Greyscale Scanning
Greyscale scanning is most often used for original photographs, but can
also be effective for maintaining the soft edge of some pencil sketches that
use lots of shading. Most flatbed desktop scanners are capable of
scanning 256 shades of grey. Some 30- and 36-bit scanners tout 4096 shades of grey.
While this may be true, you probably won't be able to use them. The current PostScript
specification only allows for 254 halftone steps (the way a shade of grey is
represented in print, 254 steps with pure white and pure black which makes 256
total). The extra grey shades (bit depth) may help to improve the scanners optical
density (OD) range if implemented properly. This can give you better shadow detail.
Unfortunately most moderately priced 30- and 36-bit desktop scanners implement the
the expanded bit depth only in color, opting for a more common lookup table (LUT)
approach to greyscale scanning.
Nevertheless, 256 shades of grey will probably be sufficient for just about all your
needs (unless you plan to scan Tomographs, or high-density negatives). Few desktop
printers can do justice to even 100 shades of grey. Even Ansel Adams landscapes
were judged good enough at 256 shades to be sold as screen savers.
Principles of Greyscale Scanning
Most of us scan at too high a resolution. As we increase scan resolution,
we capture too much detail--and the scan file can balloon to outrageous
size. The PostScript imagesetter, desktop laserprinter, or other device
must render all of the data, using lots of costly RIP time [ RIP - Raster Image Processor, the
specialized computer in your laserprinter or more expensive imagesetters ].
In addition, as you work with a scanned image, the computer constantly has to re-crunch
the scan data. Over the course of a project this time really adds up. As we count on
computer's CPU to do more and more of the work for screen presentation or ink jet
printer output we can slow our system to a crawl by using too much data.
Scans with too much data not only rob your time, they can even crash the
RIP or lock-up your computer.
Take a few moments to calculate what you need rather than scanning at a high
DPI just to be on the safe side. Capture just enough detail for what you
need to do.
Production Tips (Greyscale Scanning)
- Do a little arithmetic before you scan. Planning to reduce the image
to 50%? This doubles the resolution. Reducing to 33% triples the
resolution, and so on.
- For greyscale, a final resolution of 1.5 times the LPI works well in
most cases. For soft subjects you can get away with 1.3 times the LPI.
(i.e. - Linotronic output at 1200 dpi, 133 lpi your scan resolution should
be no greater than 200 dpi). Final scan resolution means that you've
calculated the change in spatial resolution when you size your original
scan, as discussed in the above tip) So the formula is:
DPI = LPI x %increase or decrease of original x 1.5
- Avoid data overkill!
- Clean up scans in a retouching program before using them. Adjust their
size and resolution before placing them in a layout program.
- Crop the image in a package like PhotoShop as close as possible to avoid
having the RIP compute all that white space. Avoid using
PageMaker or other layout programs ability to crop a scan, this feature
merely masks an area and does not decrease the data that the RIP must
compute.
Sharpening the image in a retouching program gives a better looking
result than using a high scan resolution. Make the on-screen image just a
little sharper than you think is needed--it softens a bit in production. The
CCD chip in your scanner that actually reads the greyscale data is linearily
balanced, meaning that the transitions between grey areas tend to be
minimized. Sharpening algorithms in a retouching program like
PhotoShop can adjust for this. Always run your program's Sharpen
routine on a scanned photo to compensate for this.
- Another little known tip is to add noise to a scan to minimize the
transitional steps between your grey scales. This is a technique often
employed by illstrators and four-color shops for smoothing a low resolution
scan. For instance if you have created a blend in your illustration package
that has only thirty steps, you can export the EPS file into your favorite
image editor and add 1 or 2 units of random noise with the noise filter to
take away hard edges and cheat your eye. This technique works well for
ultra-high and very low resolution scans too that either exceed the grey
level of your imagesetter or produce very pixelated images. Simply add a little
noise to the image and you "fool" the eye.
Common Mistakes
- Taking a big high resolution scan and resizing it in your page layout or word processing software to fill a tiny area--16
Megabytes for a postage stamp--yow! Reduce resolution by sampling down in a
retouching program or better yet re-scan at a lower resolution.
- Cropping a little piece of a big image in a page layout program--the
RIP has to process that whole big image, then throw away what's outside the
cropping window. Crop first in a retouching program, and bring what you
need of the image into the page at the proper size.
- Thinking that white borders dont count--white creates data too. Crop
the borders away while scanning to save the most time, or later in your
image editing software.
- Using a page layout program to rotate big images. Most RIPs like
image data straight. Twisting all those pixels is risky, lengthy work.
Rotate in a retouching program or rescan in the right orientation.
- Scanning Line Art as a grey scale because it has fancy stippled
shadow detail. Scan your artwork at a higher Line Art resolution rather than grey scale.
2x3 inch 150 dpi 8-bit grey scale is 2 Meg worth of data compared to 600K
for a 2x3 inch 400 dpi 1-bit Line Art scan.
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