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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.

• split video (avi/h264) on keyframe

  30 novembre 2012, par m.sr

  Hallo.

  I have a big video file. ffmpeg, tcprobe and other tool say, it is an h264-stream in an AVI-container.

  Now i'd like to cut out small chunks form the video.

  1. Problem : The index of the video seam corrupted/destroyed. I kind of fixed this via mplayer -forceidx -saveidx <indexfile> <bigvideofile></bigvideofile></indexfile>. The Problem here is, that I'm now stuck with mplayer/mencoder which can use this index file via -loadidx <indexfile></indexfile>. I have tried correcting the index like described in man aviindex (mplayer -frames 0 -saveidx mpidx broken.avi ; aviindex -i mpidx -o tcindex ; avimerge -x tcindex -i broken.avi -o fixed.avi), but this didn't fix my video - meaning that most tools i've tested couldn't search in the video file.

  2. Problem : I cut out parts of the video via following command : mencoder -loadidx in.idx -ss 8578 -endpos 20 -oac faac -ovc x264 -sws 9 -lavfopts format=mp4 -x264encopts <lotsofopts> -of lavf -vf scale=800:-10,harddup in.avi -o out.mp4</lotsofopts>. Now here the problem is, that some videos are corrupted at the beginning. I think this is because the fact, that i do not necessarily cut at keyframe.

  Questions :

  1. What is the best way to fix the index of an avi "inline" so that every tool can again work as expected with it ?

  2. How can i split at the keyframes ? Is there an mencoder-option for this ?

  3. Are Keyframes coming in a frequency ? How to find out this frequency ? (So with a bit of math it should be possible to calculate the next keyframe and cut there)

  4. Is ther perhaps some completely other way to split this movie ? Doing it by hand is no option, i've to cut out 1000+ chunks ...

  Thanks a lot !

• Need to get audio video structure for automation ffmpeg or ffprobe

  10 mai 2012, par Sapan Doshi

  I need to get video audio stream information in separate form.

  I have tried ffprobe and mediainfo, but they give full information which needs to be parsed.

  I think there could be option like, where I do not need to parse the full information and get required data.

  $ffprobe -XXX
  audio channels 8

  $ffprobe -YYY
  video_resolution 512x288

  Can anybody help with this.