MP4 to MKV Possible Aspect Ratio Distortion using MKVToolnix

[gonwk]

Hi folks,

I am not sure if it is my eyes or in fact there is a Slight distortion of image takes place when trying to remux a MP4 file into MKV.

I have a perfectly nice MP4 which also there is an external SRT subtitle with it ... so, I want to have my video and srt in one folder ... so, I use MKVToolnix to do this ...

But, when I open my original MP4 and my new MKV with VLC side by side and check several different frames ... to my eyes it seems that MKV version Aspect Ratio might be a tad (very small) off, different from the original MP4 file.

Q1: Is this possible?

Q2: How to avoid this problem?

Thanks,

G!

[jagabo]

Q1: Is this possible?

Yes, if you specified the wrong aspect ratio when you remuxed.

Q2: How to avoid this problem?

Specify the correct aspect ratio when you remux. It's in the Format Specific Options for the video stream.

[hello_hello]

I've never had an aspect ratio problem when remuxing MP4s (doesn't mean it's impossible) but it could be due to the way the aspect ratio is stored which causes the player to resize the video slightly differently, although the amount would be very small.
When you say the aspect ratio looks a little different, what about the display dimensions? If the aspect ratio is different and video fills the entire width of the screen each time, the height should be a little different.

I don't use VLC, but if you want to give MPC-HC a spin, open an MKV/MP4 and use the File/Properties menu:



The aspect ratio displayed next to "Video Size" is the "container" aspect ratio.
The aspect ratio included in the video stream is the aspect ratio written to the video stream.
Normally they'd be the same, or very, very, very close to being the same (a player needs to choose which one to obey if they're different).

If there's no aspect ratio shown (ie 16:9) then the resolution and aspect ratio are one and the same (the video has square pixels).

Sometimes the container and stream aspect ratios will be a little different..... ie the same aspect ratio might be represented with a slightly different fraction.



I'm not 100% why. It may be due to how accurately the information can be written, the program writing/reading the information, or whether it's raining, but even in the above example the difference is very small.

Anyway, maybe you can compare your original MP4 to the remuxed MKV using the above method to see if anything has changed significantly.

[gonwk]

Hi jagabo & hello_hello,

As usual Thanks folks for coming to my help ...

@ jagabo, I do check my AR as I remux to MKV ... so, that is why I am baffled by what it seems to "My Eyes" ...

@ hello_hello, OK, I will try my checking and viewing using MPC-HC ...Thanks a Bunch for detailed explanation and the snapshots, appreciate the Mini Tutorial ...

BTW, hello_hello, do you use MPC-HC x32 or x64 version? And do you use the Installed version or the Portable one?

Thanks again to both of you.

G!

[jagabo]

You can always just start two instances of VLC and check the frame sizes. Align them vertically but with a slight horizontal offset -- are the two windows the same height? Align them horizontally but with a slight vertical offset -- are they the same width? Then check the image content to make sure no rows or columns were cut off or padded at any of the edges.

If the video is larger than your screen resolution use the Video -> Zoom -> 1/2 or 1/4 option.

[hello_hello]

BTW, hello_hello, do you use MPC-HC x32 or x64 version? And do you use the Installed version or the Portable one?

32 bit. I'm still using XP.
And the portable version.
I think by default you can only open one instance of MPC-HC at a time, or when you double click on a file to open it, it opens in any instance of MPC-HC which is already running. If you want to open multiple files at the same time in different instances of the player by double clicking on them, there's a setting under View/Options/Player.

[gonwk]

Hi hello_hello & jagabo,

I have attached 2 Clips for your review and analysis ...

File #1 is "ATWI80D (1956)_Trimmed.mkv" which is the Original file.

File #2 is "ATWI80D (1956) (1)_Trimmed.mkv" which is made by MKVToolnix 7.0.0 (x64) Portable.

May be you guys can tell me if my eyes are playing Tricks on me ... but to me Original seems a tad different than the new MKVtoolnix Muxed one.

Thanks,

G!

[poisondeathray]

They are the same.

There is no bitstream SAR information, and no container AR information for either, so it's 2.25:1 = 720/320 * 1/1

[hello_hello]

I'll confess I'm not seeing a difference. What happens if you load each of these pics in a new tab or an image viewer and switch between them. Do they look the same?

I give up. I don't know why, but the forum keeps renaming the pics so I seemed to have uploaded the same one twice. In fact when I look at the attachments list there's only one pic listed. I've changed the names and it's still happening. Maybe the forum thinks the second pic is just a copy of the first.
Yes it seems that's what was happening. I resaved the second pic using slightly different jpg compression which changed the file size a little, and now the forum thinks there's two pics instead of one.

The first is before muxing. The second is after.

[jagabo]

Yes, the videos display with exactly the same AR and picture content with VLC.

[gonwk]

Hi posiondeathray, hello_hello, and jagabo,

I guess then it is my screwy eyes ... because somehow it seems to me that the Muxed version looks a very small tad different ...

Thanks all of you for taking the time to analyze the clips and reply back.

G!