bytedeco/javacv

png image stream synthesis webm video transparent channel lost

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#2,150 opened on Dec 20, 2023

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Description

I have a png pipeline stream, similar to the one below

PipeInputStream pis = new PipeInputStream()
grabber = new FFmpegFrameGrabber(pis, 0);
grabber.setImageWidth(1920);
grabber.setImageHeight(1080);
grabber.setFrameRate(25);
grabber.setImageMode(FrameGrabber.ImageMode.RAW);
grabber.setFormat("png_pipe");
//Why `PixelFormat` is bgra, because I turn Mat
grabber.setPixelFormat(avutil.AV_PIX_FMT_BGRA);
grabber.start();

FFmpegFrameRecorder

recorder = new FFmpegFrameRecorder("output.webm", 1920, 1080);
recorder.setAudioCodec(avcodec.AV_CODEC_ID_OPUS);
recorder.setVideoCodec(avcodec.AV_CODEC_ID_VP9);
recorder.setFormat("webm");
recorder.start();
Frame frame = null;
while((frame = grabber.grabImage()) != null) {
    recorder.record(frame)
}
recorder.close();
grabber.close();

Description： Images saved as local png files before being written to the pipeline have a transparent channel, but the transparent channel in the generated webm video is lost.webm video generated directly from the FFmpeg command line also has transparent channels

FFmpeg command:

ffmpeg -i %6d.png -c:v libvpx-vp9 -y output.webm

Contributor guide

Research direction
Investigate how the FFmpegFrameRecorder handles pixel formats and alpha channels. Check if the recorder is configured to preserve transparency (e.g., AV PIX FMT BGRA) and ensure the WebM container supports VP9 with alpha. Compare with the working FFmpeg command line to identify missed settings.
Tech stack
java
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
JavaFFmpeg basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70