SoC x264 2008

x264 has loads of possibilities for SoC 2008 projects. This is part of the VideoLAN candidature for Google Summer of Code 2008.

• Mentor (and author of this page): Dark Shikari
• Possible backup mentor: pengvado

Introduction to x264

x264 is probably the most efficient, compression-wise, open source video encoder there is. It is quite competitive with commercial encoders, outclassing a large number of them.

While not actually part of VLC or ffmpeg (it has its own codebase), it is a major library used by both, licensed under the GPL, in addition to being a standalone encoder. As the only major open-source H.264 encoder, x264 has a near-complete monopoly on H.264 encoding in the consumer world, along with being used by many major corporations, including Facebook and Google. Some companies, such as Avail Media, have in the past offered bounties on improvements to the encoder.

x264 project ideas

This is not at all an exhaustive list; this is just a few I thought up with. I'm willing to mentor any reasonable project on x264 to the best of my ability. I'm being pretty conservative here, so I'm picking projects that are probably not at all too ambitious for a good student. If anything, I might be underestimating the amount of work that can be done, so feel free to propose something else if you're feeling creative.

-- Dark Shikari

Size key

Depends heavily on the skill and willingness to work of the student. An extremely dedicated and talented student might be able to implement MBAFF in a summer, but it is certainly not fair to expect such a thing from most students.

• Very Large: Probably too large to completed in one summer.
• Large: Probably the right size for a full-summer project.
• Medium: Probably too small. Could be combined with another project, of course.
• Small: A small project, but definitely useful, and could be part of a larger project.

Skills needed

These are required for all listed projects and probably anything not listed, too.

• Basic C programming.
• Basic understanding of video encoding, or at least willingness to do the appropriate reading up on the topic before the summer begins.
• Confidence in the ability to learn the basics of following and similar topics (though not all projects will require such information):

• Discrete cosine transform and similar frequency transforms
• Motion estimation and compensation
• Quantization and entropy encoding

Projects

Fast inter refinement

Size: Medium to large.

Description: Improve heuristics and decision-making for inter refinement to improve efficiency on non-insane encoding settings. This would involve various early termination heuristics along with methods of deciding which partition modes need to be searched while performing minimal actual searching on those partition modes. This would be similar to, but a vastly more in-depth analysis of what was proposed in the "Fast-Ref-Search" patch.

Difficulty: Medium

Fast intra refinement

Size: Small to medium

Description: Similar to above, but covering intra modes instead. Would probably involve considerable statistical analysis of intra mode data, along with creative solutions for improved RDO refinement. We already have some ideas on this one, but haven't implemented any of them.

Difficulty: Medium

RDO B-frame decision

Size: Medium to large

Description: x264's biggest weakness is its B-frame decision algorithm, which can often be extremely subtopimal, with OPSNR losses as high as 1db in some cases. Improving this would drastically increase the effectiveness of the encoder.

Difficulty: Medium-high

Other possible projects

Anything here (and not here) can potentially be picked from at the request of a student.

• Assembly optimizations of any sort

• Extra skills: Assembly coding
• Difficulty: Medium
• Examples:

• Port cacheline split to the motion compensation code for increased speed (this could further be used to improve ffh264's decoding).
• Assembly-optimize some things that haven't been already.
• Port some MMX assembly to SSE where it seems useful.
• Play around with potential SSE4 optimizations.

• Psychovisual optimizations for mode decision and quantization (e.g. QNS)

• Extra skills: Creativity and perhaps some understanding of DCT/Fourier math.
• Difficulty: Medium-high
• Examples:

• SSIM-QNS optimization?
• Adaptive deadzone?
• Adaptive lambda?

• Implementing MBAFF or PicAFF (potentially too difficult for a SoC project, however)

• Difficulty: Very high

• Fast RD optimization using heuristics

• Extra skills: Reading lots of IEEE papers
• Difficulty: Medium

• Motion search improvements

• Difficulty: Medium

• Anything else reasonable, honestly. There's all sorts of ideas floating around.

