[FFmpeg-trac] #10200(ffmpeg:new): Piping WEBP image will produce a broken image

[FFmpeg]

#10200: Piping WEBP image will produce a broken image
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             Reporter:  lolcat         |                    Owner:  (none)
                 Type:  defect         |                   Status:  new
             Priority:  normal         |                Component:  ffmpeg
              Version:  4.3.5          |               Resolution:
             Keywords:  webp, sigpipe  |               Blocked By:
             Blocking:                 |  Reproduced by developer:  0
Analyzed by developer:  0              |
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Description changed by lolcat:

Old description:

> Hello. I've been trying to convert a video to an animated webp thumbnail
> image. This command doesn't work, because the output is piped. When I
> output to a file on disk, this problem does not occur. Here is my
> command:
> {{{#!sh
> ffmpeg -i 'https://th.bing.com/th?id=OMB1.7pHOr_m7KOKS3w&pid=2.1' -vf
> "fps=10,scale=148:83:flags=lanczos" -vcodec libwebp -lossless 0
> -compression_level 6 -q:v 50 -loop 0 -preset picture -an -f webp - >
> test.webp
> }}}
>
> But if I use the following command:
> {{{#!sh
> ffmpeg -i 'https://th.bing.com/th?id=OMB1.7pHOr_m7KOKS3w&pid=2.1' -vf
> "fps=10,scale=148:83:flags=lanczos" -vcodec libwebp -lossless 0
> -compression_level 6 -q:v 50 -loop 0 -preset picture -an -f webp out.webp
> }}}
>
> The image will load just fine in all modern browsers (notice at the end,
> I'm outputting to a file instead of a pipe). When using hexedit, I found
> out that the bytes 5 to 7 are all 0s, when it should be `88 B4 05`.
> Patching these bytes in manually magically fixes the image.
>
> Thank you!

New description:

 Hello. I've been trying to convert a video to an animated webp thumbnail
 image. This command doesn't work, because the output is piped. When I
 output to a file on disk, this problem does not occur. Here is my command:
 {{{#!sh
 ffmpeg -i 'https://th.bing.com/th?id=OMB1.7pHOr_m7KOKS3w&pid=2.1' -vf
 "fps=10,scale=148:83:flags=lanczos" -vcodec libwebp -lossless 0
 -compression_level 6 -q:v 50 -loop 0 -preset picture -an -f webp - >
 test.webp
 }}}

 But if I use the following command:
 {{{#!sh
 ffmpeg -i 'https://th.bing.com/th?id=OMB1.7pHOr_m7KOKS3w&pid=2.1' -vf
 "fps=10,scale=148:83:flags=lanczos" -vcodec libwebp -lossless 0
 -compression_level 6 -q:v 50 -loop 0 -preset picture -an -f webp out.webp
 }}}

 The image will load just fine in all modern browsers (notice at the end,
 I'm outputting to a file instead of a pipe). When using hexedit, I found
 out that the bytes 5 to 7 are all 0s, when it should be `88 B4 05`.
 Patching these bytes in manually magically fixes the image.

 UPDATE:
 I investigated the bug a little, and it seems that replacing the bytes I
 talked about only fixes the issue *sometimes*. When trying to convert this
 URL: https://th.bing.com/th?id=OMB1.7pHOr_m7KOKS3w it does not yield any
 errors in both ffmpeg and any image decoder that supports it. But if I try
 to convert the video I linked in the original question (first code
 snippet), I get a corruption in the middle of the file. I know this
 because I am piping the output of ffmpeg into the Firefox browser, and in
 the console, even though I can see that the image started animating, it
 cuts off and tells me that the file is either "truncated, or corrupt". I
 established that it was not truncated, and truly corrupted, since the
 filesize is not different from something outputted by the second command
 ...

 I'm at a lost here. This sounds like a pipe encoder issue, and I don't
 have the expertise to fix this. If this helps anyone, I will post my (PHP)
 attempt at fixing the file(s)
 {{{#!php
 <?php

 $video = escapeshellarg($_GET["v"]);

 header("Content-Type: image/webp");
 //header("Content-Type: text/plain");
 $handle = popen(
         "ffmpeg -i {$video} -vf \"fps=10,scale=148:83:flags=lanczos\"
 -vcodec libwebp -lossless 0 -compression_level 6 -q:v 50 -loop 0 -preset
 picture -an -f webp -",
         "rb"
 );

 if($handle === false){

         // ffmpeg call failed
         return;
 }

 $bytemod = false;

 while(!feof($handle)){

         if($bytemod === false){

                 echo substr_replace(fread($handle, 7), hex2bin("88b405"),
 4, 3);
                 $bytemod = true;
                 continue;
         }

         echo fread($handle, 1);
 }

 fclose($handle);
 }}}

--
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Ticket URL: <https://trac.ffmpeg.org/ticket/10200#comment:1>
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