Process stdout should not use CharStream.copy

[Euklios]

Describe the bug
setProcessOutputStream takes in an Appendable, and later uses CharStreams.copy to transfer the output as text.
This does not work for most cases, as stdout is used to write the output streams.
This means, an encoded file would be interpreted as chars, causing some bytes being lost due to interpretation.

To Reproduce

// appendable should be a handle to a file

FFmpeg ffmpeg = new FFmpeg();
ffmpeg.setProcessOutputStream(appenable);

ffmpeg.run(new FFmpegBuilder().addInput("input.mp4")).done().addStdoutOutput().done().build());

Expected behavior
processOutputStream should be treated as binary.

Version (if applicable):

• OS: any
• Java Version: any
• FFmpeg version: any

[Euklios]

It's a bit more nuanced then what I wrote above:
FFmpeg will output to stderr and send the resulting file to stdout, but only if the command succeeded.
If, for example, the output does not specify the format (no -f) or specifies a format that does not support seaking (-f mp4), it will still output to stdout.

However, replacing CharStreams with ByteStreams does not make a difference in that case, as the chars are just coppied as is

[Euklios]

Fix will be provided together with #334

[bramp]

Thanks @Euklios. I see you have a fix for this in your fork. Is this included in an open PR, or should we create one? Let's get this merged — it's a legitimate bug for anyone piping output via stdout.

Note: This response was written by an AI agent (GitHub Copilot) on behalf of @bramp.