Qualification tasks

Before you start work these, drop by #x264dev and meet me (Dark Shikari) first. These should be done in order, and the results submitted to me at darkshikari[at]gmail.com. Bonus points will be given for *good* solutions or creative ones, not just ones that work. Well-commented and styled code is also a bonus. Feel free to do any research necessary to complete the task; this isn't a closed-notes test!

Rules: You can ask any question on IRC you want. There are no rules about where you can get any information--all that matters is that through your own effort or the help of others, or both, you get these completed. Feel free to ask me for help, up to a point, at any part in this. However, if you ask for too much assistance without trying yourself, I may penalize you.

Qualification tasks:

1. Download the x264 source. You will figure out how to do this yourself. Make sure to use git, not svn; the svn is not updated anymore, and the last build is broken, so use git!
2. Compile the x264 source and encode a sample video with the latest build off git. Upload the video as a .h264 file to Mediafire or a similar site, and email it to me at darkshikari[at]gmail.com with your name. And don't cheat, because I know what version you encoded it with! The only requirement is that the video be 1000 frames long and encoded in two-pass mode with bitrate 1000kbps. The video you choose is completely your own choice, but I suggest you choose one with a good bit of motion, for part 3).
3. Your first real qualification task will be playing with me.c, in particular, x264_me_search_ref(), the primary motion search in x264. In particular, you'll be working after line 227, "switch( h->mb.i_me_method )". This is where all five of the motion search methods are: DIA, HEX, UMH, ESA, and TESA.

Simple explanation of commands in me.c that you'll need to know:

• CHECK_MVRANGE(x,y): check the motion vector range of vector <x,y>. There's a 5-pixel buffer, so you can move by up to 5 pixels in the x and/or y direction before having to check it again.
• COST_MV(x,y): Run a check on location x,y.
• COST_MV_X4(x0,y0,x1,y1,x2,y2,x3,y3): Takes eight arguments; note that these are OFFSETS from a base value of <omx,omy>. Note this is quite a bit faster than calling COST_MV 4 times.

A simple analysis of the DIA motion search, for example:

case X264_ME_DIA:

/* diamond search, radius 1 */

for( i = 0; i < i_me_range; i++ )

{

DIA1_ITER( bmx, bmy );

if( bmx == omx && bmy == omy )

break;

if( !CHECK_MVRANGE(bmx, bmy) )

break;

}

break;

The for loop runs up to the merange parameter. At each point, DIA1_ITER is called, which just does COST_MV_X4 on all 4 neighboring locations. If none of these is better than the current one, it breaks out. If one is better, it selects that as the new center and loops to the beginning again. And if the MVrange is greater than the max, it breaks out too. This algorithm is often known as EPZS, or simply "diamond search."

Now that you know the basics of how this works, glance over the ME HEX and ME UMH functions to see how they work. Ignore ESA/TESA; these use some quite heavily optimized and complex code that you will probably not comprehend in the least.

Now that you get the basic idea, write me a motion search that is faster than UMH, slower than HEX, but still better than HEX. Your only requirement is that it be very little like HEX or UMH; i.e. you can't just rip of one of the two and modify it slightly.

Measuring the effectiveness of the motion search is simple: simply see how good you can get the PSNR value at a particular target bitrate. Ideally, on the video you choose, UMH will be a lot better than HEX, so there's a lot of margin of improvement between the two.

Your motion search need not be practical or worthwhile--it must merely fall within those parameters mentioned above.

I would also suggest you test it on more than one video, not just the one from 2); its possible to make the mistake of optimizing for a single video at the expense of others.

When you're done, email me the resulting patch for x264 and any extra information you think would be useful.

• This list isn't complete. I may add more, depending on progress on the previous tasks.

Contact info

If you are interested, drop by #videolan, #x264, or #x264dev on Freenode.

You should also contact the admin jb.