SCANNING IMAGES

HALFTONES

One of the most common problems encountered in digital prepress is that images are supplied to Webcom at too low a resolution. The final resolution of scanned halftone images should generally be 300 dpi at 100% of the reproduction size. If you are in doubt about the resolution at which to scan your original image, calculate the scanning resolution using the following equation:

2 × line ruling (in lpi) × reproduction factor

The line ruling used for covers is 150 lpi and for text pages it is between 120 and 150 lpi.

To find the reproduction factor, divide the final size by the original size. For example, you have a 4" × 5" original which is to be reproduced on a cover as at 10", the reproduction factor (using the width of the original) is:

8" ÷ 4" = 2"

In this example, the image is for the cover, so the line ruling is 150 lpi. Therefore, you would need to scan the original at:

2 × 150 lpi × 2 = 600 dpi

This resolution amount may seem high, but bear in mind your 4" × 5" original is being enlarged by 200%.

If you had an 8" × 10" original that was only going to be reproduced at 2.5", then the reproduction factor would be:

2" ÷ 8" = 0.25"

You would need to scan the original at:

2 × 150 × 0.25 = 75 dpi

This resolution amount may seem low but your 8" × 10" image is being reduced to 25% of its original size.

This is generally a good way to calculate your scanning resolution, except that sometimes you have no way of knowing what the final dimensions of the image will be. If this is the case, scanning at a slightly higher resolution will allow you to enlarge the image slightly without sacrificing quality. Remember though, if you have a 300 dpi image that you enlarge to 200%, it is now only a 150 dpi image!

LINE ART

While the scanning resolution equations mentioned above are helpful for halftone scans (people, landscapes, etc.), line art graphics such as logos, type, CAD drawings, etc., are treated somewhat differently. For line art, a high resolution scan is required. They should also be scanned as Monochrome and not as Grayscale.

You may use the following chart as a quick reference for minimum and ideal image resolution. The exceptions to all of these rules are screen captures, which are 72 or 96 DPI and should remain at that resolution.

DOT GAIN CONSIDERATIONS

You may be accustomed to scanning your images while keeping dot gain on press in mind. With Webcom’s Computer-to-Plate system, this is no longer necessary. Our platesetters have calibration sets built in that will automatically compensate for dot gain as they output the plates.

Scans supplied to Webcom should not compensate for dot gain or they will look washed out due to the combined effect of the two compensations. Instead, scan your halftones with a 5% highlight dot and a 95% shadow dot. For grayscale images, the highlight is the lightest area carrying detail and the shadow is the darkest area carrying detail. For colour images, the highlight is the lightest neutral area carrying detail and the shadow is the darkest neutral area carrying detail.

The size of the midtone dot will depend on the nature of your original. If you have a high-key original (i.e. a polar bear in a snowstorm) you will want a larger midtone dot in order to provide better detail in the highlights. If you have a low-key original (i.e. a black cat in a coal bin) you will want a smaller midtone dot in order to provide better detail in the shadows.

These guidelines should be used as a starting point only and may not necessarily ensure optimum scanning results in every situation. Each original piece being scanned may require individual consideration.

CAD DRAWINGS

CAD drawings may require special handling because, in many instances, the lines that make up the drawing may be too thin to image properly. Contact your Sales Representative or Project Manager for more information on these types of graphics.

Scan your halftones with a 5% highlight dot and a 95% shadow dot.

Use EPS for vector-based graphics and TIFF for bitmap images. GIF or JPEG formats are only appropriate for use on the Web or in multimedia presentations.

IMAGE FILE FORMATS

Your image editing or scanning software allows you to save images in a variety of file formats. With so much variety, how do you know which file type suits your needs?

Basically, there are three types of graphics file formats: bitmap, vector and metafile. Pictures created as rows and columns of pixels are generally referred to as bitmap files. Each pixel is assigned a particular colour or gray value and the combination of all of the pixels make up the image. Image editing programs such as Adobe PhotoShop^®, or the software you use to run a scanner, are some of the more common applications for creating bitmaps. TIFF (tag image file format), BMP (Windows bitmap), MacPaint^®, and PCX are all examples of bitmap-format graphics files.

Another type of file format commonly used is the vector-based file format. Vector-based files contain mathematical instructions for drawing objects. These expressions describe shapes such as lines, ellipses, polygons, rectangles, and arcs. Simply put, the vector file gives the instructions, “Start this line at point A and draw to point B” or “This is a polygon made up of the following line segments.” PostScript^® paths and type, such as those you’d find in an Illustrator EPS (Encapsulated PostScript) file, are examples of vector-format graphics files.

The third popular format is called a metafile. Metafiles may contain both vector and bitmap graphics, but don’t necessarily have both – sometimes you’ll find metafiles that contain only an image. Macintosh PICT^®, CGM (Computer Graphics Metafile), and WMF (Windows Metafile) formats are all examples of metafiles. Illustrator EPS files might contain vector information and a placed bitmap image but these are not considered Metafiles.

For files being sent to Webcom, make sure to use EPS for vector-based graphics and TIFF for bitmap images. Avoid using any other file format because, although they may print well on non-PostScript^® devices, they may not print well on PostScript^® ones.

The GIF file format is only appropriate for viewing on screen – either on the Web or in multimedia presentations. If you’ve ever printed pictures from a web site, you probably have noticed that the low resolution nature of the graphics resulted in a poor quality print-out. JPEG images are sometimes used for printing but this format is a "lossy" format and should be avoided.

Most professional programs allow you to convert files from one format to another. Look for a Save As, Save a Copy or Export command.