[opendtv] Re: CMOS sensors and rolling shutters

  • From: Jeroen Stessen <jeroen.stessen@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 1 Feb 2008 11:31:05 +0100


Tom Barry: 
> So it would seem that any frame-at-a-time capture designed for a fixed 
> pixel display would not look right on a scanning CRT, and vice versa. 
> Why isn't this more of a problem?

Because, and this is important, also most matrix displays are 
scanned from top to bottom. It takes almost a frame period to build 
up a picture on an LCD. The only exception are displays that use 
pulse width modulation by means of weighted sub-fields to create a 
grey scale. The two types that I know of are plasma and DLP. 
They build up the sub-fields at a much higher frame rate, so that 
for the eye it seems as if it is generated instantaneously. 
But they come with their own class of motion artefacts, probably 
bigger than the one we are discussing here... 

So, IF the camera and the display are scanned synchronously, then 
there should not be any motion artefacts. But the artefacts will 
appear if you display the pictures in an unintended way, like 
freezing an individual frame. Then it appears to the eye as not 
being scanned at all, and you can see (only) the camera artefacts. 
Also zooming a picture (showing it in a small window on your 
monitor) disrupts the synchronisation between source and display, 
not to mention the fact that the refresh rate of the PC monitor 
is usually not synchronized to anything external. 

This is btw also how interlace gets a bad name, because people 
are capturing stills from a moving program, instead of showing 
the fields at their intended refresh rate. You don't see quite as 
many artefacts if you just view 1080i on a 1080i CRT (or ALIS). 

BTW^2, there are 3 scanning speeds to consider: the camera, the 
display, and the eye tracking. Complicated stuff.... 

-- Jeroen

| From:    Jeroen H. Stessen                |
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