Author Archives: Multimedia Mike

FATE Ends the Mac

Did you know Mac OS X can even blue-screen? To be fair, it doesn’t actually present a blue screen. But when Mac OS X encounters a kernel panic, it looks like this:



True to form, Mac just has to be prettier and glossier than other operating systems, even in the area of system crashes.

The reason I bring this up is that the FATE system is bringing down my Mac. My Mac Mini is reliably dying every single time I try to execute my FATE client Python script. Maybe the weather is getting too warm.

Update, 2010-6-8: Following advice in the comments, I tried to run Memtest86 on the Mac Mini in question. I couldn’t get the machine to boot the CD I made. As an alternative, I turned the machine off and let it rest for a night. In the morning, I turned it on and ran the FATE client script. It’s working for now.

Fighting with the VP8 Spec

As stated in a previous blog post on the matter, FFmpeg’s policy is to reimplement codecs rather than adopt other codebases wholesale. And so it is with Google’s recently open sourced VP8 codec, the video portion of their Webm initiative. I happen to know that the new FFmpeg implementation is in the capable hands of several of my co-developers so I’m not even worrying about that angle.

Instead, I thought of another of my characteristically useless exercises: Create an independent VP8 decoder implementation entirely in pure Python. Silly? Perhaps. But it has one very practical application: By attempting to write a new decoder based on the official bitstream documentation, this could serve as a mechanism for validating said spec, something near and dear to my heart.

What is the current state of the spec? Let me reiterate that I’m glad it exists. As I stated during the initial open sourcing event, everything that Google produced for the initial event went well beyond my wildest expectations. Having said that, the documentation does fall short in a number of places. Fortunately, I am on the Webm mailing lists and am sending in corrections and ideas for general improvement. For the most part, I have been able to understand the general ideas behind the decoding flow based on the spec and am even able to implement certain pieces correctly. Then I usually instrument the libvpx source code with output statements in order to validate that I’m doing everything right.

Token Blocker
Unfortunately, I’m quite blocked right now on the chapter regarding token/DCT coefficient decoding (chapter 13 in the current document iteration). In his seminal critique of the codec, Dark Shikari complained that large segments of the spec are just C code fragments copy and pasted from the official production decoder. As annoying as that is, the biggest insult comes at the end of section 13.3:

While we have in fact completely described the coefficient decoding procedure, the reader will probably find it helpful to consult the reference implementation, which can be found in the file detokenize.c.

The reader most certainly will not find it helpful to consult the file detokenize.c. The file in question implements the coefficient residual decoding with an unholy sequence of C macros that contain goto statements. Honestly, I thought I did understand the coefficient decoding procedure based on the spec’s description. But my numbers don’t match up with the official decoder. Instrumenting or tracing macro’d code is obviously painful and studying the same code is making me think I don’t understand the procedure after all. To be fair, entropy decoding often occupies a lot of CPU time for many video decoders and I have little doubt that the macro/goto approach is much faster than clearer, more readable methods. It’s just highly inappropriate to refer to it for pedagogical purposes.

Aside: For comparison, check out the reference implementation for the VC-1 codec. It was written so clearly and naively that the implementors used an O(n) Huffman decoder. That’s commitment to clarity.

I wonder if my FFmpeg cohorts are having better luck with the DCT residue decoding in their new libavcodec implementation? Maybe if I can get this Python decoder working, it can serve as a more appropriate reference decoder.

Update: Almost immediately after I posted this entry, I figured out a big problem that was holding me back, and then several more small ones, and finally decoded by first correct DCT coefficient from the stream (I’ve never been so happy to see the number -448). I might be back on track now. Even better was realizing that my original understanding of the spec was correct.

Unrelated
I found this image on the Doom9 forums. I ROFL’d:
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Monster Battery Power Revisited

So I have this new fat netbook battery and I performed an experiment to determine how long it really lasts. In my last post on the matter, it was suggested that I should rely on the information that gnome-power-manager is giving me. However, I have rarely seen GPM report more than about 2 hours of charge; even on a full battery, it only reports 3h25m when I profiled it as lasting over 5 hours in my typical use. So I started digging to understand how GPM gets its numbers and determine if, perhaps, it’s not getting accurate data from the system.

I started poking around /proc for the data I wanted. You can learn a lot in /proc as long as you know the right question to ask. I had to remember what the power subsystem is called — ACPI — and this led me to /proc/acpi/battery/BAT0/state which has data such as:

present:                 yes
capacity state:          ok
charging state:          charged
present rate:            unknown
remaining capacity:      100 mAh
present voltage:         8326 mV

“Remaining capacity” rated in mAh is a little odd; I would later determine that this should actually be expressed as a percentage (i.e., 100% charge at the time of this reading). Examining the GPM source code, it seems to determine as a function of the current CPU load (queried via /proc/stat) and the battery state queried via a facility called devicekit. I couldn’t immediately find any source code to the latter but I was able to install a utility called ‘devkit-power’. Mostly, it appears to rehash data already found in the above /proc file.

Curiously, the file /proc/acpi/battery/BAT0/info, which displays essential information about the battery, reports the design capacity of my battery as only 4400 mAh which is true for the original battery; the new monster battery is supposed to be 10400 mAh. I can imagine that all of these data points could be conspiring to under-report my remaining battery life.

Science project: Repeat the previous power-related science project but also parse and track the remaining capacity and present voltage fields from the battery state proc file.

Let’s skip straight to the results (which are consistent with my last set of results in terms of longevity):



So there is definitely something strange going on with the reporting– the 4400 mAh battery reports discharge at a linear rate while the 10400 mAh battery reports precipitous dropoff after 60%.

