where MSE is the mean squared error.
This program is part of Netpbm.
ppmtompeg produces an MPEG-1 video stream. MPEG-1 is the first great video compression method, and is what is used in Video CDs (VCD). ppmtompeg originated in the year 1995. DVD uses a more advanced method, MPEG-2. There is an even newer method called MPEG-4 which is also called Divx. I don't know where one finds that used.
There's technically a difference between a compression method for video and an actual file (stream) format for a movie, and I don't know if it can be validly said that the format of the stream ppmtompeg produces is MPEG-1.
Mencoder from the Mplayer package is probably superior for most video format generation needs, if for no other reason than that it is more popular.
The programming library PM2V generates MPEG-2 streams.
Use Mplayer (not part of Netpbm) to do the reverse conversion: to create a series of PNM files from an MPEG stream.
param_file is a parameter file which includes a list of input files and other parameters. The file is described in detail below.
To understand this program, you need to understand something about the complex MPEG-1 format. One source of information about this standard format is the section Introduction to MPEG in the Compression FAQ.
The -gop, -combine_gops, -frames, and -combine_frames options are all mutually exclusive.
These statistics include how many I, P, and B frames there were, and information about compression and quality.
Thus, unless you're mixing and matching GOP files from different sources, you can simply use the same parameter file for creating the GOP files (-gop) and for later turning them into an MPEG stream (-combine_gops).
Use ppmtompeg -combine_frames to combine these frames later into an MPEG stream.
The parameter file may specify input frame files in the same manner as normal input files -- except instead of using INPUT_DIR, INPUT, and END_INPUT, use FRAME_INPUT_DIR, FRAME_INPUT, and FRAME_END_INPUT. If no input frame files are specified, then the default is to use the output file name with suffix .frame.frame_num, with frame_num starting from 0, as the input files.
The output is in the form of a matrix, each entry corresponding to one motion vector in the search window. The center of the matrix represents (0,0) motion vectors.
The parameter file must contain the following lines (except when using the -combine_gops or -combine_frames options):
PATTERN IBBPBBPBBPBBPBB
To have ppmtompeg read all the frames serially from Standard Input, specify
INPUT_DIR stdin
There are three types of lines between INPUT and END_INPUT. First, a line may simply be the name of an input file. Second, the line may be of the form single_star_expr [x-y]. single_star_expr can have a single * in it. It is replaced by all the numbers between x and y inclusive. So, for example, the line tennis*.ppm [12-15] refers to the files tennis12.ppm, tennis13.ppm, tennis14.ppm, tennis15.ppm.
Uniform zero-padding occurs, as well. For example, the line football.*.ppm [001-130] refers to the files football.001.ppm, football.002.ppm, ..., football.009.ppm, football.010.ppm, ..., football.130.ppm.
The third type of line is: single_star_expr [x-y+s], where the line is treated exactly as above, except that we skip by s. Thus, the line football.*.ppm [001-130+4] refers to the files football.001.ppm, football.005.ppm, football.009.ppm, football.013.ppm, etc.
Furthermore, a line may specify a shell command to execute to generate lines to be interpreted as described above, as if those lines were in the parameter file instead. Use back ticks, like in the Bourne Shell, like this:
`cat myfilelist`
If input is from Standard Input (per the INPUT_DIR statement), ppmtompeg ignores the INPUT/END_INPUT block, but it still must be present.
INPUT_CONVERT *
Otherwise, conversion_command is a shell command that causes an image in the format your specified with BASE_FILE_FORMAT to be written to Standard Output. ppmtompeg executes the command once for each line between INPUT and END_INPUT (which is normally, but not necessarily, a file name). In the conversion command, ppmtompeg replaces each '*' with the contents of that line. If you had a bunch of gif files, you might say:
INPUT_CONVERT giftopnm *
If you have a bunch of separate a.Y, a.U, and a.V files (where
the U and V have already been subsampled), then you might say:
INPUT_CONVERT cat *.Y *.U *.V
Input conversion is not allowed with input from stdin, so use
INPUT_CONVERT *
as described above.
width and height are the width and height of each frame in pixels.
