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1 A Guide To Developing MPlayer Codecs
2 by Mike Melanson (melanson at pcisys dot net)
3 updated to libmpcodecs arch by A'rpi
5 SEE ALSO: libmpcodecs.txt !!!
7 Introduction
8 ------------
9 I've developed a number of open source decoders for the MPlayer project,
10 for both audio and video data. As such, I feel I'm qualified to document a
11 few notes about developing new codecs for the codebase.
13 As always, the best way to learn how to incorporate a new codec is to
14 study a bunch of existing code. This document is supplementary material to
15 the code, meant to give some tips, pointers, and a general roadmap.
17 A note about terminology: "Codec" stands for coder/decoder (or
18 compressor/decompressor, if you prefer). The term refers to a module that
19 can both encode and decode data. However, this document focuses primarily
20 on incorporating decoders. Still, the terms "decoder" and "codec" are
21 often used interchangeably.
23 Necessary Materials
24 -------------------
25 So you've decided that you want to implement a new decoder for
26 MPlayer. There are a few things you will need:
28 - Knowledge of the codec to be implemented: You will need to know the data
29 format of the chunks that MPlayer will pass to you. You will need to know
30 how to take apart the data structures inside. You will need to know the
31 algorithmic operations that need to be performed on the data in order to
32 reconstruct the original media.
34 - Sample media: Preferably, lots of it. You will need media encoded in
35 your data format and stored in a media file format that MPlayer knows how
36 to parse (these include AVI, ASF, MOV, RM, VIVO, among others). If the
37 encoded data is stored in a media file format that MPlayer doesn't
38 understand, then you will either need to somehow convert the format to a
39 media file format that the program does understand, or write your own
40 MPlayer file demuxer that can handle the data. Writing a file demuxer
41 is beyond the scope of this document.
42 Try to obtain media that stresses all possible modes of a
43 decoder. If an audio codec is known to work with both mono and stereo
44 data, search for sample media of both types. If a video codec is known to
45 work at 7 different bit depths, then, as painful as it may be, do what you
46 can to obtain sample media encoded for each of the 7 bit depths.
48 - Latest CVS snapshot: It's always useful to develop code for the very
49 latest development version of MPlayer. Be sure to update your local CVS
50 copy often.
52 - General programming knowledge, working Linux development environment: I
53 would hope that these items would go without saying, but you never know.
55 Typical Development Cycle
56 -------------------------
57 1) Set up basic infrastructure
58 First things first, there's a big song and dance to go through in order to
59 let the MPlayer program know that you have a new codec to incorporate.
61 First, modify your local copy of codecs.conf. It may be system-shared or
62 in your home directory. Add a new entry for your codec. If it's an open
63 source codec, it would be a good idea to place the new entry with the rest
64 of the open source codecs. When you're confident that you have the entry
65 right, be sure to add it to etc/codecs.conf in your workspace. See the
66 file codecs.conf.txt for a detailed description of the format of this
67 file. Create a new audiocodec or videocodec block with the proper info,
68 FOURCCs/format numbers, output formats, and a unique driver name. Remember
69 the driver name.
71 Next, create a new source file which contains the main decoding function
72 that MPlayer will call to decode data. Eventually, you may have multiple
73 files which comprise your decoder, but let's start simple here.
74 For audio codecs, see ad_sample.c skeleton. For video, choose one of the
75 existing vd_*.c files which you think is close to your codec in behaviour.
77 Next, modify the Makefile so that it will compile your new source file.
78 Also, add your codec to the array in ad.c (for audio) or vd.c (for video).
80 Next, compile the project and see if you have everything correct so far.
82 Next, you want to make sure that the encoded data is making it to your
83 decoding function in the first place. This may sound like a trivial
84 exercise, but there are a lot of things that can go wrong (and I've
85 watched most of them go wrong in my experience). At the beginning of your
86 skeleton decoder function, enter the following code:
87 int i;
88 for (i = 0; i < 16; i++)
89 printf ("%02X ", input[i]);
90 printf ("\n");
91 When you compile and run MPlayer, your decoder function will print the
92 first 16 bytes of each data chunk that it receives. Open the sample media
93 in a hex editor and reconcile what you see on the screen with what
94 you find in the file. If the decoder is printing the first 16 bytes of
95 each block, that's a good sign that you're ready to move on to step
96 2. Otherwise, you need to figure out why the data isn't getting to your
97 decoder. Is your decoder even being invoked? If not, why not?
99 2) Develop the decoder
100 Go for it. Remember to make it work, first, then make it work fast. Some
101 specific tips:
103 What output formats should you support in your decoder? Whatever makes
104 sense. YUV output is always preferable over RGB output. Generally, if a
105 codec uses a YUV data as its source data, you will be able to decode a
106 frame of YUV data. If a codec takes RGB data as its input, as many older
107 video codecs do, then there's no point in supporting YUV output; just
108 output as many RGB formats as possible.
