TSF-GreyScale

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 shrinking it to tiny size--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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