Upload File

11223344

2
I m ages i nt end ed t o be vi ew ed on a co m puter s cr een are best m easured di r ect l y i n pi xel s, because t he out put devices, m oni t ors and pr oj ectors are general l y t hought of i n t erm s of t he num ber of pi xel s t hey di spl ay , 600 x 800 f or exam pl e. D eter m i ni ng t he r esolut i on of i m ages i nt ended for pri nt m ustt ake i nto acco unt several addi t i onal cri t eri a. First of al l, an i m age on paper is m or e com m onl y t ho ught of by i t’ s phy si c a lsize. A l so t he out put devi ces, pri nt ers , are m easured i n t w o w ay s . T he m or e com m on m easure is t he si ze of t he sm al l est dot ofa si ngl e col ort hat t he pri ntercan m ake, general l y descr i bed by how m any can f i t i n an i nch, hence dot s per i nch or dpi . T hi s descr i bes how s harp t he pr intercan m ake a si ngl e color i m age in t he col or i nks av ailabl e. This is appropri ate f or i m ages r ef err ed to as l ine copy w her e t her e ar e no i nt erm edi at e t ones, onl y t he w hi t e of the paper and t he solid appl i cati on of one of t he colors of t he i nk avai l abl e, general l y cyan, m agent a, yellow and black. I m ages of t hi s t ype should be sc anned at the f ul l resoluti on of th e pri nte r . I n a c tu a l pra ctice, i t i s di f f i cult f or t he hum an eye t o t el l t he di f f erence of anyt hi ng over 400 dot s per i nch, so no m at t er w hat t he r esol uti on of t he p ri nt er , scanni ng li ne copy over 400 dot s per i nch yi el ds di m i ni shi ng returns. I n o r der t o si m ul at e gr ey t ones, pri nt er s use a schem e know n as hal f t oni ng, w here cl ust ers of t he d ots t he pri nt er i s capabl e o f are used t o si m ul at e shades of grey. I n col or printi ng the i m age i s di vi ded i nto i t’ s cy an, m agent a, yel l ow and bl ack com po nent and each i s pri nt ed in a hal f tone pat tern, w hi ch t he eye and brai n r econst ruction i nto t he f ul lspect rum of col ors . T hese hal f t one pat t erns ar e m easured in l i nes peri nch. Iti s t hi s hal f t one f requency t hat det erm ines how sharp an i m age a gi ven pr i nt er can pr oduce. It i s qui t e rare f or t hi s m easure to be publi shed i n pri nt er m anual s, and m odern col or printers especi al l y use vari ant s of t he schem e for w hi ch t his hal f t one f requency i s no tas obvi ous as w i t h a t radi t i onal pri nt ed hal f t one. T he b est s chem e is to m ake a a s s um pt ion o f w hat t he hal f t one f r equency i s based on t he f ul lr esol ut i on of t he printer. N ew laser pri nt ers w ill have a hal f tone f r equency about 85 C olor i nkj et pri nt ers pri nt i ng at 720 dpi have a hal f t one f r equency ab out 90 li nes per i nch and at 1440 dpi , about 130 lines per i nch. T he gener al recom m endat i on f or t he cor r ect resol uti on f or an i m age is one and a hal f t o t w o ti m es t he hal f t one f r eq uency , dep endi ng on how cri ti calyou are. So f or a bl ack and w hi t e l aser pri nt er , you w oul d hav e t o hav e bet w een 120 and 170 dots per i nch. A s w ith any appli cat i on, you w ant t o have enough dat a t o get a s harp i m age, butno m ore. The i m ages on t he acc om panyi ng page dem onst r at es t he pract i cali m pl i cat i ons of thi s. T he t op r ow of i m ages r ange f r om t he t op end of t he range, 180 d pi , a lit t l e m ore than t w i ce the hal f t one f r eq uency , t o 105 dpi , onl y abo ut 125% of t he hal f t one f r equency . I t’ s rather di f f i cul tto tel l th e di f f ere nce betw een these i m ages. On cl ose i nspection t he higher r esol ut i on i m ages seem sharper , but i t ’ s hard t o i dent i f y speci fic detail s t hat cont ri but e t o t hat di f f erence, and w i t hout the 180 dpi i m age t o com par e i tto, i t w oul d be eas y to as s um e t he 105 dpi i m age w asas good as i tcoul d be. T he 80 dpii m age atfi rs t gl ance l ooks pret t y go od, but on caref uli nspection, t he curves of t he l am p base and t he out er edge of t he cat ’ s eye r eveal t he st air s t ep pat tern o f t he square pi xel s t hat m ake up the i m age. The poi nt s of t he tuf t s of furon her chest are no t as s harp and w ell def ined. I n the 60 dpi i m age t he pi xel s can be seen t hroughout t he i m age t o a car ef ul vi ew er , and at 40 dpi , t hey do m inat e t he i m age. Imag e resolution for print 800 dpi 400 dpi 200 dpi 100 dpi Line copy examples

