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Stretch detection? - incans - 09.01.2021

Hardware: Core i7-4770K, 16GB RAM



OS: Windows 7 Pro 64bit



Hybrid: 2020.12.13.1 (dev patch)



My first attempt to rip and transcode a film made in Cinemascope (2.35:1 aspect ratio) was a major failure. While trying to work out how to deal with it I found this under the help in the section Base->Video->Stretch




Quote:if 'Config->Automation->Matroska Settings->Scan for streched streams' is enabled 'Stretch' shows the detected stretch for the this stream



Minor issue, there is an unecessary "the" in this text.




Larger issue, on my system the option Scan mkv for stretched streams is disabled (greyed out)?




I'm not even sure if "stretch" refers to the anamorphic projection (non 1:1 pixel aspect ratio) applied to this film, but if the option isn't available I can't test it to see. What determines whether the stretch analysis option is enabled?



[attachment=1248]


RE: Stretch detection? - Selur - 09.01.2021

Quote:Larger issue, on my system the option Scan mkv for stretched streams is disabled (greyed out)?
Option is disabled since it doesn't work properly atm.

Quote:I'm not even sure if "stretch" refers to the anamorphic projection (non 1:1 pixel aspect ratio) applied to this film, but if the option isn't available I can't test it to see.
Stretch has nothing to do with the PAR of the input.
Matroska has an option to stretch/shrink audio, read: https://gitlab.com/mbunkus/mkvtoolnix/-/wikis/Stretch-or-shrink-audio-tracks

Quote:My first attempt to rip and transcode a film made in Cinemascope (2.35:1 aspect ratio) was a major failure.
Since you didn't share in what way it was a failure, I can't really help.

My wild guess would be that your input is Cinemascope (2.35:1 display aspect ratio), but isn't properly flagged.
-> share details about your problem if you want help

Cu Selur


RE: Stretch detection? - incans - 12.01.2021

Quote:My first attempt to rip and transcode a film made in Cinemascope (2.35:1 aspect ratio) was a major failure.
>> Since you didn't share in what way it was a failure, I can't really help.

I wanted to try to make sense of it myself if I could ;-)

>>My wild guess would be that your input is Cinemascope (2.35:1 display aspect ratio), but isn't properly flagged.
Exactly so. This is what I found by experimenting with the (very useful) tools in Hybrid-

The source was a DVD of Flash Gordon (1980). The original film is a Cinemascope 2.35:1 feature, and the DVD describes itself as 16:9 anamorphic. However the mkv rip shows up in Hybrid with an aspect ratio of 16:15 (1.067:1) which is clearly nonsense.

Attempt 1
First pass I just treated this like a normal 16:9 film. I was rewarded with a picture occupying about 50% of the screen horizontally and 40% vertically. A little vertically squeezed sticking plaster of a picture.

Attempt 2
For the next attempt I did the following-
  1. Crop the picture to 720x440
  2. Applied 68 pixel letterbox bars top and bottom
  3. Tansformed the picture to 1.63:1 aspect ratio with square pixels (1.63 is 720/440)

This was better, it occupied more of the screen and the picture wasn't distorted, but it still wasn't using the full screen width on the TV. My guess was i'd failed to take account of the fact that a standard 720x576 image (1.25:1) has to have an "anamorphic" transform to fit a 1.77:1 TV screen, so setting "square pixels" was incorrect.

Attempt 3
For this one I cropped the picture down to just the 720x440 active area, then resized the result to 720x306. This worked, and actually filled the TV screen horizontally, but it struck me that I was maybe wasting CPU resizing the image during transcoding, when presumably the TV or streamer can do this on the fly pretty easily, because they do it every time you play a 720x576 DVD on a 1080p or 2160p display.

Attempt 4
After several experiments trying to make sense of the various pieces of the aspect ratio puzzle I ended up with-

Input PAR: 4x3   (= 64/45 (16:9 PAL PAR) x 15/16 (reported PAR of the rip)

Output PAR: 72x44 (active area of resulting image)
Convert output to PAR: On
No crop, no resize.

it worked! (mostly by luck...) I don't know if this recipe will work for all widescreen material but at least I understand the controls in the Crop/Resize section a bit better now.

One comment- It took me a while to understand that you could set use the Crop/Resize options to override the output PAR settings WITHOUT resizing the picture (because choosing an output PAR automatically enables resizing). Fine once I "got" it, but the fact that the PAR section was still in effect even if Picture Resizing was not enabled was not obvious to me in the user interface.

BTW sorry for the crazy spacing in the previous post, but every time I removed the extra line breaks between paragraphs the eidtor seemed to re-insert them?!  Hence the excessive spaces between paragraphs.


RE: Stretch detection? - Selur - 12.01.2021

Quote:... it worked!
Happy you found a way. Smile

I think that main problem seems to be that you seem to mix the different aspect ratio definitions. Smile
-> May be reading [INFO] About pixel aspect ratios,.. and the sources linked in that thread help a bit.

My usual approach when getting a source with wrong aspect ratio is:
  • load source
  • search for a scene where a square or round object is shown that allows me to see the effect of different aspect ratios
  • adjust the input PAR (=pixel aspect ratio) so that the observed object looks proper. (testing the standard PARs is usually a good start)
  • crop the content (= remove all black bars); this does now change the pixel aspect ratio.
  • decide what output PAR (=pixel aspect ratio) the output should have
  • if the output PAR (=pixel aspect ratio) should differ from the input PAR (=pixel aspect ratio) I enable 'Convert to PAR' and specifiy the PAR (=pixel aspect ratio) I want for the output. This automatically enabled the 'Resize', since now it's time to decide the pixel count of the output.
  • now I decide the count of pixels in width and height the output picture has (the storage aspect ratio).
  • To archive the SAR I want, I
    1. keep the 'Auto adjust' in the 'Picture Resize' active since it makes sure that the output does not get distorted.
    2. I set 'Auto adjust' to either 'height' or 'width' and specify either my target width or target height.
    3. To lastly archive the storage aspect ratio I want I enable letter box and enter my storage width and storage height.

-> Hope the link above and this helps a bit for the future.

Cu Selur


RE: Stretch detection? - incans - 12.01.2021

Nice "recipe" for the process, thanks Smile


RE: Stretch detection? - incans - 13.01.2021

One question.

On-


  1. load source
  2. search for a scene where a square or round object is shown that allows me to see the effect of different aspect ratios
  3. adjust the input PAR (=pixel aspect ratio) so that the observed object looks proper. (testing the standard PARs is usually a good start)
  4. crop the content (= remove all black bars); this does now change the pixel aspect ratio.
If I follow this process, at step 3, with input PAR set but no crop applied, the input PAR setting is not applied to the preview. For example i'm looking at a DVD of Near Dark (1987) which has a 1.89:1 (non letterboxed) stated PAR.

Hybrid calculated a default input PAR of 256x135, which is 1.89:1, but if I preview that with crop disabled I see the 5:4 aspect ratio of the 720x576 data, so the picture is squashed laterally.

If I enable Picture Crop and Crop View, even without actually setting any non-zero crop values, all of a sudden the preview is the correct aspect ratio. Is this the expected behaviour?


RE: Stretch detection? - Selur - 14.01.2021

I should have mentioned that I always usw the Vapoursynth Preview,...