Video players questions

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Joined: Sun Oct 26, 2014 2:54 am

Video players questions

Post by yaziwan » Sun Oct 26, 2014 3:32 am

Given that FFmpeg is the leading multimedia framework and most of the video/audio players uses it, I'm wondering somethings about audio/video players using FFmpeg as intermediate.

I'm studying and I want to know how audio/video players works and I have some questions.

I was reading the ffplay source code and I saw that ffplay handles the subtitle stream. I tried to use a mkv file with a subtitle on it and doesn't work. I tried using arguments such as -sst but nothing happened. - I was reading about subtitles and how video files uses it (or may I say containers?). I saw that there's two ways putting a subtitle: hardsubs and softsubs - roughly speaking hardsubs mode is burned and becomes part of the video, and softsubs turns a stream of subtitles (I might be wrong - please, correct me).

:arrow: The question is: How does they handle this? I mean, when the subtitle is part of the video there's nothing to do, the video stream itself shows the subtitle, but what about the softsubs? how are they handled? (I hard something about text subs as well). - How does the subtitle appears on the screen and can be configured changing fonts, size, colors, without encoding everything again?

:arrow: I was studying some video players source codes and some or most of them uses OpenGL as renderer of the frame and others uses (such as Qt's QWidget) (kind of or for sure) canvas. - What is the most used and which one is fastest and better? OpenGL with shaders and stuffs? Handling YUV or RGB and so on? How does that work?

:arrow: It might be a dump question but what is the format that AVFrame returns? For example, when we want to save frames as images first we need the frame and then we convert, from which format we are converting from? Does it change according with the video codec or it's always the same?

:arrow: Most of the videos I've been trying to handle is using YUV720P, I tried to save the frames as png and I need to convert to RGB first. I did a test with the players and I put at the same frame and I took also screenshots and compared. The video players shows the frames more colorful. I tried the same with ffplay that uses SDL (OpenGL) and the colors (quality) of the frames seems to be really low. What might be? What they do? Is it shaders (or a kind of magic? haha).

Well, I think that is it for now. I hope you help me with that. :D

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