Upload: omar-gonzales

Post on 02-Jun-2018

220 views

Category:

Documents


0 download

Report

TRANSCRIPT

Page 1: 11223344

8/11/2019 11223344

http://slidepdf.com/reader/full/11223344 1/2

Im ages intended to be view ed on a com puter screenare best m easured directly in pixels, because the outputdevices, m onitors and projectors are generally thoughtof in term s of the num ber of pixels they display, 600x 800 for exam ple.

D eterm ining the resolution of im ages intended for printm ust take into account several additional criteria. Firstof all, an im age on paper is m ore com m only thoughtof by it’s physical size.

A lso the output devices, printers, are m easured in tw ow ays. The m ore com m on m easure is the size of thesm allest dot of a single color that the printer can m ake,generally described by how m any can fit in an inch,hence dots per inch or dpi.

This describes how sharp the printer can m ake a singlecolor im age in the color inks available. This is appropriatefor im ages referred to as line copy w here there are nointerm ediate tones, only the w hite of the paper and

the solid application of one of the colors of the inkavailable, generally cyan, m agenta, yellow and black.

Im ages of this type should be scanned at the fullresolution of the printer. In actual practice, it is difficultfor the hum an eye to tell the difference of anythingover 400 dots per inch, so no m atter w hat the resolutionof the printer, scanning line copy over 400 dots perinch yields dim inishing returns.

In order to sim ulate grey tones, printers use a schem eknow n as halftoning, w here clusters of the dots theprinter is capable of are used to sim ulate shades ofgrey. In color printing the im age is divided into it’s cyan,m agenta, yellow and black com ponent and each is

printed in a halftone pattern, w hich the eye and brainreconstruction into the full spectrum of colors.

These halftone patterns are m easured in lines per inch.

It is this halftone frequency that determ ines how sharpan im age a given printer can produce.

It is quite rare for this m easure to be published inprinter m anuals, and m odern color printers especiallyuse variants of the schem e for w hich this halftonefrequency is not as obvious as w ith a traditional printedhalftone.

The best schem e is to m ake a assum ption of w hat thehalftone frequency is based on the full resolution ofthe printer.

N ew laser printers w ill have a halftone frequency about85

C olor inkjet printers printing at 720 dpi have a halftonefrequency about 90 lines per inch and at 1440 dpi,about 130 lines per inch.

The general recom m endation for the correctresolution for an im age is one and a half to tw otim es the halftone frequency, depending on howcritical you are.

So for a black and w hite laser printer, you w ouldhave to have betw een 120 and 170 dots per inch.

A s with any application, you w ant to have enoughdata to get a sharp im age, but no m ore.

The im ages on the accom panying page dem onstratesthe practical im plications of this.

The top row of im ages range from the top end ofthe range, 180 dpi, a little m ore than tw ice thehalftone frequency, to 105 dpi, only about 125% ofthe halftone frequency.

It’s rather difficult to tell the difference between theseim ages. O n close inspection the higher resolutionim ages seem sharper, but it’s hard to identify specific

details that contribute to that difference, and w ithoutthe 180 dpi im age to com pare it to, it w ould be easyto assum e the 105 dpi im age w as as good as it couldbe.

The 80 dpi im age at first glance looks pretty good,but on careful inspection, the curves of the lam pbase and the outer edge of the cat’s eye reveal thestair step pattern of the square pixels that m ake upthe im age. The points of the tufts of fur on her chestare not as sharp and w ell defined.

In the 60 dpi im age the pixels can be seen throughoutthe im age to a careful view er, and at 40 dpi, theydom inate the im age.

Image resolution for print

800 dpi 400 dpi

200 dpi 100 dpi

Line copy examples

Page 2: 11223344

8/11/2019 11223344

http://slidepdf.com/reader/full/11223344 2/2

180 dpi 140 dpi 105 dpi

80dpi 60dpi 40dpi


11223344 ‘s FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY

11223344 FEDERATION FEUD FEDERATION FEUD FEDERATION FEUD FEDERATION FEUD

11223344 SHRM Membership TEAM 2 TEAM 1 50 30 20 Networking w/ Others Continuing Education Access to Info CheerSilenceLoseWinBoo Round 1 What did individual

11223344 FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD FAMILY FEUD