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• Started Programming Young

  6 septembre 2011, par Multimedia Mike — Programming

  I have some of the strangest memories of my struggles to jump into computer programming.

  Back To BASIC
  I remember doing some Logo programming on Apple II computers at school in 5th grade (1987 timeframe). But that was mostly driving turtle graphics. Then I remember doing some TRS-80 BASIC in 7th grade, circa 1989. Emboldened by what very little I had learned in perhaps the week or 2 we took in a science class to do this, I tried a little GW-BASIC on my family’s “IBM-PC compatible” computer (they were still called that back then). I still remember what my first program consisted of. Even back then I was interested in manipulating graphics and color on a computer screen. Thus :

  10 color 1
  20 print "This is color 1"
  30 color 2
  40 print "This is color 2"
  ...
  

  And so on through 15 colors. Hey, it did the job– it demonstrated the 15 different colors you could set in text mode.

  What’s FOR For ?
  That 7th grade computer unit in science class wasn’t very thick on computer science details. I recall working with a lab partner to transcribe code listings into a computer (and also saving my work to a storage cassette). We also developed form processing programs that would print instructions to input text followed by an “INPUT I$” statement to obtain the user’s output.

  I remember there was some situation where we needed a brief delay between input and printing. The teacher told us to use a construct of the form :

  10 FOR I = 1 TO 20000
  20 NEXT I
  

  We had to calibrate the number based on our empirical assessment of how long it lasted but I recall that the number couldn’t be much higher than about 32000, for reasons that would become clearer much later.

  Imagine my confusion when I would read and try to comprehend BASIC program code I would find in magazines. I would of course see that FOR..NEXT construct all over the place but obviously not in the context of introducing deliberate execution delays. Indeed, my understanding of one of the fundamental building blocks of computer programming — iteration — was completely skewed because of this early lesson.

  Refactoring
  Somewhere along the line, I figured out that the FOR..NEXT could be used to do the same thing a bunch of times, possibly with different values. A few years after I had written that color program, I found it again and realized that I could write it as :

  10 for I = 1 to 15
  20 color I
  30 print I
  40 next I
  

  It still took me a few more years to sort out the meaning of WHILE..WEND, though.

• Getting artifacts when trying to read video stream from YouTube

  23 février 2012, par givi

  I'm trying to read the video frames from the RTSP stream that I get from YouTube. Here is the link for my test video :

  rtsp ://v8.cache5.c.youtube.com/CiILENy73wIaGQkJlrXMiAG8BxMYDSANFEgGUgZ2aWRlb3MM/0/0/0/video.3gp

  If I'm reading frames from the local file - everything is fine, but when read them from a stream I get nothing but lots of artifacts. I've googled around and found out that there might be a problem with UDP packets and switching to TCP may help but I really can't find where it is possible to change this.

  Here is the function for reading a frame :

  bool nextFrame(AVFormatContext *pFormatCtx, AVCodecContext *pCodecCtx, int videoStream, AVFrame *pFrame) {  AVPacket packet;
  
      int frameFinished = 0;
  
  
  
      while( !frameFinished && av_read_frame(pFormatCtx, &packet) >= 0 ) {
  
          // Is this a packet from the video stream?
  
          if( packet.stream_index == videoStream ) {
  
              // Decode video frame
  
              avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
  
          }
  
  
  
          // Free the packet that was allocated by av_read_frame
  
          av_free_packet(&packet);    
  
         }    
  
         return frameFinished!=0;  
  
  }

  I'm also getting lots of error messages in my log :

