FPS x interval between scenes

Discussion in 'Dash Cam Software / Development' started by sraposo, Apr 28, 2017.

  1. sraposo

    sraposo New Member

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    Hi, All.

    I hope that´s the most appropriate forum here, since it seems to be the more technically skilled.

    Most of 4-channel dash cam recorders claims a 30FPS frequency, some with more than that, other with at least 15FPS. I couldn´t find a vehicular system that recorded at very low rates as 2 FPS.

    Why I mentioned that?

    I´ve got a ASF format video file (extension WMV) to analyze and it presents a new scenario each 0,5s (2 Hz). This is allegedly an original video made by a dash cam recorder, (supposely) with no edition nor conversion. In this file, there´s a sudden jump in time according timestamp, 16 seconds, just few seconds before and some after an accident, strongly suggesting that there was an intentional cut. I don´t know yet what´s the dash cam recorder maker nor product type and this information may take long time to get. Thus, I´m trying to speed up this study.

    So, if there is, or there was, a 2-FPS system, it would be enough to make such a file but, as I mentioned above, dash cam recorders on the market works with at least 15FPS, so image refresh would happen each 67ms, much faster than that 500ms that can be seen on that WMV file I mentioned.
    This WMV file has a 30FPS rate, according to the specific field in the file header (ASF format) and has a 15-frame extent for each scene, sometimes 7 frames. With a 30FPS specification in the file header and 15 frames per scene, yes!, you see a movie that changes scenes each 0.5, or 2 scenes per second. That´s makes sense.
    But what DOESN´T make sense is a movie (allegedly original) that shows 2 images per second has been recorded with 14 repeated frames (1 frame with a useful image and 14 more frames with the same image), this way wasting media room and processing time.

    So, I´ve got 2 hypothesis, and that considers that this WMV file is not the original one:
    1) the original video was 15, 30, whatever (much) more than 2 FPS, but was converted to a file taking frames with a 14-frame interval. In other words, taking 1 frame and skipping 14. Most dash cam records on the market records at a 30FPS rate, some at 15FPS, others at more than 30FPS.
    2) the original recording was really done with a 2 FPS rate (although I don´t know if some dash cam recording system allows such recording rate) and a new file was created after the original has been edited, and saved with a different quality, that increased FPS (thus, resulting the replicated frames)

    Since I´m not a dash cam expert, I ask you to help me to understand this case.

    Regards!
     
    Last edited: Apr 28, 2017
  2. jokiin

    jokiin Well-Known Member Manufacturer

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    sounds like the file is from a very cheap camera, the file format and multiple repeat frames is typical of low end cameras that have interpolated or fake resolutions, far more likely than someone purposely re-encoding a file like that, would be very tedious work to make that kind of file intentionally
     
  3. sraposo

    sraposo New Member

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    Hi, Jokiin.

    Yes, it´s also possible that´s a low end video recording system with a very low FPS rate (probably 2!).
    So, the original file had an extent cut off and finally it was saved with a set of parameters that result in a 30FPS file.
     
  4. jokiin

    jokiin Well-Known Member Manufacturer

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    ASF and WMV format is typical of low end junk cameras that are most often very low res and will repeat multiple frames to try and pass as higher res product to the uninitiated, quality wise they're not much better than a wood carving or stone tablets, good luck working with those files
     
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  5. SawMaster

    SawMaster Well-Known Member

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    If the clip is related to legal evidence, you might want some more random sample clips from the same camera to show that what you have is (or isn't) normal operation for that cam. Any dashcam which is working is better than no cam at all, but this shows the reason why good cams are far better ;)

    Phil
     
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  6. sraposo

    sraposo New Member

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    You mean that such a low end cam captures and digitalize in a very low rate (like 2 FPS) and intentionally records several identical frames (with same image)?
     
  7. sraposo

    sraposo New Member

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    Hi, Sawmaster.

    Yes, this video was provided as a legal evidence and the most important stretch is no longer there, allegedly due some recording failure or other unknown reason. A video that presents 2 scenes per second but is officialy, according file header, a 30 FPS video, with 15 repeated frames for each scene, what doesn´t make sense to me, but Jokiin stated that these repeated frames are intentional on low end systems.

    I don´t know yet what´s the dash cam system maker and consequently which model is, so I ignore if such equipment records only at 2 FPS or can be set to record at such rate. The fact is: video presents images at a 2Hz frequency.
     
