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Upsampling Photographs and Line Art

More and more, interesting photos and other images are being included in books, advertising materials, and every other form of printed product. Look in any design magazine and you will see a plethora of ads for digital photo libraries from which you can purchase the rights to reproduce specialized photographic images and line art. In addition to on-line photo services, you can shoot images with digital cameras or even produce hard-copy glossy prints from film shot with traditional cameras. Should you need to prepare the image you have captured into a form suitable for offset printing, scanning and/or some form of image manipulation would be involved.

The general rule of thumb regarding scanning any photograph is to scan at twice the photo’s printed line screen. If your offset printer will print halftones (b/w or color) at 133 lpi (the number of lines of halftone dots per inch), you should scan your photos at 266 dpi (dots per inch). Anything higher is a waste of computer memory because it does not improve the photographic quality.

An important, related item is to factor in the final size of the image you scan. If your scanned image will be used at 100 percent of the size of the original, the formula noted above is adequate. However, if you scan the image and then reduce it, you can get away with less than 266 dpi. An example would be to scan the original at 133 dpi and then make your photo 50 percent smaller.

Although scanning at the final size is preferable, you can reduce or enlarge an image slightly (maybe 105 –110 percent of the original size, or 5-10 percent enlargement) without compromising the photo’s quality. Where people make the most drastic mistakes (which really compromise the final image) is by scanning a photo and then enlarging it more than a few percentage points.

When you ask Photoshop (or any other image manipulation software) to increase the size of the image once it has been scanned, and then increase the resolution as well, this is called upsampling. (An example of this is to take an Internet image saved at 72 dpi and then enlarge it and upsample it to 300 dpi). As Photoshop does this, it essentially makes up picture information that doesn’t exist. As the picture gets larger, Photoshop adds pixels (picture elements, essentially dots that are averages of the existing pixels) between those already there. Past a certain point of over-enlargement (beyond 105-110 percent of the original size), what you get is a blurred image and/or visible pixels.

The same doesn’t exactly apply for line art (drawings with only black lines and no shades of gray).

A scan of a line drawing (with no shading) should be made at 1200 dpi at the final size (100 percent size) to not show any visible pixels. However, upsampling (which is also called interpolating) is often more forgiving with line art than with halftones. Although I have never read this anywhere, I have found this to be true in my own experience, and this is my theory as to why.

When you enlarge a color or b/w image and use Photoshop to upsample, you are averaging between pixels of process color, PMS color, or various shades of gray. When you upsample a line art image, however, you are not averaging between shades of gray or between colors, since all pixels are either black or white. If you were to enlarge the image without increasing the resolution, your pixels would become visible dots or squares (this would cause undesirable patterns). But by enlarging the line art image in Photoshop and at the same time upsampling (or increasing the resolution, let’s say from 600 dpi to 1200 dpi), you make the pixels smaller (and therefore less visible, making the image look like a continuous line drawing and not a pattern of dots). In this case, unlike enlarging halftones, you are just adding or removing smaller and smaller pixels that are either black or white. If the resolution is sufficiently high, the eye will not distinguish between the separate pixels.

This has worked for me, but you should always look closely at your laser output before committing to film or plate at the offset printer.

Another thing to remember when enlarging or reducing images is that the above only pertains to bitmapped, or rasterized (dot pattern), art. Vector art, on the other hand, is created in Illustrator or Freehand and is made up of PostScript lines, curves, and fills (essentially mathematical equations) rather than a grid of dots. “Vector art” refers to drawings, not photographs. It is superior in its own right since it never has jagged edges from visible pixels, as does rasterized art. This is true whether or not it is reduced or enlarged. Vector art can therefore be reduced or enlarged to any size with no degradation in quality.


[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]

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