Burn srt to transparent video

[Taapo]

This question has been asked before on this board, but never fully answered.

Before someone says that you can hardcode subtitles directly to a movie, yes - I know that. There are use cases for what I'm trying to accomplish, but it seems difficult. If one of you could help out, that would be awesome.

What I'm trying to do is to use a SRT/ASS file and avisynth+ to create a transparent video file, with just the subtitles burnt in.

The movie should have the same length as the last timecode in the srt/ass file, and a alpha channel should be set for the background. Export it using lagarith or any other losless codec, so it would be easy to drop into a NLE (Premieren, Vegas, etc).

Is this possible with Avisynth?

[sneaker_ger]

Use MaskSub() function of (xy-)vsfilter plugin.

[Taapo]

As I can see, MaskSub needs a video to determine the length of the subtitles, correct? Is there a way to just render out the subtitles without having a resource? Can it take the length from the srt file itself?

MaskSub("subtitles.ext",video.width,video.height,video.framerate,video.framecount)

[StainlessS]

Is there a way to just render out the subtitles without having a resource? Can it take the length from the srt file itself?


You're gonna need details of source vid anyways, so whats the prob. ?

[Taapo]

There is no problem. I was just wondering if there was a way to do it without the source file?

[smok3]

Ffmpeg white on black script
https://github.com/brontosaurusrex/singularity/blob/master/bin/srt2video

[Taapo]

Ffmpeg white on black script
https://github.com/brontosaurusrex/singularity/blob/master/bin/srt2video

Now, *that's* helpful :) Finally someone who brings forward the right information. Thanks a lot!

[smok3]

Btw, you dont need to invent alpha, just drop the results to say premiere and set overlay to screen, then position stuff.

[Taapo]

Well, just when I'm getting myself in a corner again.

Everything works, but I want to use ASS formatting (border around font, and shadow) - I need an alpha channel of course. With normal white text, it would be easy, but with more advanced formatting the screen overlay wouldn't work - or am I wrong?

Currently I'm trying to do this - which works for the most part, but it leaves red traces around the font.

ffmpeg.exe -y -f lavfi -i color=s=1920x1080:color=0xff0000 -an -vf "ass=test.ass,colorkey=0xff0000:0.5:0.5,format=yuva420p" -c:v png -t "00:01:40.544" -threads 0 "video.mov" -stats

[poisondeathray]

Keying out a color will give you poor results. You're going to get overlap and ratty edges

Black/white and using white as a luma mask will only work... if you have white text. No fancy ASS subs. Otherwise you need 2 videos, 1 for the separate alpha (black/white text is the alpha channel)

Sorry, but the only way you're going to get perfect clean results is with masksub

[Taapo]

In that case, where can I find a compatible XySubFilter.dll file for use in avisynth+?

The ones at https://code.google.com/archive/p/xy-vsfilter/downloads doesn't seem to work at all ...

[Taapo]

Sorry, too fast. I found the working version: https://forum.doom9.org/showthread.php?t=168282

[Taapo]

So I managed to get things working in Avisynth with the following lines:

LoadPlugin("VSFilter.dll")
MaskSub("test.ass",1920,1080,25,3000)
FlipVertical()

I'm not sure how I can get the avs encoded to for example an avi with transparent background. Is there a line that enables saving to an external file, or should I open this in virtualdub and save it that way?

What would be the best way to get a workable file for Premiere Pro?

[poisondeathray]

yes you can use vdub if you want. Use UT or lagarith in RGBA mode

[Taapo]

I have gotten this to work perfectly with avisynth.

But I'm still wondering why it would be difficult to get it working using ffmpeg alone? Is there a way to set a background completely transparent in ffmpeg, and have the subtitle visible only?

Transparency is no problem by using these settings:
-c:v png

But is there a way to set a background to transparent using lavfi?

[Taapo]

So I tried something new: using a transparent png as input, and overlay the subtitles on it.

ffmpeg.exe -y -loop 1 -i transparent-1920x1080.png -vf "ass=test-normal.ass,format=rgba" -c:v png -t "00:00:10.000" -threads 4 "video-transparent.mov" -stats

When I play in a normal video player, I can see the subtitles on a white background. But when I open in Premiere, it shows nothing - just as if the alpha channel is applied to the whole image.

Any ideas?

[sneaker_ger]

ffmpeg -f lavfi -i "color=color=white@0.0:size=1920x1080,format=rgba,subtitles=test.ass:alpha=1" -t 10 out%d.png
https://superuser.com/questions/1242710/ffmpeg-how-to-render-subtitle-at-a-specific-time-as-an-image

[Taapo]

Awesome :) Thanks a lot! Setting the background to black is a better way, since white will show through a little bit around the fonts.

[sneaker_ger]

Eh, isn't the color irrelevant? We're rendering on alpha. I thought color is only to get the picture dimensions.

[Taapo]

Well, I thought so as well, but have been testing it here with black bordered white fonts - and it shows through. So black background with the black borders gives it maybe a subpixel more black ...

[sneaker_ger]

I don't understand. Why? Bug?

[Taapo]

Not sure, need to submit it to ffmpeg developers to know why.

[poisondeathray]

ffmpeg subtitle/ass alpha isn't preserved correctly . For example, things like drop shadow aren't preserved. Just compare it to masksub or view the alpha channel directly

[shekh]

I tried it and don`t see any problem. Large shadow with \blur10 looks ok.
Whether you need black or white background depends on application. Normally it should be black and treated as "premultiplied alpha".

However, now I think there is a problem. Alpha is squared - this is wrong.

[poisondeathray]

left = masksub ; right = ffmpeg

top is composite against a random background (teal)

bottom is alpha channel viewed directly

https://s26.postimg.org/dz0hhjr1l/image.png (https://postimages.org/)

[shekh]

Yes, I saw ffmpeg source code and it does not implement a path specific for alpha.

[Taapo]

@poisondeathray how do you show the alpha channel in your example? Is it inside editing software, or something like ffmpeg?

[poisondeathray]

@poisondeathray how do you show the alpha channel in your example? Is it inside editing software, or something like ffmpeg?

In avisynth, showalpha(). Seems appropriate since this is posted in avisynth usage forum
http://avisynth.nl/index.php/ShowAlpha

Or you can use other programs, video editors, after effects etc.. It's just viewing the "A" in RGBA . Just like you can view red, green, blue channels individually too