[opendtv] Re: State of the Art LCD 45" HDTV

Craig Birkmaier wrote:
 > The main difference with fixed pixel displays is that they are
 > capable of presenting detail that is not acceptable for video or
 > natural images, which require Nyquist filtering of the samples to
 > prevent the perception of aliasing. We can illuminate alternating
 > lines at full intensity - i.e. black white transitions. This ability
 > to present non-Nyquist filtered samples is critical when you want to
 > enhance contrast at the expense of some aliasing, as is the case with
 > the text you are reading now.

It is nice they can present non-Nyquist images.  But I'm worrying that, 
to the extent you can at all see the pixels, they can't really do 
anything else.  And that may interfere a bit with whatever sort of 
mental interpolation our brains do with eye motion tracking.

With a nice fuzzy CRT spot beam if your image moves 1/2 pixel then 
adjacent pixels will tend to blend to grey on a sharp edge, with a 
gradient between them.  But on a fixed pixel display there is more apt 
to be a sudden change of value.  Any filtering can only change the value 
of the entire tiny little square pixel and if that pixel is individually 
perceptible it may cause a visible dithering effect on some motion.

I'm not saying for sure this is a problem but I have been wondering 
about it recently.  Previously of course I've just been touting the idea 
of fixing the (possible) problem using 1080p displays but they still 
seem slow to arrive.  There are yet no xHD3's and Intel has abandoned 
LCos.  :-(

- Tom




> At 7:56 PM -0400 10/27/04, Tom Barry wrote:
> 
>>You must have very good eyes.  I've usually figured that over 4 screen
>>heights viewing distance 720p was fine, reserving 1080i/p for less than
>>that.
> 
> 
> Actually I was just being lazy. I did not want to take the time to 
> calculate the cycles per degree of potential resolution available at 
> the viewing distances I mentioned. And there are other factors that 
> complicate this discussion.
> 
> As I noted, the samples per inch of the display works out to be 49 
> samples per inch. Personal computer displays, which are nominally 
> viewed at a distance of about 30 inches typically have a DPI (same as 
> samples per inch) between 72 and 96. This suggests that the optimal 
> viewing distance is somewhere in-between that needed for a computer 
> display and that needed for a TV display. I guessed that it would be 
> 4-5 feet (48-60 inches), which is clearly much closer than most 
> people would sit to watch TV ( but conceivable for viewing of an 
> information display in a signage application).
> 
> At a more typical TV viewing distance of 7-10 feet this display is 
> overkill - it would not be possible to resolve the available detail. 
> Tom is correct, that 720P is more than adequate for a display of this 
> size when viewed at the "Lechner distance."\
> 
> 
>>But I've been wondering lately if I have to amend that a bit for fixed
>>pixel displays with well defined pixels and no implicit filtering from
>>CRT spot beam size.  The fixed pixel displays may well create some
>>artifacts that were not an issue back when the original viewing and
>>visibility tests were mostly performed.
> 
> 
> An interesting observation, but there is no need to amend your thinking.
> 
> The main difference with fixed pixel displays is that they are 
> capable of presenting detail that is not acceptable for video or 
> natural images, which require Nyquist filtering of the samples to 
> prevent the perception of aliasing. We can illuminate alternating 
> lines at full intensity - i.e. black white transitions. This ability 
> to present non-Nyquist filtered samples is critical when you want to 
> enhance contrast at the expense of some aliasing, as is the case with 
> the text you are reading now.
> 
> But every fixed pixel display can also present properly Nyqist 
> filtered samples, at the expense of perceived resolution and 
> contrast. You actually see more accurate image portrayal than with a 
> CRT because the samples are always in the right physical location and 
> the sample size (as compared to the spot beam size) is uniform 
> everywhere on the display (note that this may not be absolutely true 
> for projection systems because of the potential for lens distortions, 
> key stoning, etc.
> 
> The other obvious difference is that the "trick" that makes interlace 
> work on scanning displays, does not work with "constant illumination 
> displays." The human visual system integrates the scanning spot to 
> create the perception of moving images. By interlacing we sacrifice 
> vertical resolution in order to improve temporal resolution - the 
> human visual system integrates the constantly changing half vertical 
> resolution fields. This works quite well  if we use enough filtering 
> to hide the artifacts of interlacing.
> 
> But with displays that illuminate entire video frames continuously, 
> for most or all of the temporal sampling period, the "trick" doesn't 
> work. We must create the information missing from the interlaced 
> sampling process to create complete frames. This can be done crudely 
> with spatial/temporal filters that hide the artifacts, or we can use 
> complex motion compensated prediction techniques to "predict" the 
> missing samples. The latter will typically produce more detailed 
> images, but suffers from problems with many types of imagery that are 
> very difficult to predict - for example;
> - When new information is revealed between temporal samples (e.g. a 
> rotating globe).
> - Plastic deformations - i.e. reflections off of non-planar surfaces
> - New information entering the edges of the frame
> 
> Some of these issues can be improved with more temporal samples in 
> the prediction buffers. Some require complex calculations akin to 
> those used for the rendering of complex 3D animations.
> 
> Life is SOOOOOOO much easier when we sample images properly in the first 
> place.
> 
> Regards
> Craig
>  
>  
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