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• Inside WebM Technology : VP8 Intra and Inter Prediction

  20 juillet 2010, par noreply@blogger.com (Lou Quillio)
  Continuing our series on WebM technology, I will discuss the use of prediction methods in the VP8 video codec, with special attention to the TM_PRED and SPLITMV modes, which are unique to VP8.

  
  

  First, some background. To encode a video frame, block-based codecs such as VP8 first divide the frame into smaller segments called macroblocks. Within each macroblock, the encoder can predict redundant motion and color information based on previously processed blocks. The redundant data can be subtracted from the block, resulting in more efficient compression.

  
  Image by Fido Factor, licensed under Creative Commons Attribution License.
  Based on a work at www.flickr.com

  
  

  A VP8 encoder uses two classes of prediction :

  • Intra prediction uses data within a single video frame
  • Inter prediction uses data from previously encoded frames

  The residual signal data is then encoded using other techniques, such as transform coding.

  
  

  VP8 Intra Prediction Modes

  VP8 intra prediction modes are used with three types of macroblocks :

  • 4x4 luma
  • 16x16 luma
  • 8x8 chroma

  Four common intra prediction modes are shared by these macroblocks :
  

  • H_PRED (horizontal prediction). Fills each column of the block with a copy of the left column, L.
  • V_PRED (vertical prediction). Fills each row of the block with a copy of the above row, A.
  • DC_PRED (DC prediction). Fills the block with a single value using the average of the pixels in the row above A and the column to the left of L.
  • TM_PRED (TrueMotion prediction). A mode that gets its name from a compression technique developed by On2 Technologies. In addition to the row A and column L, TM_PRED uses the pixel P above and to the left of the block. Horizontal differences between pixels in A (starting from P) are propagated using the pixels from L to start each row.

  For 4x4 luma blocks, there are six additional intra modes similar to V_PRED and H_PRED, but correspond to predicting pixels in different directions. These modes are outside the scope of this post, but if you want to learn more see the VP8 Bitstream Guide.

  
  

  As mentioned above, the TM_PRED mode is unique to VP8. The following figure uses an example 4x4 block of pixels to illustrate how the TM_PRED mode works :

  

  Where C, As and Ls represent reconstructed pixel values from previously coded blocks, and X₀₀ through X₃₃ represent predicted values for the current block. TM_PRED uses the following equation to calculate X_ij :

  
  

  X_ij = L_i + A_j - C (i, j=0, 1, 2, 3)

  
  

  Although the above example uses a 4x4 block, the TM_PRED mode for 8x8 and 16x16 blocks works in the same fashion.

  TM_PRED is one of the more frequently used intra prediction modes in VP8, and for common video sequences it is typically used by 20% to 45% of all blocks that are intra coded. Overall, together with other intra prediction modes, TM_PRED helps VP8 to achieve very good compression efficiency, especially for key frames, which can only use intra modes (key frames by their very nature cannot refer to previously encoded frames).

  
  

  VP8 Inter Prediction Modes

  
  

  In VP8, inter prediction modes are used only on inter frames (non-key frames). For any VP8 inter frame, there are typically three previously coded reference frames that can be used for prediction. A typical inter prediction block is constructed using a motion vector to copy a block from one of the three frames. The motion vector points to the location of a pixel block to be copied. In most video compression schemes, a good portion of the bits are spent on encoding motion vectors ; the portion can be especially large for video encoded at lower datarates.

  
  

  Like previous VPx codecs, VP8 encodes motion vectors very efficiently by reusing vectors from neighboring macroblocks (a macroblock includes one 16x16 luma block and two 8x8 chroma blocks). VP8 uses a similar strategy in the overall design of inter prediction modes. For example, the prediction modes "NEAREST" and "NEAR" make use of last and second-to-last, non-zero motion vectors from neighboring macroblocks. These inter prediction modes can be used in combination with any of the three different reference frames.

