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Discussion Starter · #1 ·
I am not sure this is the place to post this but. I have a question about the way that 1080i is interlaced.



It is my understanding that 1080i never reachs full vertical detail on a fast motion image or dynamic image. I know that with a 1080i interlacing image you have two fields of 540x960 that must merge to make 1080x1920. What I have been told is that in order for the two fields to merge, that the image must use vertical shifting for the fields to merge and that vertical shifting can not take place with a fast motion image. So, on those images you are only seeing one field of 540x960. In other worlds the frist field is drawn then stops glowing on the raster then the second field is drawn.



Is that true and how does the motion of the dynamic image make that happen. Does the interlacing scanned format know it can not shift the image on a dynamic image? How does that work?



Any help would be great.



Thanks



-ZAGBY
 

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Discussion Starter · #3 ·
Thanks Tom,




From what I can tell about kell factor the interlacing scanning format goes through peeks and valleys when displaying a image. So, I am thinking with very fast movement there is a drop in resolution to 540x960 and this would be a valley. Where as when the two fields merge you would have the peek of it's resolution or 1080x1920. I know that is not the resolved resolution but that seems to me to be what I see being said in the link.
 

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You are correct that apparent resolution drops during motion, but it would drop to 1920x540 at worst (which is the resolution of one field), not 960x540. You still get the full horizontal resolution.


The kell factor described in the article relates to determining the resolution capability of the display (CRT or whatever), not to interlacing and how it affects apparent resolution.


As far as interlace shifting goes, the raster scan is shifted vertically every field whether or not there is motion. The apparent drop in resolution is due to the interplay between the shifting motion of the scanlines and the motion in the image. There are peaks and valleys in the apparent vertical resolution depending on the vertical component of the motion at any given point in the image and in time. The "valleys" (1920x540) would occur when the vertical motion was an odd integer multiple of one pixel (out of 1080) per field. The "peaks" (1920x1080) would be at even integer multiples of a pixel per field (including 0, which is still or sideways only). When the vertical component of the motion is accelerating or decelerating, you may see a shimmer as the apparent resolution cycles between the peaks and valleys. For fast or jerky vertical motion this may break down to an apparent resolution that stays at 1920x540, but motion blur will likely come into play, making it not matter so much. These things mainly apply to full rate 60Hz video. Interlacing artifacts aren't as apparent with a film source and 3:2 pulldown. They are overshadowed by temporal effects such as judder and apparent multiple images during motion.


- Dave
 
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