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VideoDecoder output guarantees

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  • ¿Algo no está bien?
#900 3 comentarios 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
5/5
Tiempo estimado
Más de una semana
Aptitud para principiantes
25/100
Tipo de issue
Nueva funcionalidad
Claridad
Bastante claro
Estado de actividad
Estancado
Área
api, audio-video-rtc

Línea de trabajo

Comienza leyendo las secciones de la especificación de VideoDecoder que cubren output, flush y optimizeForLatency. Compara su comportamiento previsto con las discusiones enlazadas en los issues 650, 698 y 206, incluida la cuestión de la hardware acceleration. Se considera completado cuando el issue documenta si los llamadores pueden saber cuándo se necesitan más paquetes, cómo afecta la optimización de la latencia a la salida y qué compensaciones de rendimiento relevantes existen.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

My use case requires me to decode multiple video streams at the same time. The source files can be of extremely high bitrate, 4K videos, so I would like to be mindful of how I use the GPU.

My root issue is that I cannot find any guarantee of when I might receive a frame or when I need to send more data to the decoder. If I investigate the packets and check the P or B frames, I see no reason why a packet would not be sent to the output. When I call flush I receive frames in the output perfectly.

Based on that experience, I thought I could decode GOPs, then call flush. In terms of decoding the bare minimum it's far from optimal, but at least it gives me some guarantee of when I can receive a frame. However, this is not trivial to do either since I work with libav under the hood, what I consider a keyframe might not be accepted by WebCodecs. Same issue as described here. The idea of flush without invalidating whatever inner state it has seemed like a good idea to me.

The answer to my question could be the optimizeForLatency, as in the specification it seems to do exactly what I want. However, as I read the threads here, I am now confused on what it does and what it doesn't. As @dalecurtis mentioned here, in Chromium, this flag does not affect flush directly, just omits using threading which can hold back frames. According to another thread though, I got the impression this only applies to software decoders, not hardware-accelerated ones. Does this have an effect in hw-accelerated decoding? If so, what are the performance drawbacks to consider?

Very different in nature but what I love about FFmpeg/libav is that this is very trivial to do:

int ret = avcodec_receive_frame(decoder->codec_ctx, decoder->frame);
if (ret == AVERROR(EAGAIN)) {
  // I know the decoder needs more frames before it can give me something
}

To summarize, my question is how can I have a guarantee that I passed enough packets (or how can I know for sure that I didn't ) to the VideoDecoder to surely have an output? Ideally, I want to decode the smallest amount possible for a guaranteed output.

Lenguaje dominante
HTML
Estrellas
1.3k
Forks
192
Merge medio
14 h 40 min
PR fusionados (30 d)
8

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  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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