Re: Why interlaced HDTV?

In article <f9dpg1dkfn59hsbsp1jcvjolulaiosvtvp@xxxxxxx>, JC <nospam@xxxxxxxxxxx> writes 

Imagine taking two separate photographs at half resolution, a short
time apart of something that's moving and then trying to merge them to
get double resolution. The moving object will be in a different
position on the second field to the first. Now imagine rapidly
flicking between these two images or showing them merged, before
moving on to the next two.

With a progressive system you just reshow the same FRAME multiple
times or in the case of a memory type display such as LCD, just change
the pixels as required.

Have you ever actually tried that and *looked* at the result? I doubt it, because if you did you would notice that it produces just the same motion deficiency that you are being so critical of interlace to progressive conversion of.

If you show that same progressive frame several times then motion produces the perception of several images of the object in the scene that is moving. For example, 50Hz-720p on a 100Hz display with the simple frame repetition that you describe, will produce double images of moving objects. This is well known and was a major limitation of early and current cheap 100Hz sets (although you, no doubt, attribute it to the interlaced source) which is why better 100Hz sets use motion compensation algorithms to fix it.

If you want to see why this happens, try drawing a graph of screen position along the direction of motion versus time. An object moving across the field of view at a constant rate will be represented by a straight line on this plot. A 50Hz camera sees the position of the object every 20mS on the time axis - so mark dots along the line representing the position that the camera sees. Now redraw those dots in the same positions some time later, representing the object position on the display. If your display is 50Hz then each camera dot produces a corresponding display dot - and your brain joins the dots up to create what appears to be continuous motion again.

If you display repeated frames at 100Hz, then each dot from the camera is displayed twice in the same position but 10mS apart on the time axis. Now natural real life objects don't exhibit regular stop-go motion at 10mS intervals, they move and your eye tracks the motion to keep the object of interest on the fovea. Your eye muscles simply can't keep up with that stop-go motion - and your brain tells it that it doesn't need to. You naturally misinterpret this display as *two* objects moving smoothly across the display, one behind the other by a distance corresponding to the 10mS time gap between display repeats - the same effect as if you had an interlace image reconstructed in a frame store and displayed progressively.

You can use the same approach to understand that motion blur on LCDs is fundamental to the continuous display, not just a consequence of LCD time constant. Just replicate each camera sample with a line extending the full 20mS on the display. Now look at how your eye interprets the continuous motion - as your eye tracks the motion of the object across the screen all that resolution that you are so keen to capture at 50Hz on the progressive camera is lost in a blur corresponding to the distance the object moves in 20mS! That is why "memory type displays" as you call them are just garbage for reproduction of motion - they trade motion for resolution all the time.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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