PC 3D formats (was: monitors & video cards)

[P3D Andrew Woods <andrew@xxxxxxxxxxxxxxxxxxxxx>]

Larry Berlin wrote:
> I have read of several systems which use an outside *black box*
> connected to the *video out* on the computer and the *video in* of the
> monitor and which provides the drive signal to the glasses. I'm not sure
> which systems actually use this method. I have no idea how current that
> information may be. I would like to hear from Jonathon Gross about how the
> Neotek system does it's thing. It may be this kind of external device....?
> Is it interlaced, or page flipped?

Ahhh, now I see where you are coming from.  

There are three main techniques of achieving field-sequential (image 
sequential, etc) with PC systems.  Here's a quick tutorial for those
of you who are still following this discussion:

(1) interlacing is probably the easiest and is what is used in 3D video
    systems (i.e. the VHS video tapes available from 3DTV, VREX etc).

    Interlacing means that first the odd lines of the image are drawn
    on the screen (the odd field) and then the even lines of the image
    are drawn on the screen (the even field).  If the image is made
    up of odd lines being the right perspective image and the even lines
    being from the left perspective image, the interlaced nature of the
    display will mean that first a right image will be displayed and
    then the left image will be displayed (left and right images will
    be alternately displayed - time or field-sequentially).
    When viewed through LC shutter glasses synchronised with the field
    rate, each eye will see a different image.

    Problems: * Some video cards won't switch to interlace mode. 
              * You only get half vertical resolution.

    Advantages: * relatively simple software-wise to implement.
                * Hardware can be relatively simple
                * very easy to display stereo images by using 3D interlaced
                  images (such as appropriately generated gifs)

(2) page flipping is primarily used with non-interlaced (or progressive scan)
    displays (which display all the lines of the frame in one go, not half
    half as in interlacing).

    Page flipping works by changing the image in video memory (of the video 
    card) every time a new image is to be displayed.  Normally what would 
    happen is that a device driver will watch the progress of the video 
    card generating the video signal.  When it sees that a frame is 
    finished being drawn it will change the image in video memory to the
    other stereo-pair image, so that you will get alternate frames being
    left and right images.

    Problems: * You need a resident device driver to do the page flipping
              * Your video card memory will need to accommodate two images
                (the left and right) to allow the page flipping to occur.
    
    Advantages: * Hardware can be relatively simple
                * full vertical resolution (if your video card has enough
                  memory).
                
(3) Sync Doubling (or sub-fields) works by dividing your existing screen
    into two halves (the top half and the bottom half).  Your software
    puts the left image of a stereo-pair into the top half of the display
    and the right image of a stereo-pair into the bottom half of the display.

    After your video card generated its normal video signal, an external 
    black box adds an extra vertical sync pulse to the video signal (at
    the vertical mid-point of the screen).  The monitor will now see the
    video signal differently from before and will display the first half
    of the video signal (which used to be the top half of your screen) 
    as a full screen image and then the second half of the video signal
    (which used to be the bottom half of your screen) as a full screen
    image.  So again, there will be left and right images being displayed
    alternately (time sequentially).  LC shutter glasses can be synchronised
    with these images to allow you to see a 3D image.

    Just to clarify Larry's question about interlacing: The sync-doubled
    output of the black box is not interlaced in the true sense of
    the word.

    Problems: * Half vertical resolution
              * Hardware is relatively complicated
              * compatability problems with existing graphical user
                interfaces (like windows).
              * images need to be squashed vertically before writing
                to video memory.

    Advantages: * software is relatively simple (apart from problems 
                  with Windows).
                * you don't need a high speed video card to display
                  flicker-free stereoscopic images.

There are some beautiful animated GIFs at the Catalyst web site which
beautifully illustrate the operation of these three systems.
If you are having problems with my descriptions, these images will
illustrate techniques to you very quickly.

  interlacing:   http://www.skypoint.com/~catalyst/interlace.html
  page flipping: http://www.skypoint.com/~catalyst/pageflip.html
  sync double:   http://www.skypoint.com/~catalyst/syncdouble.html#syncdouble

   NB: Some of their descriptions are a bit wrong in places and exhibit a 
       bias towards the sync double technique which I guess is understandable
       because their system uses the sync-double technique.

SYSTEMS AND THE TECHNIQUES THEY USE:
===================================

 To my knowledge the following systems can use either the interlace or page
   flipping methods:  3D-MAX (Kasan Electronics Korea), VR Surfer (VREX) &
   Simuleyes VR (StereoGraphics)
 The systems from 3DTV corp use the page flipping method (but there is
   nothing stopping them being used with the interlacing method too)
 Neotek uses the sync-double technique.

FLICKER:
=======

I've avoided the topic of flicker so far because it is a bit more complicated
to explain than it first may seem.

The following table illustrates what happens with the various 3D display
methods with regards to image refresh rates:  (borrowed in part from 
Catalyst's web site)

  +-------------+---------------+----------+-----------------+----------+
  |  3D Method  |Vertical Output|  Fields  |Effective Refresh| Vertical |
  |             |   Frequency   |per Second|Rate for a Stereo|Resolution|
  |             | of video card |          |   Field Pair    |          |
  +-------------+---------------+----------+-----------------+----------+
  |Interlaced   |     60Hz      |   60Hz   |      30Hz       |   half   |
  +-------------+---------------+----------+-----------------+----------+
  |Page Flipping|     60Hz      |   60Hz   |      30Hz       |   full   |
  +-------------+---------------+----------+-----------------+----------+
  |Sync-Doubled |     60Hz      |   120Hz  |      60Hz       |   half   |
  +-------------+---------------+----------+-----------------+----------+

Now the ifs and buts:
  * flicker sensitivity varies with image brightness but generally
    from about 40Hz (refresh rate of stereo field pair) the perception
    of flicker starts to drop to an acceptable level.
    The optimum is 50 to 60Hz (stereo field pair refresh rate).
  * flicker can be overcome with both the interlaced and page flipping  
    methods by increasing the refresh rate which the video card 
    generates.  BUT this depends upon the capabilities of the video 
    card (and the monitor)
  * for the same resolution and output frequency, non-interlaced modes
    will require twice the pixel rate of that needed by interlaced modes.
    Therefore you will need a higher quality card to achieve 100Hz
    stereoscopic display with an non-interlaced mode than you will for 
    interlaced.
  * The best image quality will be achieved with page flipping
    at non-interlaced 120Hz BUT you will need an impressive card to
    get those rates - especially if you also want high 
    resolution (800x600 + ) too.

Now, is there anything I've missed out?  

Oh, yes...   (C) 1997 Andrew Woods.    ;-)


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