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Progressive scan vs 24fps cinema.

Discussion in 'Projectors, Screens & Video Processors' started by cosaw, Jun 28, 2003.

  1. cosaw

    cosaw
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    How do progressive scan tellys compare to 24fps film in the cinema?

    For instance I assume its something like this order wise:

    50Hz interlaced = flicker
    50Hz progressive = less flicker
    100Hz interlaced = ?????
    100Hz progressive = less flicker

    Where does this fit:
    24fps or 24Hz cinema = ?????

    It's a while since I've been to the cinema but does film flicker at essentially what is 24Hz?

    Film has the advantafe of no scan time i.e. refreshing the screen top to bottom is instantaneous. Whereas as the electron beam scans from top to bottom on a telly the image at the top of the screen will start to die before the electron beam reaches the bottom of the screen thus reducing the theoretical resolution of each scan at any point in time.

    Any answers?

    Cosaw
     
  2. They

    They
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    50Hz interlaced = 50Hz large area flicker and 25Hz line flicker
    50Hz progressive = 50Hz large area flicker and no line flicker (in theory)
    100Hz interlaced = no large area flicker and 25Hz line flicker unless further processed
    100 Hz progressive = no large area flicker and no line flicker (in theory)

    None of these display modes are optimum and there are many other imaging issues such as interlacing artifacts, interlaced to progressive processing and its associated artifacts, motion problems and blur.

    Cinema projection shows each frame twice, thus increasing the flicker rate from 24Hz to 48Hz, which when considering the viewing environment, display process and image brightness is just good enough not to flicker too disturbingly for most people. However, repeating the frames can add motion judder and blur to a image frame rate which is already too low to portray smooth life like motion. (The same is true for TV where 24Hz film source is sped up to 25Hz then shown twice to obtain the 50Hz rate whether interlaced or progressive)

    Image reveal for each frame via a shutter arrangment is not exactly instantaneous but is faster than the frame duration as apposed to the TV scanning system.
     
  3. cosaw

    cosaw
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    Ahh, I see. Thanks for the info. I didn't know that: "cinema projection shows each frame twice".

    I understand what you mean by: "which is already too low to portray smooth life like motion" but isn't it the case that the smoothness can be emulated by the exposure time of the camera? i.e. setting the correct exposure time means that each individual frame has just enough blur to trick the eye/brain.

    As far as I understand it that's why things like nintendo games running at 25 fps can look jerky, because no motion blur is applied to each frame, something which more advanced games consoles have started to do? With PC games they've always run at a higher frame rate anyway so this hasn't mattered as much.

    Why was 24fps chosen for film in the first place? I suppose it was a physical constraint? Is it true that these IMAX film as well as having larger film frames are filmed at higher frame rates approaching 60-70fps? In this case you'd have to set a quicker exposure time when filming in order that you didn't blur up the final image?
     
  4. They

    They
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    Yes, the motion blur does help trick our visual system into accepting the motion is continuous for the most part. Typically (and I am no expert on this) I believe the exposure duration is around 50% of the frame time but this can change of course, for practical as well as artistic reasons.

    Keeping the 50% exposure duration per frame time is valid even as the frame rate increases but in that case the degree of motion captured is finer as will be the percieved sharpness and image depth (but still contain motion blur at the same proportion per frame). I read a study that concluded the optimum flicker or frame rate for the high res area of the retina is 92Hz and for the low res peripheral area is 300hz, (the latter being of lesser importance for film/TV viewing).

    Motion processing can achieve the 92Hz or greater from existing sources but can do nothing to alter the exposure and degree of motion blur that exists in the original image.

    I can't recall exactly why 24fps for cinema was chosen, but I vaguely remember it was something of a compromise between mechanical considerations (including the speed an operator could hand-crank a camera in the early days of cinema), economics of the cost per foot of film, and motion portrayal.

    I believe the frame rates used for IMAX are 24, 30, 48 & 60 and can include stereo imaging. I am not sure if any of the public motion simulator rides that use 70mm and IMAX technology use higher frame rates than 60fps.
     
