Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!sm.unisys.com!mlogic!bennett From: bennett@mlogic.UUCP (Bennett Leeds) Newsgroups: comp.graphics Subject: Re: ShowScan quality image bit rate Message-ID: <146@mlogic.UUCP> Date: 6 Feb 89 19:27:57 GMT References: <18071@glacier.STANFORD.EDU> <6652@polya.Stanford.EDU> Lines: 74 In article <6652@polya.Stanford.EDU>, djones@polya.Stanford.EDU (David Jones) writes: > In article <18071@glacier.STANFORD.EDU> jbn@glacier.UUCP (John B. Nagle) writes: > > Consider 6000 x 8000 pixels x 60frames/sec x 24 bits/pixel; > >the data rate is about 70 gigabits/sec, ... > > John Nagle > Someone else commented that at some point you'd exceed the limits > of human perception. > > Since a great deal is in fact known about how visual information is > encoded (at early stages at least), images stored as 24 bits/pixel > can be compressed by quite a bit with no perceptible loss > in image quality (provided of course that you know precisely how > the image will be viewed). It looks like there's room to better Mr Nagle's > pixel representation by 3 orders of magnitude. > > David G. Jones > djones@polya.stanford.edu Resolution should encompass more than just the number of pixels per screen dimension - it should include the range of colors (shades) displayed at each pixel. Current TV technology has at least as long a way to go in this regard (if not longer) than it does in dimensional resolution. For instance, turn your best computer monitor off. What color is the screen? Its a gray, hopefully a dark gray. This is the blackest black your monitor can produce. Now develop an unexposed roll of film, and play it through a projector in a conventional theatre. I think you'll find this black quite a bit darker. You can apply similar tests at the top end, comparing the whites. Again, the film white will be much brighter than the computer monitor's whitest white. This stuff reminds me of the debates that appear in Rec.audio about number of samples and number of bits per sample. The point is that the "dynamic range" of film is much greater than TV. And of course, real life's dynamic range is greater than film's. Our eyes can't even handle it all at once - they have to adjust (ever notice how much brighter car headlights are at night than they are during the day?). Back to the original discussion, if 24 bits per pixel is too much information for the limited dynamic range of TV, it may not be enough for film because there are values that film can reproduce that TV technology cannot. Does anybody have an idea of what the values need to be? Of course, this will vary by film emulsion, projector bulb and type, and room environment, but is there a general rule that anybody has developed by experience? For TV work, 15 bits is usually (but not always) enough if all you are doing is displaying the raw image, but you need more than that if you want to alter the image via gamma correction, etc. Finally, with a screen as large as ShowScan's (and IMAX's as well), the viewer does not take in the whole screen at once - his eye roams over it, concentrating on certain portions of it. It is these *portions* of the screen that need to be displayed at a higher resolution. If it is a normal, non-computer generated scene, areas of the image may be out of focus because they are too close or too far from the camera/lens. These areas can get by with a lower image resolution. Of course, we then need a system that can display portions of an image at differring resolutions. Perhaps if we shot movies using a system like in Nikon's N8008 35mm still camera, which uses a computer-controlled area-weighted system that figures out where your main subject is based on distance and lighting information and sets the exposure and focus (it's an auto-focus camera) automatically, we could tie that into our computer system to figure out which portions of the image we should proportion our resolution to (since we know where we focused on). Bennett Leeds -- Bennett Leeds | Media Logic, Inc., Santa Monica, Ca | These opinions are not ARPA: celia!mlogic!bennett@tis.llnl.gov | my employer's. UUCP: ...sdcrdcf!mlogic!bennett |