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Resize videos with ffmpeg – Keep aspect ratio – Telegram API

I’m trying to write a script for a telegram userbot that can convert any video/animation to a .mp4. I already have it so the script communicates and utilizes the API of cloudconvert.com.

Now my problem lies within the ffmpeg command that I’m passing to CC, as I need to keep videos below 1280 pixels on either side. It doesn’t matter if the final video is 720*1280 or 1280*1280 or something completely else, as long as neither of the two sides surpass 1280 pixels.

Here comes the tricky part, I don’t want to ruin the aspect ratio and I don’t want the video to be upscaled if it’s smaller than 1280.

The part of code that is actually relevant is the scale portion.
This following piece will resize a video to maximum 1280 pixels in height, but doesn’t take the width into account, it just keeps the ratio.

-vf "scale=min'(1280,iw)':-2"

Now how would I have to either adapt it or change it so it will actually resize depending on which side is greater than 1280 pixels?

I hope I’m being specific enough and I’m looking forward to your help.

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2

Answers


  1. 0

    The problem appears only, if the height is bigger then the width, this increases the number of limits for the width from 2 to 3:

    • width must be less or equal than 1280 (don’t exceed width)
    • width must be less or equal than initial width (don’t upscale)
    • width must be less or equal than 1280*width/height (don’t exceed height)

    To test for all cases, you would use min(1280,min(iw,round(1280*iw/ih))), creating a filter of 

    -vf "scale=min(1280,min(iw,round(1280*iw/ih))):-2"

    EDIT

    In some versions of ffmpeg the line above will not work citing self-referencing issues. In this case we can create an alternative line of thought:

    • If the width is bigger than (or equal to) the height, we scale by width using min(iw,1280)
    • If the height is bigger than the width, we scale by height using min(ih,1280)

    The expression would then be -vf 'scale=if(gte(iw,ih),min(1280,iw),-2):if(lt(iw,ih),min(1280,ih),-2)'

    Don’t forget, that you might run this through some shell parsing mechanism, which would additionally create the need to escape the commas. The expression 

    -vf 'scale=if(gte(iw,ih),min(1280,iw),-2):if(lt(iw,ih),min(1280,ih),-2)'

    Is verified to work with versions 2.7.2 to 3.4.4 on ubuntu Linux

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  2. 0 
    ffmpeg -i a.mp4 -vf "scale=:-1:force_original_aspect_ratio=decrease"
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