Page 16 - CC2019_PS_Look_Inside

Image Resolution
When an output device creates halftone dots, it calculates
the average value of a group of pixels in the raster image and
generates a spot of appropriate size. A raster image’s
resolution — measured in pixels per inch (ppi) —
determines the quantity of pixel data the printer can read.
Images need to have suffi cient resolution so the output
device can generate enough halftone dots to create the
appearance of continuous tone. In the images above, the same raster image is reproduced at 72 ppi
(left) and 300 ppi (right). Notice the obvious degradation in quality in the 72-ppi version.
Ideally, the printer will have four pixels for each halftone dot
created. In the image to the right, each white square represents a
pixel. Th e highlighted area shows the pixel information used to
generate a halftone dot. If an image only has 72 pixels per inch, the
output device has to generate four halftone dots per pixel, resulting
in poor printed quality.
Th e relationship between pixels and halftone dots defi nes the
rule of resolution for raster-based images — the resolution of a 72 ppi 300 ppi
raster image (ppi) should be two times the screen ruling (lpi) that
will be used for printing.
Note:
Open and Resize an Image For line art, the general
rule is to scan the image
Every raster image has a defi ned, specifi c resolution that is established when the at the same resolution
image is created. If you scan an image to be 3″ high by 3″ wide at 150 ppi, that as the output device.
image has 450 pixels in each vertical column and 450 pixels in each horizontal Many laser printers and
row. Simply resizing the image stretches, or compresses, those pixels into a diff erent digital presses image at
physical space, but does not add or remove pixel information. If you resize the 600–1200 dots per inch
3″ × 3″ image to 6″ × 6″ (200% of the original), the 450 pixels in each column or (dpi). Imagesetters used
row are forced to extend across 6″ instead of 3″, causing a marked loss of quality. to make printing plates
Th e eff ective resolution of an image is the resolution calculated after any scaling for a commercial press
is taken into account. Th is number is equally (perhaps more so) as important as the typically output at much
original image resolution. Th e eff ective resolution can be calculated with a fairly higher resolution —
simple equation: possibly 2400 dpi or
Original resolution ÷ (% magnifi cation ÷ 100) = Effective resolution more.

If a 300-ppi image is magnifi ed 150%, the eff ective resolution is:

300 ppi ÷ 1.5 = 200 ppi
In other words, the more you enlarge a raster image, the lower its eff ective
resolution becomes. In general, you can make an image 10% or 15% larger
without signifi cant adverse eff ects. Th e more you enlarge an image, however, the
worse the results. Even Photoshop, which off ers very sophisticated formulas (called
“algorithms”) for sizing images, cannot guarantee perfect results.
Eff ective resolution can be a very important consideration when working with
client-supplied images, especially those that come from consumer-level digital
cameras. Many of those devices capture images with a specifi c number of pixels rather
than a number of pixels per inch (ppi). In this exercise, you will explore the eff ective
resolution of an image to see if it can be used for a full-page printed magazine ad.

28 Project 1: Music CD Artwork