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- From: mdsp_ltd@uhura.cc.rochester.edu (Pooch)
- Newsgroups: comp.graphics
- Subject: Re: Question: difference in viewed and printed images
- Message-ID: <1992Sep5.030242.24988@galileo.cc.rochester.edu>
- Date: 5 Sep 92 03:02:42 GMT
- References: <1565@eagle.ukc.ac.uk> <mack.715549997@deci> <6017@m1.cs.man.ac.uk>
- Sender: news@galileo.cc.rochester.edu
- Organization: University of Rochester - Rochester, New York
- Lines: 39
- Nntp-Posting-Host: uhura.cc.rochester.edu
-
- In response to all this talk about printing black and white, grayscale, and
- color images, I'd like to offer some expertese:
-
- The PostScript language uses certain programmable algorythms to halftone
- a source image so that it can be rendered on a particular output device. This
- output device is not necessesarily 1 bit (black and white); PostScript
- can image to any output device, in any bit plane. As in the case of the
- Tektronix Phaser IId or the Phaser III, the halftoning algorythms are adjusted
- by the vendor to match the particular device: the IId can print with 8 bits
- per color, and the Phaser III has built-in error diffusion screening.
-
- Image data sent to PostScript is not pre-adjusted. It is in fact raw data, in
- as many bits per pixel as the driving application desires, limited only by mem-
- ory. PostScript level II adds the ability to specify the source image in any
- device space - RGB, CMY, CMYK, or any CIE space.
-
- Digital halftoning was devoloped to mimic the photographic halftoning process
- used in the offset plate-making process. Each digital algorythmic approximation
- has its set of attributes and drawbacks - and each will change the way an image
- is rendered from ideal. For example, the 'default' growth algorythm in the
- Apple LaserWriter series causes a harsh transition as the data crosses the 50%
- black point. This can be helped by changing the growth algorythm to the text-
- book circle, or oval, but these algorythms destroy fine hilight definition.
-
- Gamma compensation can help tremendously, as can decreasing screen frequency,
- although the latter will increase the grainyness of the image. There is a
- direct tradeoff between number of shades available at a certain screen angle,
- and the screen frequency.
-
- I hope this helps. If there are more questions that I can answer, don't
- hesitate to E-mail me.
-
-
-
- --
- +-------------------------------------------------------------------------+
- | Marc D. Spencer, C.E. WRUR-FM | Technology is a way of organizing |
- | mdsp_ltd@uhura.cc.rochester.edu | the universe so that man doesn't |
- | | have to experience it -Frisch |
-