Digital Prints

want to scale the image’s printed size during the scan, do NOT change the scanning resolution
setting like you would to change the size on the computer monitor; you’ll always
get the same printed size at 100%. Instead, leave the scanner’s resolution setting at 200 or
300 dpi (for prints), or whatever is appropriate for your printer, and instead use the
Output or Target field to modify the output size, which in turn modifies the scaling.
Keep in mind that the output resolution number is just a note or comment that is carried
along with the image. This is the scaled resolution, or the printing resolution, and it is
just a number, a numerical value, nothing more. It only matters to the printing software,
to tell it how to size the scanned image pixels on that output device later. Scaling is
intended for printing.
(Caution! You may not be aware of the actual scanning resolution when scaling. The real calculated
scanning resolution is not shown [on most scanners], and there is a risk that if you don’t
pay attention, you might exceed the scanner’s optical rating without realizing it. You can happily
set the resolution to 300 dpi and the scaling to 300% to get a larger image size. The scanner
scans at 900 ppi in that case, but you might have a 600-dpi scanner, which must interpolate
to do what you’ve asked. No hand comes out to slap you, but you’ve just exceeded the optical
capability of the scanner. A few scanners may refuse, or may show red numbers then, but most
are silent. You typically don’t have any warning of this, you just have to be careful and realize
how it works. Interpolated results won’t be as sharp as you expect.)
Resampling is very different from scaling. Scaling does not affect any pixel, but resampling
changes all pixels, and the total number of pixels in the data become different. Resampling
is a drastic procedure that actually recalculates all of the image’s pixel color data values to
produce a different size of image. Most people know that image-editing programs like
Photoshop are usually used for resampling, but scanners can resample, too. Look at a
1200-dpi flatbed scanner. This scanner has 1200 CCD cells spaced 1/1200 inch apart. A
1200-dpi scanner can therefore only scan samples at 1200 dpi horizontally. When you
scan at, say, 520 dpi, the scanner must resample the 1200-dpi scan line to 520 dpi. Some
scanners use the Bilinear resampling method (creates the new pixel to be the color interpolated
from linearly weighting the value and distance of the old pixel on either side of
the new pixel on the same row) and some use Nearest Neighbor (creates the new pixel
simply to be the same color of the one closest to the adjacent old pixel) to resample the
scan line horizontally. Some users claim it is better to always scan at full optical resolution
and then resample back to the desired dpi in an external program. Their point is that programs
like Photoshop have all of the rows and columns available for Bicubic resampling
(creates the new pixels from the color of two pixels in either direction), and your computer
has much more memory and processor power than the scanner.
Resampling is just interpolation, either downsampling (reducing image size by discarding
data and detail, replacing many pixels with a few) or upsampling (increasing size to a larger
image by fabricating additional data, replacing a few pixels with many). It is always better
to resample to reduce the image size, rather than to resample to increase the image size.
(For more about scaling and resampling when image editing, see “Sizing with Image
Editing” and also Chapter 11.)
■ What’s the deal about scanning with integer numbers? Some claim that you can scan at less
than full optical resolution, but you should scan only at values of full optical resolution
divided by integers (1, 2, 3, 4, 5, etc.). So, for a 600-dpi scanner, the idea is that you should
scan only at 300 or 150 or 100 or 75 dpi. Some scanners provide only these integer divisor
choices as presets, and others provide them mixed in with other options (see Figure
3.4). The idea is that an integer divisor makes resampling easier, with better results, because
Chapter 3 ■ Creating and Processing the Image 81