File Preparation: Scanning Tips

Scanning Tips for the Production Artist

Do a little arithmetic before you scan. If you were to use an image scaled down to 50% of original size, this would double the image's resolution. Scaling a particular image to 33% of the scan triples the resolution, etc. A final resolution of 1.5 times the LPI works well in most cases. LPI stands for lines per inch, and for offset printing the LPI is usually 133 or 150 for printing on uncoated papers and 150 to 200 for coated sheets. Resolution in terms of PPI or pixels-per-inch. Final scan resolution means that you've calculated the change in spatial resolution when you size your original scan, so the formula is: PPI = LPI x % increase or decrease of original x 1.5.

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 an image editing application such as Adobe PhotoShop™ as closely as possible to avoid having the RIP compute alot of white space. Avoid using Adobe 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 linearly balanced, meaning that the transitions between grey areas tend to be minimized. Sharpening algorithms in an application like PhotoShop™ can adjust for this. Always run your program's Sharpen routine on a scanned photo to compensate for this.

Another tip is to add noise to an image to minimize the transitional steps between gradations. For instance if you have created a blend in your illustration application that has thirty steps, you can export the EPS file into an image editor and add 1 or 2 units of random or "gaussian" noise with the noise filter to take away the hard edges between steps.

Descreening & Scanning

If you frequently scan printed matter that is screened at 133 LPI or above, you probably see a moire pattern when you view the file in a photo manipulation application or attempt to print the picture. Try scanning at 718 dpi (with descreening on if your scanner has this function), apply PhotoShop's Noise: Median filter with a 2 pixel radius and reduce the resolution to 225 or 300dpi. then apply an unsharp mask with the settings at 50 percent, 3 pixels and a threshold of 5. You may have to play around with these settings, depending upon the degree of moire in your image, but you should get something acceptable.

Line Art Scanning

Line Art scanning is best defined as simply transferring pen and ink style drawings, clip art, and some pencil sketches into the computer system. Line Art has no shading nor color other than black. Line Art scanning is considered 1-bit scanning, that is the computer sees the image as either black or white. Line Art scans, since they can only be black or white, do not have the luxury of anti-aliasing (a technique that makes smooth transitions between black and white by adding varying steps of grey), and therefore are prone to having jagged edges around curves when scanned at lower resolutions than the final output device. Therefore, you should not use Line Art style scanning to scan photographs or textured art or art that is not black and white.

Line Art should be scanned at as high a resolution as the final output device to eliminate the "jaggie" effect. In other words if you have original art that is 5x7 and you're going to output at 5x7 on a 1200 dpi imagesetter, ideally it should be scanned at 1200ppi to achieve the best possible image. Note that if your original is larger than the final size, you should adjust your original scan resolution by the same ratio so as to not have more resolution than necessary.

Line art improves the output clarity on most laser printers without having the laser printer print the whole page at high resolution. This helps when you have halftones and line art scans on the same page. The bitmap items can be selected for spot coloring when imported into many page layout programs. When a line art image is required from art that is not black and white (for instance a signature drawn in blue ink), you may accomplish this by scanning the object in as grey scale at 1200 ppi, then use contrast and brightness in an image editing program until you get a 100% black and white image. Then convert the result to a bitmap image. If you will then be using it with a spot color in a page layout program, you should should save it as a TIFF (Tagged Image File Format).

The majority of line art that is scanned is usually some simple drawings or sketch used as a template for a more exact computer illustration. Using Corel Trace™, Adobe Streamline™, or the AutoTrace function in Macromedia FreeHand™, you can convert the line art scan into an EPS (Encapsulated PostScript) file and thus allow you to increase the image to any size with out the fear of the "jaggies." Of course, this is not a flawless step, you may well have to spend a considerable amount of time correcting for obscure autotracing errors.

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