At 8:42 PM -0500 1/13/05, Tom Barry wrote: >Darn it Craig, that is just silly. > >You keep going on about 1970's studies that claimed that, compared to >the expectations of the day, 22 cycles per degree of resolution seemed >"very sharp". And yet your same article said people might be able to >see up to 40. > >"Very sharp" is not the best there is. The best occurs when there are >no visible artifacts (including obvious image softness) even when you >sit close enough the screen subtends at least 30 degrees of your visual >field. Sorry Tom. It may sound silly, but it is factual. Perhaps I need to do a better job of explaining... First, some of these studies are MUCH older than the '70s. Scientists have been studying the human visual system for centuries and the notion of measuring visual acuity in terms of cycles per degree probably goes back to the 19th century. What is important here is to understand the relationship between screen resolution and human perception. When NTSC was designed, it was well understood what it takes to present an image to the human observer that is perceived as being sharp. Human perception is a very complex subject with multiple parameters that must be considered. Staring at a high resolution still image versus a video stream is a good case in point. With a still image you can focus on a particular area of the image - assuming it is not vibrating or moving, your foveal vision will be able to stay focused on fine details long enough for a high resolution view to be constructed in your brain. This takes several hundred milliseconds. A good way to look at this from an information theory perspective is that you need only collect that highly detailed view once. Extending this to video compression, since there is no temporal element, you can send the frame once and you are done. With a video stream we add a temporal dimension, and this changes everything. You can focus on fine detail in one area of the image, but the samples are likely to keep changing, literally in front of your eyes. If the display size and viewing distance are well matched to the central foveal image receptors, as was the case for NTSC, the human visual system can capture the entire image without having to dart around to capture high resolution views. It is not a coincidence that the <11 degree field of view for NTSC is very close to the area that can be captured by foveal receptors. When we extend that field of view we now start stimulating extra foveal receptors which direct eye movement. A big part of the induction effect for HDTV is related to covering that wide field of view so that you are forced to use directed eye movements. Unfortunatley induction typically does not occur if the viewer is very close to the display - this may have something to do with the fact the the eye is forced to focus on the near field. NHK did most of its HDTV research in the mid and late '70s. In most of the papers I have read, they used film based images to determine the correct parameters to stimulate the perception of a high resolution image AND the induction effect. One of the major reasons for this was to eliminate the effect of a scanning raster on the studies. After determining the resolution requirements they then calculated the number of lines needed to provide the desired level of detail, WITHOUT the perception of the scanning raster. It was determined that a progressively scanned image with 700 lines was sufficient. Unfortunately, they were designing this system for satellite distribution using MUSE, and there was NOT enough bandwidth to deliver something akin to 720P. So they resorted to the same trick used to deal with the bandwidth limits for NTSC - interlace. You seem disturbed by the notion that some people can resolve fine details at resolutions as high as 40 cycles per degree. But there are some important caveats here. For most forms of natural imagery, the average viewer can resolve sin the range of 22 to 30 CPD. Above this average you need VERY high contrast for the eye to respond. My favorite example is a blinking red traffic light off in the distance. If it is daylight, you may not be able to resolve the light until you are fairly close - say 1/4 mile; if the sun is right behind the light you will not see the red light at all as it will be overwhelmed by the intensity of the sun. But at twilight, or after dark, you can perceive the light at much greater distances(i.e. your eye is reacting to a stimulus that may be in the range of 30-40 CPD. This is possible because of the high contrast, and the fact that the light itself is the source of that contrast. If the light is blinking, you may even perceive it sooner as the temporal element helps with perception. Mark Schubin likes to talk about another aspect of this issue....the broken pixel. What happens when there is one pixel that is permanently set to some value, say red? If the pixels that surround it are highly contrasting you may see that red pixel; if the contrast level is small, or the hue is close you probably will not see it. One advantage of having samples that are smaller than you can resolve is that a bad sample will not be readily apparent. But for normal conditions, an HD display need only provide enough resolution to satisfy the average viewer in terms of acuity. That is why i typically say 22 CPD or greater. Now what happens when you increase the viewing distance relative to the designed viewing