shep@datacube.UUCP (09/20/85)
Solarization and Posterization. We here these terms and most of us familiar with them picture the effect mentally: An image with some type of luminance or chromanance quantization and/or mapping. But what do they REALLY mean and what are their origins? I believe solarization came from the photographic effect of exposing emulsion to light sometime during development. I have no idea where the term posterization came from. Can anyone answer these questions? I worked on a popular television special effects box that has what I called solarization and posterization. The solarization name was given to all the lookup table effects performed on only the y (luminance) signal. As we quantized luminance to progressively fewer states, we said the image was more "solarized". Similarly, the "posterization" term was applied to quantization of the chromanance difference components. I recently received mail from a friend asking me about the subject... >Subject: solaraposterization >Just what are solarization and posterization? You are the world expert on >these matters now that you have become an ancient one. ... >Is a table look-up on luma and chroma independently enough or do >you have to provide LSD-like cross leakage between luma and chroma? and I remembered that I had found references that spoke of quantization in polar (hue/sat/val) space. Does anyone know of these references or algorithms? Shep Siegel UUCP: ihnp4!datacube!shep Datacube Inc.; 4 Dearborn Rd.; Peabody, Ma. 01960; 617-535-6644
david@ukma.UUCP (David Herron, NPR Lover) (09/22/85)
In article <6700033@datacube.UUCP> shep@datacube.UUCP writes: >I believe solarization came from the photographic effect of exposing >emulsion to light sometime during development. >I have no idea where the term posterization came from. Yes. Solarization is a photographic effect created while developing a print. You simply turn on the lights halfway through the development. It turns the light areas in the picture dark, *except* for a band around any dark objects. Obviously, there's some undeveloped silver left in these light areas, which get exposed letting them turn dark. Now that I think about it, there'd be some interesting effects by doing the same thing with negatives. I'm unsure why the band around dark objects doesn't get exposed though. I recall "posterization" vaguely in the same context. Like, maybe it is a more technical/specific term for the same thing, y'know, one of those things where everybody calls it one thing but it's really something else. Actually, I remember somebody telling me that the thing I called solarization was really something else (posterization? might be where the "post" prefix came from, because it's a form of "post-processing"). And, some other technique was *really* solarization. -- --- David Herron --- ARPA-> ukma!david@ANL-MCS.ARPA --- UUCP-> {ucbvax,unmvax,boulder,oddjob}!anlams!ukma!david --- {ihnp4,decvax,ucbvax}!cbosgd!ukma!david Hackin's in me blood. My mother was known as Miss Hacker before she married!
alan@sun.uucp (Alan Marr) (09/24/85)
Photographically speaking, there is a third term, "Sabattier
Effect" (sah-BAH-tchi-a, French), that is the correct term for
what is often called solarization.
Posterization is where the myriad tones (color or B&W) present
in a photo are reduced to a small number of tones, typically
only three or four because the photographic process is time
consuming since you must make internegatives at selected
exposure levels on high contrast film and register them
carefully when printing. High contrast images (two tones only)
are a subset of posterized images. Posterization is one of the
simplest image processing techniques for computers (often
called bit-slicing) and in computer graphics similar effects
are achieved with colormap manipulations.
Solarization is the process whereby film is exposed in the
camera to such massive overexposure that partial reversal of the
image occurs. The exposure is about 1000 times normal. It may
not work with modern films.
The sabattier effect is a darkroom manipulation that can use
paper or film. When I did it I exposed a print normally, and
then after about 15 secs development (out of 90 secs normal) I
flashed it with about a second of room lights. Electronic flash
can be used and is easier to calibrate. The print develops
very quickly thereafter and in 15 seconds I very fluidly whip it
into a fresh tray of stop bath to arrest the development quickly
without getting uneven development streaks.
Partial reversal takes place, but so do Mackie lines between
highly contrasting tonal regions. This is because the
development that occurs in dark areas generates a lot of bromide
byproduct that restrains the development. Agitation in normal
processing distributes the bromide from the print into the
solution so with the Sabattier effect it is important that the
developer be still at the moment of flash exposure and not be
agitated afterwards. The Mackie lines are quite beautiful,
especially where there is organized fine detail as in hair.
Stunning effects can be obtained with film by doing sequences of
Sabattier effects, building up the transformations.
I found that the best papers for Sabattier effects were high
contrast papers, especially Agfa Brovira #6. I had very
interesting results with a box of years-old Kodabromide #5 which
would yield orange and purple colors in the re-exposure areas.
These were sensitive to light so I didn't show them much (I
fixed them minimally). The colors were undoubtably due to
finely divided silver that was coming out, very similar to the
"print-out" papers that used to be used for contact printing.
---
{ucbvax,decwrl}!sun!alan
"Extraordinary how potent cheap music is." Noel Cowardkurtk@tektronix.UUCP (Kurt Krueger) (09/24/85)
Solarization is, as has been stated, the exposure of a partially developed print or negative to light. The reason that it works is that the silver's sensitivity to light changes depending on how much density (i.e. how black it is) has been built up. I don't know why. It has also been my experiance that is works better with negatives than with prints. Posterization is the quantization of grey scale, color etc. A posterized b&w print has a finite number of grey tones, rather than a continuous tone. I've been told that posters (you know, those large printed pictures) can be made this way. It is a trade off between resolution and grey scale. You can either get your grey scale by doing a half tone representation (and create a grainey picture) or by quantizing the grey scale (no grain, sharp edges, but limited grey scale).