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• Cloaked Archive Wiki

  16 mai 2011, par Multimedia Mike — General

  Google’s Chrome browser has made me phenomenally lazy. I don’t even attempt to type proper, complete URLs into the address bar anymore. I just type something vaguely related to the address and let the search engine take over. I saw something weird when I used this method to visit Archive Team’s site :

  
  
  

  There’s greater detail when you elect to view more results from the site :

  
  
  

  As the administrator of a MediaWiki installation like the one that archiveteam.org runs on, I was a little worried that they might have a spam problem. However, clicking through to any of those out-of-place pages does not indicate anything related to pharmaceuticals. Viewing source also reveals nothing amiss.

  I quickly deduced that this is a textbook example of website cloaking. This is when a website reports different content to a search engine than it reports to normal web browsers (humans, presumably). General pseudocode :

  PLAIN TEXT

  C :

  1. if (web_request.user_agent_string == CRAWLER_USER_AGENT)
  2.  return cloaked_data ;
  3. else
  4.  return real_data ;

  You can verify this for yourself using the wget command line utility :

  <br />
$ wget --quiet --user-agent="<strong>Mozilla/5.0</strong>" \<br />
 http://www.archiveteam.org/index.php?title=Geocities -O - | grep \&lt;title\&gt;<br />
&lt;title&gt;GeoCities - Archiveteam&lt;/title&gt;

  $ wget —quiet —user-agent="Googlebot/2.1"
  http://www.archiveteam.org/index.php?title=Geocities -O - | grep \<title\>
  <title>Cheap xanax | Online Drug Store, Big Discounts</title>
  

  I guess the little web prank worked because the phaux-pharma stuff got indexed. It makes we wonder if there’s a MediaWiki plugin that does this automatically.

  For extra fun, here’s a site called the CloakingDetector which purports to be able to detect whether a page employs cloaking. This is just one humble observer’s opinion, but I don’t think the site works too well :
  
  

  
  
  

• ffmpeg command to is throwing error

  25 février 2013, par Bhargav Shah

  I am new to ffmpeg usage.
  I am trying to merge two video file.
  The below bullets will provide you more details about it.

  1. I-ball usb camera
  2. Screen capture utility named UScreenCapture.

  The below command i am using on DOS.

  ffmpeg -f dshow  -i video="iBall Face2Face Webcam C12.0" -f dshow -i video="UScreenCapture"  -r 25 -vcodec mpeg4 -q 12 -f mpegts test.ts

  This command captures only from Uscreencapture source.

  while grabbing frames from Camera it is giving me an error saying that
  real-time buffer 90% full ! frame dropped !
  real-time buffer 121% full ! frame dropped !

  Can any one provide me the solution for this issue ?

• RoQ on Dreamcast

  18 mars 2011, par Multimedia Mike — Sega Dreamcast

  I have been working on that challenge to play back video on the Sega Dreamcast. To review, I asserted that the RoQ format would be a good fit for the Sega Dreamcast hardware. The goal was to play 640x480 video at 30 frames/second. Short version : I have determined that it is possible to decode such video in real time. However, I ran into certain data rate caveats.

  First off : Have you ever wondered if the Dreamcast can read an 80mm optical disc ? It can ! I discovered this when I only had 60 MB of RoQ samples to burn on a disc and a spindle full of these 210MB-capacity 80mm CD-Rs that I never have occasion to use.

  
  
  

  New RoQ Library
  There are open source RoQ decoders out there but I decided to write a new one. A few reasons : 1) RoQ is so simple that I didn’t think it would take too long ; 2) it would be nice to have a RoQ library that is license-compatible (BSD-like) with the rest of the KallistiOS distribution ; 3) the idroq.tar.gz distribution, while license-compatible, has enough issues that I didn’t want to correct it.

  Thankfully, I was correct about the task not being too difficult : I put together a new RoQ decoder in short order. I’m a bit embarrassed to admit that the part I had the most trouble with was properly converting YUV -> RGB.
  
  About the approach I took : While the original idroq.tar.gz decoder maintains YUV 4:2:0 codebooks (which led to chroma bugs during motion compensation) and FFmpeg’s decoder maintains YUV 4:4:4 codebooks, this decoder is built to convert the YUV 4:2:0 vectors into RGB565 vectors during the vector unpacking phase. Thus, the entire frame is rendered in RGB565 — no lengthy YUV -> RGB conversion after decoding — and all pixels are shuffled around as 16-bit units (minor speedup vs. shuffling everything as bytes).

  I also entertained the idea of maintaining YUYV codebooks (since the DC supports that colorspace as a texture format). But I scrapped that idea when I remembered it would lead to the same chroma bleeding problem seen in the original idroq.tar.gz decoder.

  Onto The Dreamcast
  I developed the library on a Linux computer, allowing it to output a series of PNM files for visual verification and debugging. Dropping it into a basic DC/KOS-compatible program was trivial and the first order of business was profiling.

  At first, I profiled the entire decode operation : open file, then read and decode each chunk while tossing away the results. I was roundly disappointed to see that, e.g., an 8.5-second RoQ sample needed a little more than 20 seconds to complete. Not real time. I performed a series of optimizations on the decoding library that netted notable performance gains when profiling on Linux. When I brought these same optimizations over to the DC, decoding time didn’t improve at all. This was my first suspicion that perhaps my assumptions regarding the DC’s optical drive’s data rate were not correct.

  Dreamcast Data Rate Profiling
  Let’s start with some definitions : In terms of data rate, an ’X’, i.e., 1X is the minimum data rate needed to read CD quality audio from a disc. At that speed, a drive should be able to stream 75 sectors each second. When reading mode 1/form 1 CD-ROM data, each sector has 2048 bytes (2 kbytes), so a single-speed data rate should achieve 150 kbytes/sec.

  The Dreamcast is supposed to possess a 12X optical drive. This would imply a maximum data rate of 150 kbytes/sec * 12 = 1800 kbytes/sec.

  Rigging up a trivial experiment using the RoQ samples burned on a few different CD-R discs, the best data rate I can see is about 500-525 kbytes/sec, or around 3.5X.

  Where’s the discrepancy ? My first theory has to do with the fact that not all optical media is created equal. This is why optical drives often advertise a slew of numbers which refer to the best theoretical speed for reading a CD vs. writing a CD-R vs. writing a CD-RW, etc. Perhaps the DC drive can’t read CD-Rs very quickly. To test this theory, I tried streaming a large file from a conventionally mastered CD-ROM. This worked well for the closest CD-ROM I had on hand : I was able to stream data at a rate that works out to about 6.5X.

  I smell a science project for another evening : Profiling read speeds from a mastered CD-ROM, burned CD-R, and also a mastered GD-ROM, on each of the 3 Dreamcast consoles I possess (I’ve heard that there’s variance between optical drives depending on manufacturing run).

  The Good News
  I added a little finer-grained code to profile just the video decoding functions. The good news is that the decoder meets my real time goals : That 8.5-second RoQ sample encoded at 640x480x30fps makes its way through the video decoding functions on the DC in a little less than 5 seconds. If the optical drive can supply the data fast enough, the video decoder can take care of the rest.

  The RoQ encoder included with FFmpeg does not honor any bitrate parameters. Instead, I encoded the same file at 320x240. It reportedly decoded in real time and can be streamed in real time as well.

  I say "reportedly" because I’m simply working from textual output at this point ; the next phase is to hook the decoder up to the display hardware.