110 The most preferred output format for video data is YV12. This is because
111 MPlayer supports a multitude of hardware devices that can display, scale,
112 and filter this type of data directly. MPlayer also has a bunch of
113 optimized conversion functions that can convert YV12 data to any other
114 type of output data.
116 If you do take the RGB output route, you should be aware that MPlayer
117 actually orders packed RGB data as BGR. If you're decoding into a BGR24
118 buffer, the output will look like:
119 B G R B G R B G R B ...
120 If you're decoding into a BGR32 buffer, there will need to be an
121 additional (unused) byte after each BGR triplet:
122 B G R - B G R - B G ...
124 Make liberal use of sanity checks. Start by including the file mp_msg.h at
125 the start of your decoder. Then you can use the mp_msg() function as you
126 would a normal printf() statement. Whenever your decoder notices a strange
127 bit of data or an odd condition, print a message such as:
128 mp_msg(MSGT_DECVIDEO, MSGL_WARN, "Odd data encountered: %d\n", data);
129 Obviously, you should make the message a little more
130 descriptive, for your benefit. MSGL_WARN is a good message level for this
131 type of information. Look in mp_msg.h for all of the error levels. You can
132 even make MPlayer bail out completely by using MSGL_FATAL, but that should
133 never be necessary at the data decoder level.
135 What conditions should trigger a warning? Anything, and I mean *anything*
136 out of the ordinary. Many chunks of compressed video data contain headers
137 with data such as width, height, and chunk size. Reconcile these fields
138 with the parameters passed into the decoding function (if you set it up to
139 take those parameters). Such data should match up. If it doesn't, issue a
140 warning and make an executive decision in the code about which data to
141 believe (personally, I always lend more weight to the data that was passed
142 into the decoder function, the data that comes from the container file's
143 header). If there's supposed to be a magic number embedded in, or computed
144 from, the chunk's header, issue a warning if it isn't correct.
146 Whenever you're about the index into a memory array with an index that
147 could theoretically be out of range, then test that the index is in range,
148 no matter how tedious it seems. Accessing outside of your memory range is,
149 after all, the number 1 cause of segmentation faults. Never trust that all
150 the data passed to you will be correct. If an array index suddenly winds
151 up out of range, it's probably best to issue a warning about it and bail
152 out of the decoder (but not the whole application).
154 Writing all of these warning statements may seem insipid, but consider
155 that if you don't do it when you start writing your decoder, you'll
156 probably end up doing it later on when your decoder isn't working properly
157 and you need to figure out why (believe me, I know).
159 3) Debug and test the decoder
160 If you're extremely lucky, the decoder will work the first time. If you're
161 very lucky, it will work after you've reviewed your code a few times and
162 corrected a few obvious programming mistakes. Realistically, you will
163 write the decoder, review it many times and fix many obvious and subtle
164 programming errors, and still have to go through an elaborate debug
165 process in order to get the decoder to a minimally functional state.
167 Big hint: Ask for all warnings. You know, the -Wall option in
168 gcc? It's very useful to develop your codec while running in debug
169 mode. In order to compile MPlayer with debug support (which includes -Wall
170 for all gcc operations), use the --enable-debug option when configuring
171 the project. Pay attention to all warnings and make it a goal to get
172 rid of every single one. I'll never forget when the compiler warned me
173 that there was no point in clamping a signed 16-bit variable within a
174 signed 16-bit range (the calculation to be clamped was supposed to be
175 stored in a signed 32-bit variable and then stored in the signed 16-bit
176 variable). I sat stunned for a moment, feeling like I had just dodged a
177 bullet as I knew that would have taken me hours to debug that kind of
178 mistake.
180 4) Contribute decoder to codebase
181 Create a patch with the "diff -u" format and email it to the MPlayer
182 development team for approval. You will likely need to diff the following
183 files:
184 - Makefile
185 - etc/codecs.conf
186 - ad.c or vd.c
187 Of course, you will need to include your newly-created file(s):
188 vd_<name>.c -OR- ad_<name>.c. If you contribute enough decoders, the
189 development team may even grant you write privileges to the CVS repository.
191 5) Wait for bug reports to start rolling in
192 You may think you're finished when you release the codec and if you're
193 extremely lucky, you will be right. However, it's more likely that people
194 will start throwing all kinds of oddball media at your decoder that it
195 never counted on. Cheer up; take comfort in knowing that people are
196 testing your code and attempting to use it as a real world
197 application. Download the problem media that people upload to the MPlayer
198 FTP site and get back to work, implementing fixed code that addresses the
199 issues. Contribute more patches and encourage people to hammer on your
200 decoder even more. This is how you make your decoder rock-solid.
202 EOF