Must I set those frame rate, pixel format etc?

[baxterjimuk]

On Windows, sometimes I downloaded some TV show which normally are 1080p, encoded in h264, 1 audio stream (usually aac). 1 episode is around 3-4 Gb. Now usually after I watch them, I just delete them but sometimes I do kept them but for that size it's quite big so I normally would use ffmpeg and do a simple transcode to h265/hevc using the following very simple command and that'll bring the size to half (1.5-2Gb).

ffmpeg -i input.mkv -c:v libx265 -c:a copy output.mkv

The result is satisfactory for a TV show episode. If there's any quality loss, I can't really tell so it's good. No playback issue whatsoever, and audio and video are synced nicely.

I'm trying to achieve the same using JavaCV and its built in ffmpeg. I refer to this code in here but it includes the following:

recorder = new FFmpegFrameRecorder(outputFile, grabber.getImageWidth(), grabber.getImageHeight());
recorder.setVideoCodec(grabber.getVideoCodec());
recorder.setFormat("mp4");
recorder.setFrameRate(grabber.getFrameRate());
recorder.setPixelFormat(grabber.getPixelFormat());

if (grabber.getAudioChannels() > 0) {
    recorder.setAudioChannels(grabber.getAudioChannels());
    recorder.setAudioCodec(grabber.getAudioCodec());
    recorder.setSampleRate(grabber.getSampleRate());
}

I'm wondering if those setFrameRate, setPixelFormat, and setSampleRate are really necessary. Wouldn't by default it'll just follow the input if I didn't specify it (like my simple command)? Or if there's even more things like following that I need to do just to achieve the same as the simple ffmpeg command?

recorder.setSomething(grabber.getSomething());

If I we're to proceed, maybe I'll just use setVideoCodec, setFormat, and setAudioCodec but how would I know what else need to be set apart from shown by the example. I'm worried if I don't set according to the example, problem will occur later like playback issue, audio/video out-of-sync etc.

Another sample code that I found by googling also suggest the same (except this one there's no setPixelFormat);

import org.bytedeco.javacv.*;
import org.bytedeco.javacv.Frame;

public void trimVideo(String inputFile, String outputFile, long startTimeUs, long endTimeUs) {
    try {
        FFmpegFrameGrabber grabber = new FFmpegFrameGrabber(inputFile);
        grabber.start();

        FFmpegFrameRecorder recorder = new FFmpegFrameRecorder(outputFile, 
            grabber.getImageWidth(), grabber.getImageHeight(), grabber.getAudioChannels());
        
        // Copy original video/audio settings
        recorder.setVideoCodec(grabber.getVideoCodec());
        recorder.setAudioCodec(grabber.getAudioCodec());
        recorder.setFrameRate(grabber.getFrameRate());
        recorder.setSampleRate(grabber.getSampleRate());
        recorder.start();

        // Move to start position
        grabber.setTimestamp(startTimeUs);

        Frame frame;
        while ((frame = grabber.grab()) != null) {
            // Check if frame is within the cut range
            if (grabber.getTimestamp() >= endTimeUs) {
                break;
            }
            recorder.record(frame);
        }

        recorder.stop();
        grabber.stop();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

So...idk. For things like these I hope there's a default (that matches the source video) if I don't set them via code.

[saudet]

We can easily accomplish this with the ffmpeg program:
http://bytedeco.org/javacpp-presets/ffmpeg/apidocs/org/bytedeco/ffmpeg/ffmpeg.html