Path: utzoo!attcan!utgpu!watserv1!watcgl!imax!dave From: dave@imax.com (Dave Martindale) Newsgroups: comp.graphics Subject: Re: COLOR SCANNING. Message-ID: <1989Dec12.170641.16020@imax.com> Date: 12 Dec 89 17:06:41 GMT References: <13428@s.ms.uky.edu> <3184@com50.C2S.MN.ORG> Reply-To: dave@imax.com (Dave Martindale) Organization: Imax Systems Corporation, Oakville Ontario Lines: 79 In article <3184@com50.C2S.MN.ORG> chris@com2serv.c2s.mn.org (Chris Johnson) writes: > >Maybe this goes without saying in this newsgroup, but 24 bits of color >value for RGB might be called near photographic quality, as far as color >rendition goes, but 5 bits (32 levels) probably is not very close. Well, actually, even 8 bits isn't enough to handle the brightness range that film does (particularly negative film) without getting "banding" effects due to quantization error. 12 bits per component is about right. (Actually, 8 bits would be OK if there was a linear-to-log converter in front of the A/D, but I haven't seen a commercial digitizer that does this. I wonder why?) >And that's only color rendition. Even a 400dpi image is not remotely >close to photographic quality in resolution. I don't have any exact >figures at hand, but I would suspect something like Kodachrome could >do well over 10,000dpi. Of course, an ISO 1600 film might be only >600dpi! :-) At high contrast, most colour films can resolve 100 or 125 line pairs (at 2 pixels/line pair) per mm. 100 lp/mm is about 5000 pixels/inch. There are slow B&W films (Tech Pan) that have 2 or 3 times this resolution when exposed and developed appropriately. To be fair, this is the upper resolution limit of film, where it can just barely resolve something that had a 1000:1 contrast ratio in the original. Most film will resolve only about 30 lp/mm at low contrast. So, if an electronic image is to look "as sharp" as a film one, the number of pixels you need depends on whether the image has sharp, high-contrast edges or not. Another factor is that, with movies, a series of frames projected at 24 FPS looks sharper than any single frame from the series does - the noise due to film grain averages out. Projecting at 60 FPS (e.g. Showscan) gets a further gain this way. Electronic images won't gain in the same way. When comparing film to electronic imagery, you also have to take into account the magnification you will subject the film image to. For example, a 35mm slide is 36mm wide. At 30 lp/mm, that's 2160 pixels wide. If you enlarge that to 7.2 inches wide on a print, it's only 300 dpi. (Well, except that the high-contrast features have an effective resolution of 1000 dpi). But if 35mm doesn't give you a sharp enough image at the enlargement you want, you just use a bigger piece of film. That's what's so nice about film - its flexibility. If you want more resolution, you just use more film area and change the lens focal length. With electronic imagery, doubling the linear resolution (and quadrupling the pixel count) is a horrendous problem. For some real-world comparisons: A reasonable figure for the resolution of film when projected on screen in a theatre is perhaps 36 lp/mm. This is what the audience can actually see, and includes losses in photography, printing, and projection. Then normal 35mm 4 perf film, as seen in your local theatre, gives an image that contains about 1447 x 1094 "resolvable pixels" (assuming that the resolution at the edge of the screen is the same as the centre - it isn't). 16mm film gives about 690 x 505 pixels. 35mm 8-perf, the format used by still cameras, gives 2600 x 1730 pixels 70mm 5 perf (theatrical "70 mm" format and Showscan) is capable of 3500 x 1590 "resolvable pixels". And IMAX (70mm 15 perf) gives 5010 x 3490 pixels. NTSC video is capable of at most 370 x 350 (remember, this is what the viewer can resolve, not what your frame buffer stores for NTSC output). (And NTSC's colour resolution is far worse than its luminance resolution, by a factor of 3 to 9.) HDTV resolution has been measured at about 1030 x 1030 "resolvable pixels". In other words, HDTV has finally surpassed 16mm film in quality, but does not equal even moderate-quality 35mm. For a flatbed scanner, you would have to put a resolution test target on it and see what the finest pattern it can actually resolve is. Although a scanner may be 300 DPI, it is not likely to actually resolve 150 lp/inch, the theoretical limit, for a number of reasons. My guess would be that you might actually see 100 lp/inch, for about 2000 "resolvable pixels" across a 10-inch image width. So a 300 dpi 8x10 inch flatbed scanner might have better "information capacity" than 35mm movie film, but less than a 35mm still-camera slide.