  8. jokiin

    jokiin Well-Known Member Manufacturer

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    there are cameras around that are this bad, you may of course be dealing with something that has been tampered with intentionally

    next step would be finding out the make and model of the camera

    does the footage have any embedded data in the video at all, time etc? the maker can sometimes be identified by this info
     
  9. sraposo

    sraposo New Member

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    No metadata at all. To check this was one of the first things I did.

    I still find astonishing the fact of some cam makers intentionally record repeated frames, so wasting mass memory room, just to fake a higher fps.
     
  10. jokiin

    jokiin Well-Known Member Manufacturer

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    I wasn't referring to metadata but rather anything that was embedded in the video, if there's any timestamp etc in the video the model or what platform it is from can sometimes be identified based on the font type etc

    asking what make and model the camera is would be better, if they don't want to tell you that could raise some concerns

    it happen in low end junk cameras
     
  11. sraposo

    sraposo New Member

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    Yes, I´ve got what you said, but there´s not any information of this kind, unfortunately. I´ve already inspected the file with a hex editor.


    Sure! This and other questions are already prepared to be asked during lawsuit.
     
  12. jokiin

    jokiin Well-Known Member Manufacturer

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    does the file creation time and date match the incident time and date?
     
  13. sraposo

    sraposo New Member

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    Nope! This is also one more fact that makes this WMV file suspicious. Accident happened on february, 6th 2016 and "modified" date is 11th. In the WMV header file, field "Creation Time" contains "2016-2-11 10:23:12.686 "
    Some possibilities:
    1) this WMV file was resulted of an edition that was saved on february 11th;
    2) dash cam system created a video file from a bunch of separated frames/little extents movie when a copy was requested.
     
  14. jokiin

    jokiin Well-Known Member Manufacturer

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    the file creation date could be incorrect if the system clock was wrong, typical behaviour when cheap cameras have time/date issues is they will reset to some default value like 01/01/2000 or similar so being 5 days fast is not something you would typically see

    that said though in some low end models that use odd file systems and formats they sometimes have some sort of software for exporting clips, it's possible that this type of software could give a creation date that doesn't match the event date and that would occur after the fact, you're sort of obliged to give people the benefit of the doubt but when they can't or won't provide answers to questions that would give further details about the file it does create more doubt about what's going on, does sound suss from what you have to work with
     
  15. sraposo

    sraposo New Member

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    > One more information to be checked: if timestamp in the video is correct.



    That´s my #2 hypothesis.



    A lot of questions is to be made to the other party during the lawsuit in order to make the truth arises. For while, I´ve got some clues (to be further checked) that lead me to consider this WMV a not an original material:
    - it´s a WMV file;
    - it is 30FPS movie file although image presentation rate is 2FPS;
    - a 16-second extent is missing, exactly the stretch that would have registered the accident;
    - a file generation date that is five days after the accident;
    - no metadata
     
  16. jokiin

    jokiin Well-Known Member Manufacturer

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    Not looking good for whoever presented this as evidence, a lot more questions than answers
     
  17. sraposo

    sraposo New Member

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    The good part in this case is this WMV was presented as an evidence by the other party, a transportation company that owns the bus that has crashed my client´s vehicle.
     
  18. jokiin

    jokiin Well-Known Member Manufacturer

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    it would seem to imply they may have something to hide, not a good look
     
  19. SawMaster

    SawMaster Well-Known Member

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    I doubt it will fly as evidence. The date-time stamp can well be wrong with these cams, but when operating and recording it is rare for a clip to be missing a segment unless the power to it was interrupted. And during that recording session date-time will be a continuous string, even if it is not calendar-and-clock correct, and even with a power interruption. The "FPS issue" and multiple framing is common on cheap cams; they can do that cheaply and claim higher specs than you actually get. Advertising gimmick, that's all.

    I still think you should obtain other recordings from this particular cam as those will show what it's normal operation is like and that will compare to what you've got. You're probably entitled to request that. And I'd also let them know why I was asking and that you feel sure there's been evidence tampering, along with all which that entails. My guess is at that point they will know they've been busted and will be asking for your figures regarding a settlement ;)

    Phil
     
  20. jokiin

    jokiin Well-Known Member Manufacturer

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    quite common, is that the type of camera they would be using in a bus though, highly unlikely I would think
     

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