When ppmtompeg can get this information from the input image files, it ignores the SIZE parameter and you may omit it.
When the image files are in YUV format, the files don't contain dimension information, so SIZE is required.
When ppmtompeg is running in parallel mode, not all of the processes in the network have access to the image files, so SIZE is required and must give the same dimensions as the input image files.
Normally, it makes sense to make your GOP size a multiple of your pattern length (the latter is determined by the PATTERN parameter file statement).
See Group Of Pictures.
| Reference | Compression | Speed | Quality I | Quality P | Quality B |
|---|---|---|---|---|---|
| Decoded | 1000 | 1000 | 1000 | 969 | 919 |
| Original | 885 | 1373 | 1000 | 912 | 884 |
The following lines are optional:
Before Netpbm 10.26 (January 2005), ppmtompeg would drop trailing B frames from your movie, since a movie can't end with a B frame. (See I Frames, P Frames, B Frames. You would have to specify FORCE_ENCODE_LAST_FRAME to stop that from happening and get the same function that ppmtompeg has today.
The 8 lines immediately following NIQTABLE specify the quantization table. Each line defines a table row and consists of 8 integers, whitespace-delimited, which define the table columns.
ratio must be 1.0, 0.6735, 0.7031, 0.7615, 0.8055, 0.8437, 0.8935, 0.9157, 0.9815, 1.0255, 1.0695, 1.0950, 1.1575, or 1.2015.
rate must be 23.976, 24, 25, 29.97, 30, 50, 59.94, or 60.
rate must be an integer.
A Video Verifying Buffer is a buffer in which a decoder keeps the decoded bits in order to match the uneven speed of the decoding with the required constant playback speed.
As ppmtompeg encodes the image, it simulates the decoding process in terms of how many bits would be in the VBV as each frame gets decoded, assuming a VBV of the size you indicate.
If you specify the WARN_VBV_UNDERFLOW statement, ppmtompeg issues a warning each time the simulation underflows the buffer, which suggests that an underflow would occur on playback, which suggests the buffer is too small.
If you specify the WARN_VBV_OVERFLOW statement, ppmtompeg issues a warning each time the simulation overflows the buffer, which suggests that an overflow would occur on playback, which suggests the buffer is too small.
These options were new in Netpbm 10.26 (January 2005). Before that, ppmtompeg issued the warnings always.
If you specify FORCE_I_ALIGN, ppmtompeg will increase the test frames value enough to maintain the alignment.
If there aren't enough frames for every slave to have the indicated number of test frames, ppmtompeg will give some slaves fewer.
Smaller values of t increase communication, but improve load balancing. The default is 30 seconds.
You may specify only one of PARALLEL_TIME_CHUNKS, PARALLEL_CHUNK_TAPER, and PARALLEL_PERFECT. PARALLEL_CHUNK_TAPER is usually best.
You may specify only one of PARALLEL_TIME_CHUNKS, PARALLEL_CHUNK_TAPER, and PARALLEL_PERFECT. PARALLEL_CHUNK_TAPER is usually best.
This has the advantage of minimal scheduling overhead. Where slaves have different speeds, though, it makes inefficient use of the fast ones. Where slaves are the same speed, it also has the disadvantage that they all finish at the same time and feed their output to the single Combine Server in a burst, which makes less efficient use of the Combine Server and thus can increase the total elapsed time.
You may specify only one of PARALLEL_TIME_CHUNKS, PARALLEL_CHUNK_TAPER, and PARALLEL_PERFECT. PARALLEL_CHUNK_TAPER is usually best.
Be sure to set up .rhosts files or SSH key authorizations where needed. Otherwise, you'll have to type in passwords.
On some HP machines, rsh is the restricted shell, and you want to specify remsh.