  5. cosaw

    cosaw
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    92Hz optimal rate for high res area of retina. That will explain then why the programme I watched about IMAX (some eons ago) said that as you approached these higher frequencies/frame rates the effect was to key in with the brain in some sort of hypnotic fashion. The eyes being an extension of the brain - this would seem reasonable.

    During a degree in computer science and a particular module on human computer interaction we learnt about the maximum rate of data input processable by the human brain through any one of the senses (or all) at any one time. Kilo Bytes per second, that sort of thing analogous to how a computer might go about the process of sensing. There was obviously a limit (I can't remember the figure) the adavantage was that it could, theoretically, be directed to any one of the senses at any one time in almost isolation. This might fit in with what you say about the 92Hz, (92 frames x theoretical amount of data per frame) = no of KB/s processable or maybe this figure would just be the limit on the eye as a device/piece of hardware? I can't remember.

    Anyway I've yet to see one of these IMAX presentations, I'll have to get some research done!
     
  6. digitalsafari

    digitalsafari
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    Film changed from 18fps to 24fps with the introduction of sound.

    24fps was chosen as the lowest framerate that worked for lip sync.

    IMAX (15perf 65mm) is normally 24fps, there are some special 48fps installations.

    Showscan is 5perf 65mm at 60fps and is format that looks REAL.
     
  7. cosaw

    cosaw
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    I see. Forgive my ignorance but what do you mean by "perf"

    eg IMAX (15perf 65mm)

    Showscan, I've not heard of that. I'll have to look into it! :lesson:
     
  8. They

    They
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    Number of perforations per film frame, i.e the sprocket holes along the sides of the film.
     
  9. cosaw

    cosaw
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    Ah I see. Supposedly this is what has a bearing on the frame rate? - If the projector has to run through a set amount of perforations per second then 15 perf would be a lower frame rate than 5 perf. But could that work as you'd need different style sprockets for different perfs? I suppose you could have a sproket with teeth that plunge in where no perfs are on the film!
     
  10. Mr.D

    Mr.D
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    The perf count is a reference to the height ( or width in Imax and visatvision as they run horizontally) of the film frame. Its an indication of frame size rather than rate.

    You can add motionblur to existing footage by employing optical flow (motion analysis) techniques and rerendering the footage with a larger shutter angle simulation. It works surprisingly well. Most commonly used when speeding up footage to prevent the movement from strobing.
     
  11. cosaw

    cosaw
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    That would be somthing similar to the motion blur they emulate in the more advanced video games these days to get smoother video!
     
  12. Mr.D

    Mr.D
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    I'm pretty sure they just render the CG with motionblur in the case of computer games. What I'm talking about is analysing already shot footage and effectively tracking the motion in the plate. This generates vectors for specific areas of the plate ( someone moving their arm for example and if you were to say speed it up by a factor of two rather than removing every other frame it will generate new frames taking into account the retiming of the action. ie it happens twice as quick and gets more motionblur. Its effectively a sophisticated warp.
     
  13. cosaw

    cosaw
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    Yes I uderstand what your saying with regard to film. And your correct that CG in video games would be rendered with motionblur. i.e. the CG in pre-rendered cutscenes (and the like) is just that - pre-rendered. I should have made the use of the word video a bit clearer! :blush: No what I mean is there is evidence of a technique very similar to that which you described for film in video games as you play them in real time. i.e. there is some real time motionblur rendering applied: subject to the movement of the character which you control and the environment with which you interact. Example: you move your characters gaze left and motion blur is applied to make the scenery smoothly scroll at 25fps on your telly! You'll see this on some of the newer consoles like X-box and GameCube. One classic example is a game called Pikmin on the GameCube. It employs this method at 30fps (that would be the american version NTSC). You can move multiple characters fairly rapidly round the screen at once (collectively) and subtle motionblur is evident in smoothing their motion out.
     

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