distance? For example, Bert and I sit down to watch his new 26" panel. Bert, to prove a point chooses to sit at three picture heights. I choose to sit at 5 picture heights. If our visual acuity is the same, he is going to be able to resolve more detail. At five picture heights some of the fine details will be too small for me to resolve. This is where the decisions of the content producer come into play. If it is KNOWN that the viewers will all have high resolution displays that cover 30 degrees of the field of view, you can sit on wide shots and let the viewer decide what to focus on. But this is not going to work very well for me, sitting at five picture heights. I will see a very sharp picture with a bunch of little ants running around on the football field. If you watch a football game in NTSC, what do you see? Wide shots are used only to "establish." After that, everything is close-ups because of the limited size and resolution of the NTSC display. This is just one of many reasons why it is difficult, if not impossible to shoot once and distribute to displays with significant differences in size and resolution. This is why HDTV is developing as a complementary service to SDTV. IF the content does not need the extra resolution, as is the case for most motion pictures, sitcoms and episodic shows, you can get away with delivering the same content to displays with different capabilities, but when you are optimizing for and selling resolution you by necessity sub-optimizing for less capable displays. > >You also quote studies where people would try to move back to their >habitual (in screen heights or furniture feet) viewing distance up until >the point where they were presented with theater sized screens. Please >try the same experiment again with people already accustomed to viewing >HDTV displays and tell me how it works out. The studies of preferred viewing distance, these are much more recent. The studies I have been referring to took place in the late '80s and early '90s. I suspect that if they were repeated today with the displays that are now available the results might be somewhat different. The major reason for this is that most of the studies were done with CRT based displays, both direct view and projection. As we all know now, these displays typically do not offer the desired combination of brightness and contrast that we are now seeing with LCD, DLP etc. And they also introduce raster artifacts that impair the ability to resolve fine details. I agree that people may want to sit a bit close to a good HDTV display, but I would not expect a huge change in the results. Similar "studies" are done everyday across America. Walk into any theater, and see how the seats fill up. The front rows are always the last to fill up. People are uncomfortable sitting too close to the screen. So in the end, the correct way to buy an HDTV display is to start with the viewing conditions. That is, what will be the average viewing distance. From there is is easy to determine how large a screen you will need, and at what resolution to deliver what is perceived as a sharp image. And anything that is at 22CPD or greater will look sharp to virtually everyone. I'm not certain that experience watching an HD display is going to change things either. I have seen very few HDTV installations where the primary seating is at 3 picture heights. This is not surprising for several reasons; Room size, furniture placement and screen size. The majority of people are buying BIG screens because that is what HDTV is all about. Now we are beginning to learn how people will use small high resolution screens. I agree that the jury is still out on this one. >You already have posted that the viewing distance you would use >personally for picture quality is closer than your wife would allow >based upon furniture dynamics & traffic patterns. It seems likely you >will adjust but, if not, then next time you will probably buy a larger >TV. But still not 100 inches. A reasonable assessment. By the way, the furniture managed to creep back in while she was sleeping, and we are now talking about replacing the coffee table, which is too big and the cause of some of the traffic flow problems. A larger screen is out of the question because of fireplace. The 50" screen is more than adequate for all of the possible viewing locations in the room. I am very comfotable with the 12 foot primary viewing distance for this screen (about 5 picture heights). If i get a Mac Mini that can drive the full 1280 x 720, it will be intersting to see how well the Mac GUI works at this distance. >Very few have 100 inch TV's and yet many of use DO occasionally see the > WOW factor and reap the benefits of HD. On sets no larger than yours. Go back and look at what I have been writing...again. A 100" screen is overkill for most homes. I have consistently indicated that the HDTV viewing experience begins with screens larger than 40". A screen between 40" and 100" with 1280 x 720 resolution will deliver a sharp image to the viewer at viewing distances equal to or greater than 3 picture heights. This pretty well covers >99% of all installations. Only when the screen size exceeds 100" do you need additional samples to prevent the perception of the raster or a a soft image. This is why I believe 1920 x 1080 is a waste of time for ANY commercial DTV deployment. Regards Craig ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.