : aspect ratio (lower number 1st!) (edit: 16:9 * 3:4 = 4:3) Changes are directly applied to "VideoFile.mp4", no copy

To verify before and after: ffmpeg -i VideoFile.mp4

5

ffmpeg 4.3.2 -bsf:v h264_metadata=sample_aspect_ratio=x/y gives an error on DNxHR coded Quicktime files while MP4Box writes a second line of PAR into the metadata.

Width : 720 pixels Height : 540 pixels Display aspect ratio : 16:9 Original display aspect ratio : 4:3

I tested multiple players and even an NLE and so far all display the correct aspect ratio.

Use this command line for changing the resolution and display aspect ratio.

ffmpeg -i "input file" -vf scale=$w:$h -aspect $w:$h "output file"

or

ffmpeg -i "input file" -vf -vf scale=$w:$h -aspect x:y "output file"

$w - resolution width
$h - resolution height
x - aspect ratio width
y - aspect ratio height

Notes:

• If you put -aspect x:y without -vf scale=$w:$h, the output will only have the change on display aspect ratio.

• If you put -vf scale=$w:$h without -aspect x:y, the output will only have the change on the resolution.

• The command -vf scale=$w:-1 or scale=-1:$h will not always provide the expected output so to make sure you get both change on resolution and aspect ratio, you can just use the above command line.

** I only tried this with input.mkv and output.mkv files since this is only what I am working on at the moment. Feel free to try this with the other file types.

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