  [h263 @ 0x7804c00] warning: first frame is no keyframe
  
  [h263 @ 0x7804c00] illegal ac vlc code at 6x1
  
  [h263 @ 0x7804c00] Error at MB: 18
  
  [h263 @ 0x7804c00] concealing 99 DC, 99 AC, 99 MV errors
  
  [h263 @ 0x7804c00] I cbpy damaged at 10 4
  
  [h263 @ 0x7804c00] Error at MB: 58
  
  [h263 @ 0x7804c00] concealing 99 DC, 99 AC, 99 MV errors
  
  [h263 @ 0x7804c00] I cbpy damaged at 6 6
  
  [h263 @ 0x7804c00] Error at MB: 78
  
  [h263 @ 0x7804c00] concealing 76 DC, 76 AC, 76 MV errors
  
  [h263 @ 0x7804c00] I cbpy damaged at 5 5
  
  [h263 @ 0x7804c00] Error at MB: 65
  
  [h263 @ 0x7804c00] concealing 88 DC, 88 AC, 88 MV errors
  
  [h263 @ 0x7804c00] illegal ac vlc code at 7x5
  
  [h263 @ 0x7804c00] Error at MB: 67
  
  [h263 @ 0x7804c00] concealing 86 DC, 86 AC, 86 MV errors

  ...and so on

  edit : this is 99.9% a UDP-TCP problem. I've found this link :

  rtsp ://195.200.199.8/mpeg4/media.amp

  This is some test camera available online. It streams with artifacts. However if it has get parameter 'tcp' and if I use this

  rtsp ://195.200.199.8/mpeg4/media.amp ?tcp

  everything works without artifacts.

  So to correct my question : is there any way to force YouTube or ffmpeg to use TCP ?

• Google’s YouTube Uses FFmpeg

  9 février 2011, par Multimedia Mike — General

  Controversy arose last week when Google accused Microsoft of stealing search engine results for their Bing search engine. It was a pretty novel sting operation and Google did a good job of visually illustrating their side of the story on their official blog.

  This reminds me of the fact that Google’s YouTube video hosting site uses FFmpeg for converting videos. Not that this is in the same league as the search engine shenanigans (it’s perfectly legit to use FFmpeg in this capacity, but to my knowledge, Google/YouTube has never confirmed FFmpeg usage), but I thought I would revisit this item and illustrate it with screenshots. This is not new information— I first empirically tested this fact 4 years ago. However, a lot of people wonder how exactly I can identify FFmpeg on the backend when I claim that I’ve written code that helps power YouTube.

  Short Answer
  How do I know YouTube uses FFmpeg to convert multimedia ? Because :

  1. FFmpeg can decode a number of impossibly obscure multimedia formats using code I wrote
  2. YouTube can transcode many of the same formats
  3. I screwed up when I wrote the code to support some of these weird formats
  4. My mistakes are still present when YouTube transcodes certain fringe formats

  Longer Answer (With Pictures !)
  Let’s take a video format named RoQ, developed by noted game designer Graeme Devine. Originated for use in the FMV-heavy game The 11th Hour, the format eventually found its way into the Quake 3 engine as well as many games derived from the same technology.

  Dr. Tim Ferguson reverse engineered the format (though it would later be open sourced along with the rest of the Q3 engine). I wrote a RoQ playback system for FFmpeg, and I messed up in doing so. I believe my coding error helps demonstrate the case I’m trying to make here.

  Observe what happened when I pushed the jk02.roq sample through YouTube in my original experiment 4 years ago :

  
  
  

  Do you see how the canyon walls bleed into the sky ? That’s not supposed to happen. FFmpeg doesn’t do that anymore but I was able to go back into the source code history to find when it did do that :

  
  
  

  Academic Answer
  FFmpeg fixed this bug in June of 2007 (thanks to Eric Lasota). The problem had to do with premature colorspace conversion in my original decoder.

  Leftovers
  I tried uploading the video again to see if the problem persists in YouTube’s transcoder. First bit of trivia : YouTube detects when you have uploaded the same video twice and rejects the subsequent attempts. So I created a double concatenation of the video and uploaded it. The problem is gone, illustrating that the backend is actually using a newer version of FFmpeg. This surprises me for somewhat esoteric reasons.

  Here’s another interesting bit of trivia for those who don’t do a lot of YouTube uploading— YouTube reports format details when you upload a video :

  
  
  

  So, yep, RoQ format. And you can wager that this will prompt me to go back through the litany of unusual formats that FFmpeg supports to see how YouTube responds.