  
  

  In addition, VP8 has a very sophisticated, flexible inter prediction mode called SPLITMV. This mode was designed to enable flexible partitioning of a macroblock into sub-blocks to achieve better inter prediction. SPLITMV is very useful when objects within a macroblock have different motion characteristics. Within a macroblock coded using SPLITMV mode, each sub-block can have its own motion vector. Similar to the strategy of reusing motion vectors at the macroblock level, a sub-block can also use motion vectors from neighboring sub-blocks above or left to the current block. This strategy is very flexible and can effectively encode any shape of sub-macroblock partitioning, and does so efficiently. Here is an example of a macroblock with 16x16 luma pixels that is partitioned to 16 4x4 blocks :

  
  

  

  
  

  where New represents a 4x4 bock coded with a new motion vector, and Left and Above represent a 4x4 block coded using the motion vector from the left and above, respectively. This example effectively partitions the 16x16 macroblock into 3 different segments with 3 different motion vectors (represented below by 1, 2 and 3) :

  
  

  

  
  

  Through effective use of intra and inter prediction modes, WebM encoder implementations can achieve great compression quality on a wide range of source material. If you want to delve further into VP8 prediction modes, read the VP8 Bitstream Guide or examine the reconintra.c and rdopt.c files in the VP8 source tree.

  
  

  Yaowu Xu, Ph.D. is a codec engineer at Google.

  
  

  

• FATE’s New Look

  4 août 2010, par Multimedia Mike — FATE Server

  The FATE main page exposes a lot of data. The manner in which it is presented has always been bounded by my extremely limited web development abilities. I wrestled with whether I should learn better web development skills first and allow that to inform any improved design, or focus on the more useful design and invest my web development learning time towards realizing that design.

  Fortunately, Mans solved this conundrum with an elegantly simple solution :

  
  
  

  The top of the page displays a status bar that illustrates — at a glance — how functional the codebase is. The web page source code identifies this as the failometer. It took me a few seconds to recognize what information that status bar was attempting to convey ; maybe it could use a succinct explanation.

  Mini-Book Review
  
  Before Mans took over, I thought about this problem quite a bit. I needed inspiration for creating a better FATE main page and aggregating a large amount of data in a useful, easily-digested form. Looking around the web, I see no shortage of methods for visualizing data. I could start shoehorning FATE data into available methods and see what works. But I thought it would be better to take a step back and think about the best way to organize the data. My first clue came awhile ago in the form of an xkcd comic : Blogofractal. Actually, the clue came from the mouseover text which recommended Edward Tufte’s "The Visual Display of Quantitative Information".

  
  
  

  I ordered this up and plowed through it. It’s an interesting read, to be sure. However, I think it illustrates what a book on multimedia and compression technology would look like if authored by yours truly— a book of technical curiosities from epochs past that discusses little in the way of modern practical application. Tufte’s book showed me lots of examples of infographics from decades and even centuries past, but I never concisely learned exactly how to present data such as FATE’s main page in a more useful form.

  Visualization Blog
  More recently, I discovered a blog called Flowing Data, authored by a statistics Ph.D. candidate who purportedly eats, sleeps, and breathes infographics. The post 11 Ways to Visualize Changes Over Time : A Guide offers a good starting point for creating useful data presentations.

  I still subscribe to and eagerly read Flowing Data. But I might not have as much use for data visualization now that Mans is on FATE duty.

• Sorenson Video 3 For Posterity

  12 août 2010, par Multimedia Mike — Software Museum

  On a recent dumpster dive, I procured a complete, never-been-opened copy of Sorenson Video 3 Professional Edition Compressor for Windows. And because I’m me, I thought it would be interesting to document it here :

  
  

  
  

  Not opened until now, anyway :

  
  
  

  I chuckled at the thought that I might open the user manual and find the complete data stream format listed therein. As expected, the guide offers a broad overview of video compression concepts (lossy vs. lossless, inter/intraframes, that kind of thing) and goes on to describe general guidelines for compressing different types of data. Then there is the feature reference. There are standard features and professional features (the latter includes things like bidirectional prediction, masking, and watermarking).

  I was hoping to figure out how to encode some video with this software. But I think I need full Quicktime Pro in order to do that. If you’re interested, here’s the user manual in PDF format : Sorenson Video 3 User Guide (626 Kbytes). Here’s an interesting claim from the chapter on audio compression :

  Most movies are made up of two parts, video and audio. Historically, the video portion of a digital movie was so large that the audio was only a minor piece of the puzzle. However, with Sorenson Video’s excellent compression capabilities it is possible to create a file where audio is the largest portion.

  I know it’s possible to do that, but is it really recommended ? I’m sure I have some samples in my vast repository where this is the case but it still doesn’t strike me as optimal for network delivery.