FFMpeg-仅用于Android项目学习

##

An

ffmpeg and SDL Tutorial

Part 1

Part 2

Part 3

Part 4

Part 5

Part 6

Part 7

Part 8

encode several other formats.

The only prolem is that documentation is basically nonexistent. There is a

single tutorial that shows the basics of ffmpeg and auto-generated doxygen

documents. That's it. So, when I decided to learn about ffmpeg, and

in

the

process about how digital video and audio applications work, I decided to

document the process and present

There is a sample program that comes with ffmpeg called ffplay.

C program that implements a complete video player using ffmpeg. This tutorial

will begin with

updated version of the original tutorial, written by Martin

Bohme (I have stolen liberally borrowed from that work), and work from there

will have a working video player written

in

less than 1000 lines of code!

In

making the player, we will be using SDL to output the audio and video of

the media file. SDL is

used

to download and install the SDL development libraries for your system

in

order

to compile the programs

in

this tutorial.

This tutorial is meant for people with a decent programming background. At the

very least you should know C and have some idea about concepts like queues,

mutexes, and so on.

You

should know some basics about multimedia; things like

waveforms and such, but you don't need to know a lot,

as

I explain a

lot

of

those concepts

There are printable HTML files along the way

well

files.

can also get a tarball of the text files and source code or just

the source.

You

can get a printable page of the full thing

in

HTML or

in

text.

**UPDATE:** I've fixed a code error

in

Tutorial 7 and 8,

as

-lavutil.

## Tutorial 01: Making Screencaps

Code: tutorial01.c

### Overview

**container**, and the type of container determines where the information in

the file goes. Examples of containers are

bunch of **streams**; for example, you usually have

video stream. (A "stream" is just a fancy word for "a succession of data

For our purposes, each packet contains complete frames, or multiple frames in

At its very basic level, dealing with video and audio streams is very easy:

10 OPEN video_stream FROM video.avi

30 IF frame NOT COMPLETE GOTO 20

40 DO SOMETHING WITH frame

50 GOTO 20

Handling multimedia with ffmpeg is pretty much

as

simple

as

this program,

although some programs might have a very complex "DO SOMETHING" step. So in

this tutorial, we're going to open a file, read from the video stream inside

it, and our DO SOMETHING is going to be writing the frame to a PPM file.

First, let's see how we open a file

in

the first place. With ffmpeg, you have

#include <avformat.h>

...

int

main(int argc, charg *argv[]) {

av_register_all();

This registers all available file formats and codecs with the library so they

opened. Note that you only need to call av_register_all() once, so we do it

here

file formats and codecs, but there's usually no reason why you would have to

do that.

// Open video file

We

header and stores information about the file format

file format, buffer siz e, and format options, but by setting this to NULL or

0, libavformat will auto-detect these.

This function only looks at the header, so next we need to check out the

stream information

in

the file.:

// Retrieve stream information

if(av_find_stream_info(pFormatCtx)<0)

This function populates pFormatCtx->streams with the proper information.

We

Now pFormatCtx->streams is just

an

array of pointers, of size

pFormatCtx->nb_streams, so let's walk through

it

until we find a video stream.

// Find the first video stream

videoStream=-1;

// Get a pointer to the codec context for the video stream

pCodecCtx=pFormatCtx->streams[videoStream]->codec;

The stream's information about the codec is

in

what we call the "codec

using, and now we have a pointer to it. B ut we still have to find the actual

codec and open it:

// Find the decoder for the video stream

pCodec=avcodec_find_decoder(pCodecCtx->codec_id);

if(pCodec==NULL) {

}

// Open codec

parts to this code: adding CODEC_FLAG_TRUNCATED to pCodecCtx->flags and adding

a hack to correct grossly incorrect frame rates. These two fixes aren't in

ffplay.c anymore, so

I

There's another difference to point out since we removed that code:

pCodecCtx->time_base now holds the frame rate information. time_base is a

struct that has the numerator and denominator (AVRational).

frame rate

### Storing the Data

Now we need a place to actually store the frame:

Since we're planning to output PPM files, which are stored

24-bit RGB,

we're going to have to convert our frame from its native format to RGB. ffmpeg

will do these conversions for us. For most projects (including ours) we're

going to want to convert our initial frame to a specific format. Let's

allocate a frame for the converted frame now.