Another curious item is that my script broke at first when there was 20% power remaining which, as you can imagine, is a really annoying time to discover such a bug. At that point, the “time to empty” reported by devkit-power jumped from 0 seconds to 20 hours (the first state change observed for that field).

Here’s my script, this time elevated from Bash script to Python. It requires xdotool and devkit-power to be installed (both should be available in the package manager for a distro).
Continue reading →

VP8 And FFmpeg

UPDATE, 2010-06-17: You don’t need to struggle through these instructions anymore. libvpx 0.9.1 and FFmpeg 0.6 work together much better. Please see this post for simple instructions on getting up and running quickly.

Let’s take the VP8 source code (in Google’s new libvpx library) for a spin; get it to compile and hook it up to FFmpeg. I am hesitant to publish specific instructions for building in the somewhat hackish manner available on day 1 (download FFmpeg at a certain revision and apply a patch) since that kind of post has a tendency to rise in Google rankings. I will just need to remember to update this post after the library patches are applied to the official FFmpeg tree.

Statement of libvpx’s Relationship to FFmpeg
I don’t necessarily speak officially for FFmpeg. But I’ve been with the project long enough to explain how certain things work.

Certainly, some may wonder if FFmpeg will incorporate Google’s newly open sourced libvpx library into FFmpeg. In the near term, FFmpeg will support encoding and decoding VP8 via external library as it does with a number of other libraries (most popularly, libx264). FFmpeg will not adopt the code for its own codebase, even if the license may allow it. That just isn’t how the FFmpeg crew rolls.

In the longer term, expect the FFmpeg project to develop an independent, interoperable implementation of the VP8 decoder. Sometime after that, there may also be an independent VP8 encoder as well.

Building libvpx
Download and build libvpx. This is a basic ‘configure && make’ process. The build process creates a static library, a bunch of header files, and 14 utilities. A bunch of these utilities operate on a file format called IVF which is apparently a simple transport method for VP8. I have recorded the file format on the wiki.

We could use a decoder for this in the FFmpeg code base for testing VP8 in the future. Who’s game? Just as I was proofreading this post, I saw that David Conrad has sent an IVF demuxer to the ffmpeg-devel list.

There doesn’t seem to be a ‘make install’ step for the library. Instead, go into the overly long directory (on my system, this is generated as vpx-vp8-nopost-nodocs-generic-gnu-v0.9.0), copy the contents of include/ to /usr/local/include and the static library in lib/ to /usr/local/lib .

Building FFmpeg with libvpx
Download FFmpeg source code at the revision specified or take your chances with the latest version (as I did). Download and apply provided patches. This part hurts since there is one diff per file. Most of them applied for me.

Configure FFmpeg with 'configure --enable-libvpx_vp8 --enable-pthreads'. Ideally, this should yield no complaints and ‘libvpx_vp8’ should show up in the enabled decoders and encoders sections. The library apparently relies on threading which is why '--enable-pthreads' is necessary. After I did this, I was able to create a new webm/VP8/Vorbis file simply with:

 ffmpeg -i input_file output_file.webm

Unfortunately, I can’t complete the round trip as decoding doesn’t seem to work. Passing the generated .webm file back into FFmpeg results in a bunch of errors of this format:

[libvpx_vp8 @ 0x8c4ab20]v0.9.0
[libvpx_vp8 @ 0x8c4ab20]Failed to initialize decoder: Codec does not implement requested capability

Maybe this is the FFmpeg revision mismatch biting me.

FFmpeg Presets
FFmpeg features support for preset files which contain collections of tuning options to be loaded into the program. Google provided some presets along with their FFmpeg patches:

  • 1080p50
  • 1080p
  • 360p
  • 720p50
  • 720p

To invoke one of these (assuming the program has been installed via ‘make install’ so that the presets are in the right place):

 ffmpeg -i input_file -vcodec libvpx_vp8 -vpre 720p output_file.webm

This will use a set of parameters that are known to do well when encoding a 720p video.

Code Paths
One of goals with this post was to visualize a call graph after I got the decoder hooked up to FFmpeg. Fortunately, this recon is greatly simplified by libvpx’s simple_decoder utility. Steps:

  • Build libvpx with --enable-gprof
  • Run simple_decoder on an IVF file
  • Get the pl_from_gprof.pl and dot_from_pl.pl scripts frome Graphviz’s gprof filters
  • gprof simple_decoder | ./pl_from_gprof.pl | ./dot_from_pl.pl > 001.dot
  • Remove the 2 [graph] and 1 [node] modifiers from the dot file (they only make the resulting graph very hard to read)
  • dot -Tpng 001.dot > 001.png

Here are call graphs generated from decoding test vectors 001 and 017.


Like this, only much larger and scarier (click for full graph)


It’s funny to see several functions calling an empty bubble. Probably nothing to worry about. More interesting is the fact that a lot of function_c() functions are called. The ‘_c’ at the end is important– that generally indicates that there are (or could be) SIMD-optimized versions. I know this codebase has plenty of assembly. All of the x86 ASM files appear to be written such that they could be compiled with NASM.

Leftovers
One interesting item in the code was vpx_scale/leapster. Is this in reference to the Leapster handheld educational gaming unit? Based on this item from 2005 (archive.org copy), some Leapster titles probably used VP6. This reminds me of finding references to the PlayStation in Duck/On2’s original VpVision source release. I don’t know of any PlayStation games that used Duck’s original codecs but with thousands to choose from, it’s possible that we may find a few some day.