This document used to say there was an argument to FORCE_I_ALIGN which was the number of frames ppmtompeg would use (and was required to be a multiple of the pattern length). But ppmtompeg has apparently always ignored that argument, and it does now.
This is mostly useful for debugging.
This works only if you're using a shared filesystem to communicate between the servers.
This option was new in Netpbm 10.26 (January 2005).
If you use the -combine_gops option, then you need to specify only the SIZE and OUTPUT values in the parameter file. In addition, the parameter file may specify input GOP files in the same manner as normal input files -- except instead of using INPUT_DIR, INPUT, and END_INPUT, use GOP_INPUT_DIR, GOP_INPUT, and GOP_END_INPUT. If you specify no input GOP files, then ppmtompeg uses by default the output file name with suffix .gop.gop_num, with gop_num starting from 0, as the input files.
If you use the -combine_frames option, then you need to specify only the SIZE, GOP_SIZE, and OUTPUT values in the parameter file. In addition, the parameter file may specify input frame files in the same manner as normal input files -- except instead of using INPUT_DIR, INPUT, and END_INPUT, use FRAME_INPUT_DIR, FRAME_INPUT, and FRAME_END_INPUT. If no input frame files are specified, then the default is to use the output file name with suffix .frame.frame_num, with frame_num starting from 0, as the input files.
Any number of spaces and tabs may come between each option and value. Lines beginning with # are ignored. Any other lines are ignored except for those between INPUT and END_INPUT. This allows you to use the same parameter file for normal usage and for -combine_gops and -combine_frames.
The file format is case-sensitive so all keywords should be in upper case.
The statements may appear in any order, except that the order within a block statement (such as INPUT ... END INPUT) is significant.
ppmtompeg is prepared to handle up to 16 B frames between reference frames when encoding with input from stdin. (To build a modified ppmtompeg with a higher limit, change the constant B_FRAME_RUN in frame.c and recompile).
The quantization scale values (qscale) give a trade-off between quality and compression. Using different Qscale values has very little effect on speed. The qscale values can be set separately for I, P, and B frames.
You select the qscale values with the IQSCALE, PQSCALE, and BSCALE parameter file statements.
A qscale value is an integer from 1 to 31. Larger numbers give
better compression, but worse quality. In the following, the quality
numbers are peak signal-to-noise ratio, defined as:
where MSE is the mean squared error.
Flower garden tests:
| Qscale | I Frames | P Frames | B Frames |
|---|---|---|---|
| 1 | 43.2 | 46.3 | 46.5 |
| 6 | 32.6 | 34.6 | 34.3 |
| 11 | 28.6 | 29.5 | 30.0 |
| 16 | 26.3 | 26.8 | 28.6 |
| 21 | 24.7 | 25.0 | 27.9 |
| 26 | 23.5 | 23.9 | 27.5 |
| 31 | 22.6 | 23.0 | 27.3 |
| Qscale | I Frames | P Frames | B Frames |
|---|---|---|---|
| 1 | 2 | 2 | 2 |
| 6 | 7 | 10 | 15 |
| 11 | 11 | 18 | 43 |
| 16 | 15 | 29 | 97 |
| 21 | 19 | 41 | 173 |
| 26 | 24 | 56 | 256 |
| 31 | 28 | 73 | 330 |
There are several different motion vector search techniques available. There are different techniques available for P frame search and B frame search. Using different search techniques present little difference in quality, but a large difference in compression and speed.
There are 4 types of P frame search: Exhaustive, TwoLevel, SubSample, and Logarithmic.
