带有AVSEEK_FLAG_ANY的ffmpeg av_seek_frame导致灰屏 [英] ffmpeg av_seek_frame with AVSEEK_FLAG_ANY causes grey screen

问题描述

问题: omxplayer的源代码使用AVSEEK_FLAG_BACKWARD标志调用ffmpeg av_seek_frame()方法.尽管不确定100％，但我认为这是针对最接近的i帧.相反，我想寻找精确的位置，因此我修改了源代码，以使av_seek_frame()方法现在使用AVSEEK_FLAG_ANY标志.现在，在电影放映后，我会出现灰屏，通常持续1秒钟，在此期间我可以听到音频.我已经在多台计算机上进行了尝试(实际上，我也在同时同步它们)，所以这不是一次孤立的事件.我的猜测是，寻找非i帧在计算上会更加昂贵，从而导致最初的灰屏.

Problem: omxplayer's source code calls the ffmpeg av_seek_frame() method using the AVSEEK_FLAG_BACKWARD flag. Although not 100% sure, I believe this seeks to the closest i-frame. Instead, I want to seek to exact locations, so I modified the source code such that the av_seek_frame() method now uses the AVSEEK_FLAG_ANY flag. Now, when the movie loads, I get a grey screen, generally for 1 second, during which I can hear the audio. I have tried this on multiple computers (I am actually synchronizing them, therefore, at the same time too) so it is not a n isolated incident. My guess is that seeking to non i-frames is computationally more expensive, resulting in the initial grey screen.

问题:如何使用ffmpeg指示音频等待视频就绪，然后再继续操作.

Question: How, using ffmpeg, can I instruct the audio to wait until the video is ready before proceeding.

推荐答案

实际上， 表示您要查找最接近的关键帧，其时间戳小于要查找的时间戳.

Actually, AVSEEK_FLAG_BACKWARD indicates that you want to find closest keyframe having a smaller timestamp than the one you are seeking.

通过使用 AVSEEK_FLAG_ANY 完全到您要求的时间戳.但是此帧可能不是关键帧，这意味着无法对其进行完全解码.这说明了您的灰色屏幕"，直到出现下一个关键帧为止.

By using AVSEEK_FLAG_ANY, you get the frame that corresponds exactly to the timestamp you asked for. But this frame might not be a keyframe, which means that it cannot be fully decoded. That explains your "grey screen", that appears until the next keyframe is reached.

因此，解决方案是使用AVSEEK_FLAG_BACKWARD 向后搜索，然后从该关键帧读取下一帧(例如，使用av_read_frame())，直到获得与时间戳相对应的帧.此时，您的帧将被完全解码，并且不再显示为灰色屏幕".

The solution would therefore be to seek backward using AVSEEK_FLAG_BACKWARD and, from this keyframe, read the next frames (e.g. using av_read_frame()) until you get to the one corresponding to your timestamp. At this point, your frame would be fully decoded, and would not appear as a "grey screen" anymore.

注意::由于某种原因，当我要查找的帧恰好是之前的帧时，使用AVSEEK_FLAG_BACKWARD的av_seek_frame()返回 next 关键帧.这个关键帧.否则，它将返回上一个关键帧(这就是我想要的).我的解决方案是更改我给av_seek_frame()的时间戳，以确保它会在我要查找的帧之前 返回关键帧.

NOTE: It appears that, for some reason, av_seek_frame() using AVSEEK_FLAG_BACKWARD returns the next keyframe when the frame that I am seeking is the one directly before this keyframe. Otherwise it returns the previous keyframe (which is what I want). My solution is to change the timestamp I give to av_seek_frame() to ensure that it will return the keyframe before the frame I am seeking.

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