[opendtv] Re: Math of oversampling - a REALITY CHECK

  • From: Tom Barry <trbarry@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Mon, 02 May 2005 09:03:28 -0400

Craig -

You sort of touched on it but let's also not forget the optimal 
difference between the delivery and display resolutions.

Most effectively you can hide the pixel/raster structure by having 
a higher resolution display even though both the horizontal and 
vertical display rez are maybe each 50% higher than the actual 
images being broadcast.  The results end up being similar to the 
various upscaled images of Jeroen's.  Extra softness becomes 
noticeable in extreme conditions but not, say, when viewing a 720p 
image on a 1080p display at a viewing distance designed for the 
display instead of the image.

All this just adds a bit more wiggle room.

- Tom

Craig Birkmaier wrote:

> I mistakenly sent this message only to jeroen yesterday. He has since 
> covered some of wthe issue I write about here.
> Craig
> At 2:24 PM +0200 4/29/05, Jeroen Stessen wrote:
>>See below ! (Was not posted.)
>>> To be honest, it took me a few moments to see the
>>> difference between the 576P and 720P images.
>>You will now !!
>>Be careful that you don't see only what you want to see.
>>It depends on the image content, and on the display, but
>>maybe also on any prejudice that you might have...
>>BTW, this 20" CRT monitor at 1600x1200 is rasor sharp, better
>>than any LCD that I might find in this place. Don't worry.
>>- Tom
> Let's take the prejudice out of it and consider the realities of 
> delivering high quality images to millions of consumers who will 
> watch their expensive new TVs in a manner that is not the same as 
> that we are using to evaluate these images.
> To this point we have been looking at only a single important factor. 
> Jeroen's latest images do a good job of demonstrating the information 
> carrying capacity of rasters at various spatial resolutions. As THEY 
> SHOULD, we see more detail as the density of the raster is increased. 
> But it is important to remember that we are NOT using real world 
> analysis techniques.
> We are looking at a single frame of information for extended periods 
> of time on a high resolution display viewed at a close distance, 
> where we can change the ability to see small details simply by moving 
> our heads in and out over a range of a 15" to 30" viewing distance. 
> Some of us are even zooming into the image to see the underlying 
> sampling structure.
> Here is a simple test that tells the real story. Open up the 1080P 
> file and place it in a window on the top half of your high resolution 
> display - make certain that it is displaying the samples in a 
> one-to-one relationship with the display samples (100% or actual size 
> depending on the viewer you are using). Now open one of the lower 
> resolution files, I used the 576P file, and place it below the 1080P 
> file so that the same region of the image is visible in both windows.
> What do you see. The answer depends on the distance your eyes ar from 
> the display.
> At a viewing distance of 15 inches the 1080P image looks 
> significantly sharper. At a distance of 30 inches it is still 
> sharper, but the difference is much smaller. At three feet, most of 
> us will not be able to tell the difference.
> What we are talking about here is display scalability. There are 
> three critical factors at work here:
> 1. The actual resolution of the display - with modern "lithographed' 
> display technologies the actual number of pixels is important, but 
> there may be other factors that influence the delivered resolution, 
> including:
>    a. any effects from optical filters (the lenses and screens used 
> for projection systems;
>    b. Internal image processing circuitry - the bandpass of the 
> amplifiers, and any
>        inter-format conversions that may be taking place;
>    c. Diaplay gamma and related issues of how the display generates 
> and maps pixel
>       values to the screen;
>    c. User adjustments to contrast, brightness and sharpness that affect the 
> way
>        samples are mapped to the display.
> 2. Screen size - How big are the individual pixels? For a panel 
> display that has a 1280 x 720 raster, the pixels are much larger on a 
> 40" screen than a 20 inch screen. Generally speaking larger screens 
> need more samples, but this is directly related to the third 
> criteria, the viewing distance. Those big stadium displays often have 
> pixels that are measured in inches rather than millimeters.
> 3. Viewing distance - how far is the viewer from the display. I noted 
> above that it is easy to cut the viewing distance to a computer 
> display by half, simply by moving one's head in and out. At the 
> Lechner distance (~9 feet) moving one's head has little if any impact 
> on the ability to resolve more detail. The process of choosing a 
> display with the appropriate level of resolution BEGINS by 
> determining the designed viewing distance for the application.
> What all this boils down to is that one can accurately determine the 
> resolution requirements for ANY display application if we know the 
> screen size and viewing distance. What we are seeking is to have the 
> display match, or slightly exceed the visual acuity of the viewer in 
> order to deliver a sharp image without the perception of artifacts, 
> including the ability to see individual lines and/or samples in the 
> displayed image.
> We also need to factor in the reality that for entertainment viewing, 
> the designed viewing distance, measured in multiples of screen 
> height, decreases as the display subtends a larger portion of our 
> field of view. On average we tend to sit 7-10 picture heights from 
> small "low resolutions displays such as NTSC and PAL televisions. For 
> 720P displays between 40 and 60 inches diagonal we tend to sit 4-6 
> picture heights. For 1080P displays we may sit 3-4 picture heights 
> when the screen size exceed 100 inch diagonal, but will typically sit 
> further away from smaller screens.
> This is the crux of the question originally posed by Bob Miller. How 
> much resolution do we need to deliver through the emission system to 
> create the perception of a sharp picture on the average range of 
> screen sizes that will be used to view the content?
> The reality is that 576P is adequate for most of the screens that are 
> being purchased in Europe and the U.S. today. You need a screen 
> larger than 40" diagonal before 720P delivers a perceptible 
> advantage. And you will see NO DIFFERENCE between 720P and 1080P 
> until the screen size is larger than 70 inch diagonal at typical 
> viewing distances.
> But there is another critical factor that must be considered as well. 
> What is the quality of the samples that are actually delivered 
> through the emission system? have they been impaired by the 
> compression system, as we saw in the images that Jeroen sent to us?
> At NAB there were three companies showing very sophisticated image 
> processing/de-interlacing chips that are capable of outputting 
> 1080@60P images to high resolution displays. These processors do a 
> good job with interlaced SDTV source. They do a fantastic job with 
> progressively scanned 720P source. This leaves us to consider where 
> we need to optimize the system.
> For the vast majority of consumers, 1080P is overkill. Even 720P is 
> beyond the requirements for the displays that will dominate the mass 
> markets. Given the same channel bandwidth (say 10-12 Mbps) and H.264 
> compression, which will deliver the highest quality samples to the 
> image processor inside all of those new consumer displays:
> 1. 1920 x 1080 @ 60i
> 2. 1280 x 720 @ 60P
> The answer is obvious. We need to focus on using digital compression 
> tools properly to deliver high quality samples, and let powerful 
> image processing chips deal with the up conversions for really big 
> screens.
> Regards
> Craig
> P.S. It is also important to note that the "Philips" image" that 
> Jeroen used in this test was not a frame from a sampled video image. 
> It was composed in a computer - probably at a resolution 
> SIGNIFICANTLY higher than 1920 x 1080, then output to film for 
> printing purposes.
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