There are 3 types of B frame search: Exhaustive, Cross2, and Simple. The recommended search techniques are TwoLevel and Logarithmic for P frame search, and Cross2 and Simple for B frame search. Here are some numbers comparing the different search methods:
| Technique | Compression1 | Speed 2 | Quality 3 |
|---|---|---|---|
| Exhaustive | 1000 | 1000 | 1000 |
| SubSample | 1008 | 2456 | 1000 |
| TwoLevel | 1009 | 3237 | 1000 |
| Logarithmic | 1085 | 8229 | 998 |
| Technique | Compression1 | Speed2 | Quality3 |
|---|---|---|---|
| Exhaustive | 1000 | 1000 | 1000 |
| Cross2 | 975 | 1000 | 996 |
| Simple | 938 | 1765 | 991 |
For some reason, Simple seems to give better compression, but it depends on the image sequence.
Select the search techniques with the PSEARCH_ALG and BSEARCH_ALG parameter file statements.
A Group of Pictures (GOP) is a roughly independently decodable sequence of frames. An MPEG video stream is made of one or more GOP's. You may specify how many frames should be in each GOP with the GOP_SIZE parameter file statement. A GOP always starts with an I frame.
Instead of encoding an entire sequence, you can encode a single GOP. To do this, use the -gop command option. You can later join the resulting GOP files at any time by running ppmtompeg with the -combine_gops command option.
A slice is an independently decodable unit in a frame. It can be as small as one macroblock, or it can be as big as the entire frame. Barring transmission error, adding slices does not change quality or speed; the only effect is slightly worse compression. More slices are used for noisy transmission so that errors are more recoverable. Since usually errors are not such a problem, we usually just use one slice per frame.
Control the slice size with the SLICES_PER_FRAME parameter file statement.
Some MPEG playback systems require that each slice consist of whole rows of macroblocks. If you are encoding for this kind of player, if the height of the image is H pixels, then you should set the SLICES_PER_FRAME to some number which divides H/16. For example, if the image is 240 pixels (15 macroblocks) high, then you should use only 15, 5, 3, or 1 slices per frame.
Note: these MPEG playback systems are really wrong, since the MPEG standard says this doesn't have to be so.
The search window is the window in which ppmtompeg searches for motion vectors. The window is a square. You can specify the size of the square, and whether to allow half-pixel motion vectors or not, with the RANGE and PIXEL parameter file statements.
In MPEG-1, a movie is represented as a sequence of MPEG frames, each of which is an I Frame, a P Frame, or a B Frame. Each represents an actual frame of the movie (don't get confused by the dual use of the word "frame." A movie frame is a graphical image. An MPEG frame is a set of data that describes a movie frame).
An I frame ("intra" frame) describes a movie frame in isolation -- without respect to any other frame in the movie. A P frame ("predictive" frame) describes a movie frame by describing how it differs from the movie frame described by the latest preceding I or P frame. A B frame ("bidirectional" frame) describes a movie frame by describing how it differs from the movie frames described by the nearest I or P frame before and after it.
Note that the first frame of a movie must be described by an I frame (because there is no previous movie frame) and the last movie frame must be described by an I or P frame (because there is no subsequent movie frame).
Beyond that, you can choose which frames are represented by which types. You specify a pattern, such as IBPBP and ppmtompeg simply repeats it over and over throughout the movie. The pattern affects speed, quality, and stream size. Here is a chart which shows some of the trade-offs:
| Frame Type | Size | Speed | Quality |
|---|---|---|---|
| I frames | 1000 | 1000 | 1000 |
| P frames | 409 | 609 | 969 |
| B frames | 72 | 260 | 919 |
A standard sequence is IBBPBBPBBPBBPBB.
Select the sequence with the PATTERN parameter file statement.
Since the last MPEG frame cannot be a B frame (see above), if the pattern you specify indicates a B frame for the last movie frame of the movie, ppmtompeg makes it an I frame instead.
Before Netpbm 10.26 (January 2005), ppmtompeg instead drops the trailing B frames by default, and you need the FORCE_ENCODE_LAST_FRAME parameter file statement to make it do this.
