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• Naive Sorenson Video 1 Encoder

  12 septembre 2010, par Multimedia Mike — General

  (Yes, the word is “naive” — or rather, “naïve” — not “native”. People always try to correct me when I use the word. Indeed, it should actually be written with 2 dots over the ‘i’ but who has a keyboard that can easily do that ?)

  At the most primitive level, programming a video encoder is about writing out a sequence of bits that the corresponding video decoder will understand. It’s sort of like creating a program — represented as a stream of opcodes — that will run on a given microprocessor or virtual machine. In fact, reading a video codec bitstream specification will reveal a lot of terminology along the lines of “transmitting information to the decoder” or “signaling the decoder to do xyz.”

  Creating a good encoder that will deliver decent quality at a reasonable bitrate is difficult. Creating a naive encoder that produces a technically compliant bitstream, not so much.

  
  
  

  When I wrote an FFmpeg encoder for Sorenson Video 1 (SVQ1), the first step was to just create a minimally compliant bitstream. The coarsest encoding mode that SVQ1 allows is to encode the average (mean) of each 16×16 block of samples. So I created an encoder that just encoded the mean of each block. Apple’s QuickTime Player was able to play the resulting video in all of its blocky glory. The result rather reminds me of the Super Nintendo’s mosaic effect.

  Level 5 blocks (mean-only 16×16 encoding) :
  

  
  
  

  Level 3 blocks (mean-only 8×8 encoding) :
  

  
  
  

  It’s one thing for your own decoder (in this case, FFmpeg’s own decoder) to be able to decode the data. The big test is whether the official decoder (in this case, Apple QuickTime Player) can decode the file.

  
  
  

  Now that’s a good feeling. After establishing that sort of baseline, it’s possible to adapt more and more features of the codec.

• Ideas for new HTML5 apps

  10 mars 2011, par silvia

  At the recent Linux conference in Brisbane, Australia, I promised a free copy of my book to the person that could send me the best idea for an HTML5 video application. I later also tweeted about it. While I didn’t get many emails, I am still impressed by the things people want to do. Amongst (...)

• On WebP and Academic Exercises

  2 octobre 2010, par Multimedia Mike — General

  Yesterday, Google released a new still image format called WebP. To those skilled in the art, this new format will be recognizable as a single VP8 golden frame with a 20-byte header slapped on the front (and maybe a little metadata thrown in for good measure). We have a MultimediaWiki page and a sample ready to go.

  Further, I submitted a patch to ffmpeg-devel for FFmpeg’s img2 handling system to decode these files. FFmpeg should support processing these files soon… if anyone cares. This leads into…

  The Point, or Lack Thereof
  Since yesterday’s release, I have read a whirlwind of commentary about this format, much of it critical and of the “what’s the point ?” variety. For my part, I can respect academic exercises, a.k.a., just trying random stuff to see if you can make it work. That’s pretty much this blog’s entire raison d’être. But WebP transcends mere academic exercise ; Google seems to be trying to push it as a new web standard. I don’t see how the format can go anywhere based on criticisms raised elsewhere — e.g., see Dark Shikari’s thoughtful write-up — which basically boil down to WebP not solving any real problems, technical, legal, or otherwise.

  How did WebP come to be ? I strongly suspect some engineers noticed that JPEG is roughly the same as an MPEG-1 intraframe, so why not create a new still frame format based on VP8 intraframes ? Again, I can respect that thinking– I have pondered how a still image format would perform if based on VP3/Theora or Sorenson Video 1.

  Technically
  Google claims a significant size savings for WebP vs. standard JPEG. Assuming that’s true (and there will be no shortage of blog posts to the contrary), it will still be some time before WebP support will find its way into the majority of the web browser population.

  But this got me thinking about possible interim solutions. A website could store images compressed in both formats if it so chose. Then it could serve up a WebM image if the browser could support it, as indicated by the ‘Accept’ header in the HTTP request. It seems that a website might have to reference a generic image name such as <img src="some-picture.image"> ; the web server would have to recognize the .image extension and map it to either a .jpg or a .webp image depending on what the browser claims it is capable of displaying.

  Leftovers
  I appreciate that Dark Shikari has once again stuck his neck out and made a valiant — though often futile — effort to educate the internet’s masses. I long ago resigned myself to the fact that many people aren’t going to understand many of the most basic issues surrounding multimedia technology (i.e., moving pictures synchronized with audio). But apparently, this extends to still image formats as well. It was simultaneously humorous and disheartening to see commenters who don’t even understand the application of, e.g., PNG vs. JPEG : Ahem, “We already have a great replacement for jpg : .PNG”. Coupled with the typical accusations of MPEG tribalism, I remain impressed D. Shikari finds the will to bother.

  Still, I appreciate that the discussion has introduced me to some new image formats of which I was previously unaware, such as PGF and JPEG XR.