The MPEG frames don't appear in the MPEG-1 stream in the same order that the corresponding movie frames appear in the movie -- the B frames come after the I and P frames on which they are based. For example, if the movie is 4 frames that you will represent with the pattern IBBP, the MPEG-1 stream will start with an I frame describing movie frame 0. The next frame in the MPEG-1 stream is a P frame describing movie frame 3. The last two frames in the MPEG-1 stream are B frames describing movie frames 1 and 2, respectively.
Specify the input frame images with the INPUT_DIR, INPUT, END_INPUT, BASE_FILE_FORMAT, SIZE, YUV_FORMAT and INPUT_CONVERT parameter file statements.
Specify the output file with the OUTPUT parameter file statement.
ppmtompeg can generate a variety of statistics about the encoding. See the -stat, -snr, -mv_histogram, -quiet, -no_frame_summary, and -bit_rate_info options.
You can run ppmtompeg on multiple machines at once, encoding the same MPEG stream. When you do, the machines are used as shown in the following diagram. We call this "parallel mode."
To do parallel processing, put the statement
PARALLEL
in the parameter file, followed by a listing of the machines, one
machine per line, then
END_PARALLEL
Each of the machine lines must be in one of two forms. If the machine
has filesystem access to the input files, then the line is:
machine user executable
The executable is normally ppmtompeg (you may need to give the complete path if you've built for different architectures). If the machine does not have filesystem access to the input files, the line is:
REMOTE machine user executable parameter file
The -max_machines command option limits the number of machines ppmtompeg will use. If you specify more machines in the parameter file than -max_machines allows, ppmtompeg uses only the machines listed first. This is handy if you want to experiment with different amounts of parallelism.
In general, you should use full path file names when describing executables and parameter files. This includes the parameter file argument on the original invocation of ppmtompeg.
All file names must be the same on all systems (so if e.g. you're using an NFS filesystem, you must make sure it is mounted at the same mountpoint on all systems).
Because not all of the processes involved in parallel operation have easy access to the input files, you must specify the SIZE parameter file statement when you do parallel operation.
The machine on which you originally invoke ppmtompeg is the master machine. It hosts a "combine server,", a "decode server," and a number of "i/o servers," all as separate processes. The other machines in the network (listed in the parameter file) are slave machines. Each hosts a single process that continuously requests work from the master and does it. The slave process does the computation to encode MPEG frames. It processes frames in batches identified by the master.
The master uses a remote shell command to start a process on a slave machine. By default, it uses an rsh shell command to do this. But use the RSH parameter file statement to control this. The shell command the master executes remotely is ppmtompeg, but with options to indicate that it is to perform slave functions.
The various machines talk to each other over TCP connections. Each machine finds and binds to a free TCP port number and tells its partners the port number. These port numbers are at least 2048.
Use the PARALLEL_TEST_FRAMES, PARALLEL_TIME_CHUNKS, and PARALLEL_PERFECT parameter file statements to control the way the master divides up work among the slaves.
Use the -nice command option to cause all slave processes to run "nicely," i.e. as low priority processes. That way, this substantial and long-running CPU load will have minimal impact on other, possibly interactive, users of the systems.
Here is a look at ppmtompeg speed, in single-node (not parallel) operation:
| Machine Type | Macroblocks per second1 |
|---|---|
| HP 9000/755 | 280 |
| DEC 3000/400 | 247 |
| HP 9000/750 | 191 |
| Sparc 10 | 104 |
| DEC 5000 | 68 |
The measurements in the table are with inputs and outputs via a conventional locally attached filesystem. If you are using a network filesystem over a single 10 MB/s Ethernet, that constrains your speed more than your CPU speed. In that case, don't expect to get better than 4 or 5 frames per second no matter how fast your CPUs are.
Network speed is even more of a bottleneck when the slaves do not have filesystem access to the input files -- i.e. you declare them REMOTE.
Where I/O is the bottleneck, size of the input frames can make a big difference. So YUV input is better than PPM, and JPEG is better than both.
When you're first trying to get parallel mode working, be sure to use the -debug_machines option so you can see what's going on. Also, -debug_sockets can help you diagnose communication problems.